Abstract:
We describe a soil chemistry sensor for in-situ soil chemistry sensing, the sensor comprising a probe incorporating a first, ion-selective electrode and a second, reference electrode, wherein said ion-selective electrode comprises a first-electrode housing defining a first lumen having an ion-selective plug towards a distal end, said first-electrode including a first conductor in a first electrolyte, wherein said reference electrode comprises a second electrode housing defining a second lumen having a porous reference electrode plug towards a distal end, said second electrode including a second conductor in a second electrolyte, wherein said ion-selective plug and said porous reference electrode plug are within 10 mm of one another, and wherein said porous reference electrode plug and said ion-selective plug each comprise a polymer.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to soil chemistry sensors, in particular for sensing nitrate levels, and to the fabrication and use of such sensors. 
       BACKGROUND TO THE INVENTION 
       [0002]    Inorganic fertilisers, in particular nitrogen-based fertilisers, have revolutionised agriculture. However effective management of the amount of fertiliser used on land is important for many reasons including economic efficiency and environmental management—excess nitrogen run off from agriculture can enter drinking water causing significant human health concerns, and can also damage fresh water and marine eco-systems. Various techniques are known for monitoring the nitrate level in soil. Typically a soil core is taken, the nitrogen is extracted, and this is then used to predict the amount of nitrogen available to the roots of plants in the soil. However this is a time consuming process typically involving taking a number of soil cores and waiting on average two weeks for the results to come back from the laboratory. Based on the results a farmer will decide, for example, how much nitrate-based fertiliser to use in a spring and/or autumn application. 
         [0003]    This procedure is cumbersome, slow and expensive and there is a need for improved techniques, and in particular effective but relatively inexpensive techniques. Such techniques would also be of use in the developing world. 
         [0004]    Adamchuk et al (Adamchuk, V., Lund, E., Sethuramasamyraja, B., Morgan, M., Dobermann, A. and Marx, D., 2005. Direct Measurement of Soil Chemical Properties on-the-Go using Ion-Selective Electrodes.  Computers and Electronics in Agriculture , vol. 48, no. 3, pp. 272-294) provides a relatively recent review of soil sensing techniques, but all the techniques described there involve extracting a soil sample and analysing it afterwards in an aqueous solution. Ito et al (Ito, S., Baba, K., Asano, Y., Takesako, H., 1996. Development of a Nitrate Ion-Selective Electrode Based on an Urushi Matrix Membrane and its Application to the Direct Measurement of Nitrate Nitrogen in Upland Soils.  Talanta , vol. 43, no. 11, pp. 1869-1881) describes a nitrate ion-selective electrode based on Urushi, a natural oriental lacquer. Earlier work by one of the inventors (Miller, A. J. and Zhen R-G.,  Measurement of intracellular nitrate concentrations in Chara using nitrate - selective microelectrodes , Planta 1991 184:47-52) describes the fabrication of nitrate-selective microelectrodes for measuring nitrate concentrations in plant cells. 
         [0005]    There is a need for improvements in measuring/monitoring the levels of nitrate (and other plant nutrients). 
       SUMMARY OF THE INVENTION 
       [0006]    According to the present invention there is therefore provided [as claim  1 ]. 
         [0007]    The inventors have established that for in-situ sensing of soil chemistry, that is sensing by inserting the probe directly into the soil, one key difficulty in practice is ensuring effective operation of the reference electrode. In embodiments this is addressed by fabricating the reference electrode plug or membrane in a similar manner to that used for the ion-selective electrode plug/membrane, in particular by solvent-casting a polymer material into the end of the lumen, for example high molecular weight PVC (polyvinyl chloride). In some preferred embodiments this material includes one or more additives to lower its electrical resistance. 
         [0008]    It has also been found to be important to keep the ion-selective and reference electrodes, more particularly their porous plugs, adjacent to one another, for example separated by less than 20, 15, 10, 5, 3, or most preferably 2 millimetres. This is because soil moisture content can vary and with wider spacings the results can be unreliable, becoming overly-dependent on soil moisture level. In preferred embodiments a fastening means is provided around the distal ends of the electrodes to inhibit these from moving apart during insertion into the soil. 
         [0009]    For related reasons it has also been found to be important to make the sensor waterproof (except for the porous plugs). Thus preferably the ion-sensing and reference electrodes are sealed within a waterproof enclosure, in embodiments fabricated from cold-shrink tubing: such material is able to achieve a tight seal without the application of heat, which is important for practical fabrication of the probe. 
         [0010]    In some preferred embodiments the second, reference electrode is a double junction electrode, in particular including a second electrode chamber connecting with the first via a second porous plug, preferably also fabricated from a polymer material, in a similar manner to the outer porous reference electrode plug. In practical trials the use of a double junction has been found to provide more stable results (sensing signals) and improved probe lifetime, as well as better protection for the probe overall when used in soil. 
         [0011]    In embodiments the soil chemistry sensor, more particularly the probe, incorporates soil moisture sensing means such as a pair of exposed electrical conductors. This allows the soil moisture content to be sensed so that, optionally, the signal from the ion-selective electrode may be compensated for the amount of soil moisture present. In principle, however, such soil moisture compensation may be performed using a separate sensor. 
         [0012]    In some preferred implementations the soil chemistry sensor is provided in combination with a high input impedance voltage sensor to sense the voltage from the probe. Advantageously a probe may be provided with a wireless data transmitter, for example a Wi-Fi and/or mobile phone network transmitter to communicate sensor data from the probe. Where a wired connection to the probe is employed this can tend to guide rainwater (along the outside of the wire) affecting the local nitrate levels. 
         [0013]    In one preferred implementation a plurality of probes is connected to a shared data logger, which may have a (shared) wireless network connection for exporting captured data, via one or more voltage sensors (one sensor may be multiplexed across a plurality of probes). This facilitates, for example, the soil chemistry to be measured at multiple different depths within the soil, say 30 cm, 60 cm and 90 cm, and/or at multiple different locations within a field. This can provide improved data for more efficient applications of fertiliser by varying the application of fertiliser with location within a field and/or dependent on a measured profile of soil chemistry according to depth. In embodiments a probe or set of probes may include a temperature sensor for adjusting or compensating measurement signals for temperature for improved accuracy. 
         [0014]    The invention also provides a method of collecting soil chemistry data directly from soil, in particular by inserting a probe as described above into the soil. 
         [0015]    The use of a probe, or multiple probes, as described above facilitates measuring/monitoring the actual levels of a plant nutrient in the soil, which can change quite dynamically over time and/or other conditions such as moisture level and the like. 
         [0016]    A low-cost sensor is desirable, for example for developing world applications, and also when wishing to deploy multiple sensors over an area of agricultural land. It has been found that effective electrodes can be fabricated from a disposable plastic pipette tip (for example of, PVC or polypropylene). Preferably the plugs are then formed by solvent-casting a polymer again, for example PVC, as previously described. In such a case it has been found that increased reliability can be obtained by treating the lumen of an electrode in which the plug is being formed so as to increase its internal hydrophobicity. This may be achieved in various ways including, for example, by plasma treatment but in one straightforward approach an emulsion of polydimethylsiloxane, silica filler and non-ionic emulsifier (Dow Corning® Repelcote) is employed. 
         [0017]    Some preferred embodiments of the above described soil chemistry sensor are used for in-situ soil nitrate level sensing, but depending upon the ion-selective material employed in the first electrode plug other chemicals relevant to environmental monitoring and/or plant nutrition may also be sensed including, but not limited to: ammonium, potassium and phosphorous; variants of the sensor may also be employed for sensing soil pH. 
         [0018]    In embodiments, the above-described soil chemistry sensor is used for identifying a soil chemistry depletion zone around a plant root. A nutrient depletion zone develops around a root when nutrients are removed from the soil solution faster than they can be replaced by the movement of nutrients through the solution. For example, ions which have low mobility in the solution may produce a sharp/narrow depletion zone close to the root. 
         [0019]    In embodiments, a plurality of soil chemistry sensors are used for determining soil chemistry data at a plurality of depths in a soil sample. This may enable determination of how chemicals move through soil (without any plants), and/or how chemicals are absorbed by plant root systems. For example, roots close to the surface of soil may absorb chemicals in the soil at a different rate to those deeper in the soil. 
         [0020]    The soil chemistry sensors described thus far are capable of measuring the soil chemistry in situ at a certain distance below the surface of the soil, where the distance itself (i.e. depth) is limited by the length of sensor/sensor electrodes. It may be desirable to measure the soil chemistry deeper in a soil structure in a field, and/or to measure the soil chemistry at multiple different depths in a soil structure simultaneously, and/or to obtain real-time soil chemistry data in situ. If real-time measurements indicate that fertilizer needs to be added to the top of the soil, but that the soil is prone to leaching, then a farmer may use the information to decide when to irrigate after applying fertilizer to his crops to minimise the effect of the water washing away the nitrates from near the top of the soil surface. 
         [0021]    Thus, an aspect of the present invention provides a probe for in situ soil chemistry sensing, the probe comprising: a longitudinally extending housing with a tip for penetrating soil, said housing having at least one sensing region, wherein said sensing region comprises at least one sensing membrane to permit ions from said soil to enter said probe, and wherein said sensing region is on a lateral wall of said housing displaced away from (i.e. near but not on) said tip of said probe. 
         [0022]    In embodiments, the sensing region comprises a pair of said membranes, an ion-selective membrane and a reference membrane, for respective ion-selective and reference electrodes of a sensor within said probe. 
         [0023]    In embodiments, the sensor comprises conductive electrolyte or gel between each respective membrane and a respective electrode connection. 
         [0024]    A further related aspect of the invention provides a sensor for in situ soil chemistry sensing comprising a pair of membranes in a common surface, an ion-selective membrane and a reference membrane for respective ion-selective and reference electrodes of the sensor and conductive electrolyte or gel between each respective membrane and a respective electrode connection. 
         [0025]    An aspect of the invention provides a probe for in-situ soil chemistry sensing, said probe comprising: one or more soil chemistry sensors, wherein each of said one or more soil chemistry sensors comprises: a first reference membrane and a second ion-selective membrane, a first electrical connection coupled to said first reference membrane; and a second electrical connection coupled to said second ion-selective membrane; wherein said first and said second electrical connections provide a soil chemistry sensing signal; and a tube comprising one or more sensing apertures along a length of said tube, wherein one of said one or more sensors is provided in one of said one or more apertures, and wherein said probe is capable of said soil chemistry sensing at one or more depths below a soil surface. 
         [0026]    In embodiments, each of the soil chemistry sensors is detachably attached to said tube in said apertures. 
         [0027]    In preferred embodiments, the first reference membrane and the second ion-selective membrane are close together, preferably within 2 mm of one another. 
         [0028]    In a related aspect of the invention, there is provided a method of providing nutrients to a plant, said method comprising: inserting a probe, in particular as described above, vertically into soil around said plant; sensing soil chemistry at one or more depths below a surface of said soil; identifying depletion of said nutrients at said one or more depths below said surface of said soil; applying nutrients to said soil in response to said depletion. 
         [0029]    In embodiments, the sensing occurs at a first depth close to said surface of said soil and a second depth further away from said surface of said soil, and further wherein said nutrients are depleted if said nutrient concentration at said first depth is relatively less than said nutrient concentration at said second depth, preferably 10-fold less. 
         [0030]    A further aspect of the invention provides use of a soil chemistry sensor/probe, for determining when to apply fertilizer to a plant, the method comprising: sensing a level of nutrient at a plurality of different depths in the vicinity of said plant, using the sensor/probe, wherein said depths identify a nutrient depletion zone, said nutrient depletion zone comprising a region, defined by depth, where uptake of said nutrient by said plant is relatively greater; determining a ratio of sensed nutrient levels between at least two of said different depths to determine an uptake of said nutrient by said plant from a relative depletion of said nutrient at one of said depths with respect to another; and determining when to apply said fertilizer based on said determined ratio. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]    These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which: 
           [0032]      FIG. 1  shows a soil chemistry sensor according to an embodiment of the invention (the inset shows an actually-constructed embodiment); 
           [0033]      FIG. 2  shows a calibration curve for the sensor of  FIG. 1 ; 
           [0034]      FIG. 3  shows a sensor system incorporating multiple soil chemistry sensors of the type shown in  FIG. 1 ; 
           [0035]      FIGS. 4   a  to  4   c  illustrate soil chemistry sensors in a soil column; 
           [0036]      FIGS. 5   a  and  5   b  show soil nitrate data from measurements taken using the soil column of  FIGS. 4   a - c;    
           [0037]      FIG. 6   a  shows soil nitrate depletion (uptake) data for two different cultivars; 
           [0038]      FIG. 6   b  shows a variation of nitrate concentration in a vermiculite column at two different column depths; 
           [0039]      FIG. 7  shows a schematic of a soil chemistry sensor for measuring chemical depletion (uptake) around plant roots; 
           [0040]      FIG. 8   a  shows a variation of soil nitrate depletion (uptake) over time at different points around plant roots; 
           [0041]      FIG. 8   b  shows a variation of soil nitrate depletion (uptake) over time at different points around plant roots, and a variation in light over time; 
           [0042]      FIGS. 9   a  and  9   b  depict a soil chemistry sensor according to embodiments of the invention; 
           [0043]      FIGS. 9   c  and  9   d  depict a soil chemistry sensor comprising side membranes according to an alternative embodiment of the invention; 
           [0044]      FIGS. 9   e  and  9   f  illustrate construction steps to form the side membranes of  FIG. 9   d;    
           [0045]      FIGS. 9   g  and  9   h  illustrate construction steps to seal the soil chemistry sensor of  FIG. 9   d;    
           [0046]      FIG. 10   a  shows a probe comprising a vertical soil chemistry sensor for measuring soil chemistry within a soil structure, according to an embodiment of the invention; 
           [0047]      FIGS. 10   b  and  10   c  illustrate construction of the probe of  FIG. 10   a;    
           [0048]      FIG. 10   d  shows a probe comprising multiple vertical soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure, according to an embodiment of the invention; 
           [0049]      FIG. 11   a  shows a probe comprising multiple horizontal soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure, according to an alternative embodiment of the invention; 
           [0050]      FIGS. 11   b  and  11   c  depict two different arrangements for the horizontal soil chemistry sensors of  FIG. 11   a;    
           [0051]      FIG. 12   a  illustrates a probe comprising multiple ‘button-style’ soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure, according to an embodiment of the invention; 
           [0052]      FIG. 12   b  shows a zoomed-in view of the ‘button-style’ soil chemistry sensor of  FIG. 12   a;    
           [0053]      FIG. 12   c  illustrates a probe comprising multiple ‘button-style’ soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure, according to an alternative embodiment of the invention; and 
           [0054]      FIG. 12   d  shows a zoomed-in view of the ‘button-style’ soil chemistry sensor of  FIG. 12   c.    
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0055]      FIG. 1  shows a nitrate (NO 3 )-selective soil chemistry sensor  100  according to an embodiment of the invention. The sensor comprises a nitrate-selective electrode  102  and a double junction reference electrode  104 . The ion-selective electrode comprises a plastic lumen at  106 , in embodiments fabricated from a disposable pipette tip or plastic syringe such as a polypropylene Distritip® syringe, for low cost. An ion-selective membrane  108  is fabricated at the end of the electrode by solvent-casting a polymer such as high molecular weight PVC in combination with an ion carrier. For example for nitrate sensing a suitable ion carrier is tridodecylmethylammonium (TDDMA) nitrate, although additionally or alternatively other ion-selective components may be employed to additionally or alternatively sense other plant nutrients. The membrane composition may be dissolved, for example, in tetrahydrofuran (THF). 
         [0056]    An example ion-selective composition for fabricating a nitrate-selective membrane is as follows: 
         [0000]    1.50 wt % Tridodecylmethylammonium nitrate
 
16.25 wt % 2-Nitrophenyl octyl ether
 
1.93 wt % Nitrocellulose, 35% in isopropanol
 
0.25 wt % Methyltriphenylphosphonium bromide
 
5.75 wt % Poly(vinyl chloride) high molecular weight
 
       74.32 wt % Tetrahydrofuran 
       [0057]    Preferably the plastic interior of the lumen is treated with a silanizing solution such as Dow Corning® repelcote to improve the hydrophobicity of the surface of the plastic, to improve the sealing between the solvent-cast membrane and lumen. After this has thoroughly dried, the end portion of the lumen is filled with the dissolved ion-selective composition and the THF allowed to evaporate gradually over a number of hours at room temperature. The electrode is then filled with an electrolyte  110  comprising, for example, 100 mM KCl+100 mM KNO 3 , in which is located an Ag/AgCl (chlorided silver) electrode wire  112 . 
         [0058]    Preferably the reference electrode  104  is a double junction reference electrode comprising a pair of plastic lumen  114   a,b , each with a solvent-cast reference membrane  116   a,b  fabricated as previously described except that the TDDMA nitrate ion carrier is omitted. The reference membrane composition may also include one or more additives to improve the electrical conductivity of this membrane. 
         [0059]    In the double junction reference electrode different outer  118   a  and inner  118   b  filling solutions are employed. The inner filling solution may be 100 mM KCl; the outer filling solution may comprise, for example, 100 mM ammonium sulphate (for nitrate sensing) or 100 mM magnesium sulphate (for NH 4  sensing) or 100 mM sodium chloride (for potassium sensing). Further examples of solution compositions, for sensing different ions, and pH, are given later. 
         [0060]    In some preferred embodiments the tips of the ion-selective and reference electrodes are fastened at  120  together by tape and/or rubber sleeves; preferably the membrane tips are only 1-2 mm apart. It is also important for the probe to be waterproof, and this was achieved by sealing the probe with cold-shrink tubing  122  (a pre-stretched elastomer which shrinks upon removal of the supporting core during application), in combination with silicone sealant. This provided an effective seal without the need for heating, which can damage the electrodes. 
         [0061]    As schematically illustrated in  FIG. 1  the silver/silver chloride electrode wires  112  are coupled to a high import impedance volt meter, for example a differential electrometer. The potential across the wires  112  is a function of the selected, for example nitrate ion concentration. The closer to 100 mM nitrate (the level of nitrate ions in the solution in the ion-selective electrode) the closer the potential is to 0V. When the nitrate (or other ion) concentration in the external environment decreases, the voltage increases, according to a variation of the Nernst equation, the Nickolsky-Eisenman equation (which takes into account interference from ions other than the target ion): 
         [0000]        E=K +(2.303 RT/z   i   F )log( a   i   +k   ij   a   j   Zi/Zj ) 
         [0062]    where E is the potential, z i  and a i  are the charge and activity of the ion of interest, K is a constant dependent on the probe design, R is the gas constant, T is temperature in Kelvin, F is the Faraday constant, j labels the interfering ions and k ij  is the selectivity coefficient—a quantitative measurement of the ability of the electrode to discriminate against interfering ion j. 
         [0063]    Prior to use the probe is calibrated using a set of nitrate solutions which have a constant background ionic strength, for example with nitrate activity of 100 mM (pNO 3 ), 10 mM, 1 mM, 0.1 mM and 0.01 mM.  FIG. 2  shows an example calibration curve in which the points are the measured voltages at the different nitrate concentrations and the line is a Nickolsky-Eisenman curve fitted to the equation below: 
         [0000]        f=P 1+ P 2*log(10̂(− x )+ P 3)
 
         [0064]    In this equation the slope, P2 is approximately 58 mV and the value of P3 defines the limited detection of the probe (in M).  FIG. 2  shows two curves, a second curve with 10 mM of KCl added to each nitrate solution. Chloride ions can interfere with nitrate-selective membranes but the two curves in  FIG. 2  are substantially coincident, showing that the presence of chloride ions has almost no effect on measurements from embodiments of the probe. 
         [0065]    The table below shows one example of a set of soil chemistry measurements made using an embodiment of the probe—stable, consistent readings were obtained. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
               
               
                 Parameters 
                 EMF = P1 + P2 · log(a + P3) 
                 NO3 Measures 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 P1 
                 −26.2614 
                   
               
               
                 P2 
                 −54.1356 
                   
               
               
                 P3 
                 3.89E−06 
               
               
                   
               
             
          
         
       
     
         [0066]    Time series measurements could establish the effectiveness of uptake of nitrate by plants in the soil under test. Calibrations before and after time series measurements indicated little or no drift in the probe data. 
         [0067]    Monitoring the actual levels of nitrate in soil shows that these vary quite substantially with time and a location, even within a single field. It is desirable to be able to track these variations and/or to monitor nitrate (or other plant nutrient) levels at different depths within the soil, for example 30 cm, 60 cm and 90 cm deep. 
         [0068]      FIG. 3  shows an embodiment of a system  300  including a plurality of probes  100   a - n  each as previously described, each coupled to a respective voltage amplifier  302   a - n  which provides voltage data to a data logger  304 . In this example the probes are coupled to the data logger via a wired connection which is used to power the probes/amplifiers, but in other embodiments a wireless connection may be employed. The data logger  304  preferably incorporates non-volatile memory  306  to store the collected data and an RF transceiver  310 , for example, to communicate with a computer or mobile phone network, to provide a link to a remote data collection/analysis computer/network. A power supply  308  for the system may comprise, for example, a rechargeable battery, optionally powered from a renewable energy source such as wind or solar power. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                   
                   
                 Nitrate 
                   
                   
               
               
                 Date 
                   
                 Electrode 
                 Electrode 
                 Electrode 
                 Average 
                 activity 
                 Temp 
                 Temperature 
               
               
                 Place 
                 Time 
                 1 
                 2 
                 3 
                 (mV) 
                 (mM) 
                 given 
                 celcius 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Water 
                 270 
                 270 
                 270 
                 270 
                 −0.000523128 
                   
                   
               
               
                 Silver sand 
                 205 
                 205 
                 205 
                 205 
                 0.049578314 
               
               
                 washed + water 
               
               
                 Silver sand 
                 201 
                 201 
                 201 
                 201 
                 0.059495063 
               
               
                 washed + 0.1 
               
               
                 mM NO3 
               
               
                 Linosa (3 pt) + 
                 199 
                 199 
                 199 
                 199 
                 0.065123179 
               
               
                 water 
               
               
                 Linosa (3 pt) + 
                 180 
                 180 
                 180 
                 180 
                 0.150958781 
               
               
                 0.1 mM NO3 
               
               
                 Linosa (3 pt) + 
                 196 
                 196 
                 196 
                 196 
                 0.074516048 
               
               
                 water 
               
               
                 Linosa (3 pt) + 
                 196 
                 196 
                 196 
                 196 
                 0.074516048 
               
               
                 0.1 mM NO3 
               
               
                 Piriddu (3 pt) + 
                 173 
                 173 
                 173 
                 173 
                 0.204661734 
               
               
                 water 
               
               
                 Piriddu (3 pt) + 
                 187 
                 187 
                 187 
                 187 
                 0.111084466 
               
               
                 0.1 mM NO3 
               
               
                 Linosa (1 pt) + 
                 171 
                 171 
                 171 
                 171 
                 0.22317925 
               
               
                 water 
               
               
                 Linosa (1 pt) + 
                 178 
                 178 
                 178 
                 178 
                 0.164707993 
               
               
                 0.1 mM NO3 
               
               
                 UC82 (1 pt) + 
                 141 
                 141 
                 141 
                 141 
                 0.809547861 
               
               
                 water 
               
               
                 UC82 (1 pt) + 
                 156 
                 156 
                 156 
                 156 
                 0.425885519 
               
               
                 0.1 mM NO3 
               
               
                   
               
             
          
         
       
     
       Further Example Compositions 
       [0069]    Further example ion-selective compositions for fabricating other ion-selective membranes are given below. Preferably High molecular weight poly(vinyl chloride) (PVC) is used in all the membranes. 
       Ammonium 
       [0070]    An example ammonium sensor component is nonactin, sold by Sigma-Aldrich (Ammonium ionophore I). The electrode detection limit 11 μM ammonium, but high K +  can interfere with the response. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—nonactin 
                  5% 
               
               
                   
                 2-nitrophenyl octyl ether 
                 64% 
               
               
                   
                 (plasticiser) 
                   
               
               
                   
                 Potassium tetrakis (4- 
                  1% 
               
               
                   
                 Chlorophenyl) borate (additive) 
                   
               
               
                   
                 PVC matrix 
                 30% 
               
               
                   
                   
               
             
          
         
       
     
       Calcium 
       [0071]    An example calcium sensor component is again sold by Sigma-Aldrich as calcium ionophore II (product number 21193). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—ETH129 
                   1% 
               
               
                   
                 2-nitrophenyl octyl ether 
                 65.6% 
               
               
                   
                 (plasticiser) 
                   
               
               
                   
                 Potassium tetrakis (4- 
                  0.6% 
               
               
                   
                 Chlorophenyl) borate (additive) 
                   
               
               
                   
                 PVC matrix 
                 32.8% 
               
               
                   
                   
               
             
          
         
       
     
       Potassium 
       [0072]    An example ammonium sensor is 90% (w/w) potassium ionophore cocktail and 10% (w/w) PVC polymer. The potassium ionophore cocktail contains: 
         [0000]    5 wt % potassium ionophore (Sigma product number 60403) valinomycin.
 
93 wt % 1,2 dimethyl-3-notrobenzene (Sigma product number 40870).
 
2 wt % potassium tetrakis(4-chlorophenyl) borate (Sigma product number 60591).
 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—valinomycin 
                  5% 
               
               
                   
                 1,2 dimethyl-3-nitrobenzene 
                 93% 
               
               
                   
                 (plasticiser) 
                   
               
               
                   
                 Potassium tetrakis (4- 
                  2% 
               
               
                   
                 Chlorophenyl) borate (additive) 
               
               
                   
                   
               
             
          
         
       
     
       Phosphate 
       [0073]    An example phosphate sensor component may be based on the phosphate sensor of Carey C M &amp; Riggan W B Anal Chem. 1994 Nov. 1; 66(21):3587-91, a cyclic polyamine ionophore for use in a dibasic phosphate-selective electrode. Further details can be found in EP2376442A, to which reference may be made. The composition is similar to that used for nitrate. 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—Carey &amp; Riggan 1994 
                  6% 
               
               
                   
                 2-nitrophenyl octyl ether 
                 65% 
               
               
                   
                 (plasticiser) 
                   
               
               
                   
                 Methyltriphenylphosphonium 
                  1% 
               
               
                   
                 (additive) 
                   
               
               
                   
                 PVC matrix 
                 23% 
               
               
                   
                 nitrocellulose 
                  5% 
               
               
                   
                   
               
             
          
         
       
     
         [0074]    In addition reference may be made to Kim et al. 2007 Transactions of ASABE 50(2):415-425 which shows that similar cobalt sensors may be fabricated. 
       pH (Protons) 
       [0075]    One example pH sensor molecule is ETH1907 (Hydrogen ionophore II), sold commercially by Sigma-Aldrich. Like the nitrate sensor mix, this pH cocktail is made with a PVC and nitrocellulose solid matrix, 62% pH cocktail, 28% PVC and 10% blotting nitrocellulose (Sigma-Aldrich product code N8267 Sigma). 
         [0076]    A suitable cocktail mix (as tabulated below) is sold by Sigma-Aldrich (product code 95297). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—ETH1907 
                 6% 
               
               
                   
                 2-nitrophenyl octyl ether 
                 93%  
               
               
                   
                 (plasticiser) 
                   
               
               
                   
                 Potassium tetrakis (4- 
                 1% 
               
               
                   
                 Chlorophenyl) borate (additive) 
               
               
                   
                   
               
             
          
         
       
     
       Sodium 
       [0077]    For sodium reference may be made to Carden et al. 2001 (J. Exp. Bot. 52: 1353) which describes an improved sodium sensor (improved Na +  to K +  selectivity). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                   
               
               
                   
                 Component (wt %) 
                 % 
               
               
                   
                   
               
             
             
               
                   
                 Sensor—sodium ionophore* 
                   1% 
               
               
                   
                 Bis (1-butylpentyl) adipate 
                 65.9% 
               
               
                   
                 (plasticiser)—(Sigma product 
                   
               
               
                   
                 Number 02150) 
                  0.6% 
               
               
                   
                 Potassium tetrakis (4- 
                   
               
               
                   
                 Chlorophenyl) borate (additive) 
                   
               
               
                   
                 PVC matrix 
                   33% 
               
               
                   
                   
               
             
          
         
       
     
       Use of Soil Chemistry Sensor to Detect Chemical Variation at Different Depths: 
       [0078]    Turning now to  FIGS. 4   a  to  4   c , these illustrate a soil column system  400 , according to an embodiment of the invention. 
         [0079]    As shown in  FIG. 4   a , the soil column system  400  comprises a soil column  412  which is used to contain a volume of soil  414 . One or more holes  410   a, b, c  are formed in the soil column  412  at different points along the length of the column. The one or more holes  410   a, b, c  are provided for one or more soil chemistry sensors. The soil column system enables the measurement of soil chemistry at different depths in the soil  414 . Advantageously, this may enable determination of how chemicals move through soil (without any plants), and/or how chemicals are absorbed by plant root systems. For example, roots close to the top of the soil column  412  may absorb chemicals in the soil  414  at a different rate to those deeper in the soil column. 
         [0080]    A soil sensor comprising a nitrate-selective electrode and a double junction reference electrode (as described above) is inserted into a hole  410  to enable measurement of the soil chemistry. In preferred embodiments, the soil column  412  comprises three holes  410   a, b, c  and a soil sensor is inserted into each hole to enable soil chemistry measurements to be taken at three depths, e.g. close to the top of the volume of soil  414  (i.e. for shallow/surface roots), mid-way along the length of the soil column  412  and close to the bottom of the volume of soil  414  (i.e. for deep roots). Preferably, the soil chemistry sensors are placed at 30 cm, 60 cm and 90 cm beneath the surface of the soil. As shown in  FIG. 4   a , hole  410  is adapted to the shape of the soil sensor electrodes, such that one electrode can be inserted into one part  416   a  of the hole, and the second electrode can be inserted into a second part  416   b  of the hole. Thus, the hole  410  substantially matches the size of the plastic lumen of the electrodes, to help to seal the hole and minimise any soil or soil water from leaking out of the hole  410 . 
         [0081]      FIG. 4   b  shows a soil column system  400  comprising three soil sensors  418   a, b, c  at three different positions along the soil column.  FIG. 4   c  illustrates the soil column system  400  being used to measure soil chemistry at different depths. The silver/silver chloride electrode wires of each sensor are connected to a data logger and a high import impedance volt meter  420  (e.g. a differential electrometer). 
         [0082]      FIGS. 5   a  and  5   b  show soil nitrate data from measurements taken using the (three sensor) soil column system of  FIGS. 4   a - c . The soil column system was used to compare the nitrate uptake of two wheat cultivars:  Robigus , which is known to have a high nitrate uptake efficiency, and  Maris  Widgeon, which is known to be have a low nitrate uptake efficiency.  FIGS. 5   a  and  5   b  respectively depict the variation of nitrate depletion in the soil column over time at different depths in a column containing the  Maris  Widgeon, cultivar and in a column containing the  Robigus  cultivar. In both cases, soil nitrate concentrations were measured by the soil column system at three depths over a period of 14 hours. The soil column system not only verify the known nitrate uptake efficiency of the two cultivars, but advantageously also enables a better understanding of how the root systems of the two cultivars behave. For example, the locations of the soil nitrate sensors in the soil column system show that the roots of the  Robigus  wheat variety appear to absorb nitrates via their shallow roots (i.e. those at the top of the soil column). This may be useful information to wheat farmers, as it indicates that for particularly varieties of wheat, nitrate-rich fertilizer should be added frequently to the soil surface, whereas this may not be necessary for other varieties. 
         [0083]    Thus, knowing the nitrate level at different depths in the soil/growth medium can help to optimise the amount of fertilizer to add to the soil to promote growth while also minimising leaching (i.e. loss of water-soluble nutrients from the soil). For example, a low nitrate concentration measured close to the top of the soil surface (e.g. 30 cm from the soil surface), may trigger the need for more fertilizer to be added. In particular, a trigger to add more fertilizer may be that the nitrate concentration 30 cm below the soil surface is, for example, 10-fold less than that at 60 cm or 90 cm below the soil surface. However, a high nitrate concentration (e.g. 5 times above the expected concentration of 15 mM) at 90 cm may also be indicative of leaching. Thus, the nitrate levels measured by the soil chemistry sensors may also be used to make decisions about irrigation. For example, if measurements indicate fertilizer needs to be added to the top of the soil, but that the soil is prone to leaching, then a farmer may use the information to decide when to irrigate after applying fertilizer to minimise the effect of the water washing away the nitrates from near the top of the soil surface. 
       Use of Soil Chemistry Sensor to Detect Chemical Variation in a Hydroponic Culture: 
       [0084]    The above-described soil chemistry sensor may in embodiments be used to measure uptake of nutrients in a hydroponic growth culture.  FIG. 6   a  shows nitrate depletion (uptake) data for two different cultivars in a hydroponic culture, i.e. a mineral nutrient solution (as opposed to in soil). In such embodiments, it is essential that the sensor is waterproof (except for the porous plugs), and the ion-sensing and reference electrodes may be sealed within a waterproof enclosure (for example, an enclosure fabricated from cold-shrink tubing). 
       Use of Soil Chemistry Sensor to Detect Chemical Variation in an Artificial Growth Medium: 
       [0085]    In embodiments, the soil column system  400  shown in  FIG. 4   a  may be adapted for a vermiculite growth medium and the soil chemistry sensors used to detect chemical variation in the vermiculite column at different depths. Typically, exfoliated vermiculite (a hydrous, silicate mineral), is combined with other materials such as peat or pine bark to form a soil-less growth media. Vermiculite-based media are known to promote faster root growth because the mixture helps retain air, plant food and moisture and releases them as the plant requires them. Thus, the vermiculite column system was used to determine how nitrates move through the profile of the artificial growth medium. 
         [0086]      FIG. 6   b  shows a variation of nitrate concentration in a vermiculite column at two different column depths. The data was collected using a column without any plants in order to determine the change in nitrate concentration due to movement through the growth medium alone (i.e. without the effect of uptake by a plant). Nitrate was added to the top of the vermiculite column at time t=0. The data indicates that nitrates flow relatively quickly through the growth medium to reach the middle of the column. 
         [0087]    Consequently, as described above, the soil chemistry sensor can be used to: 
         [0000]    Provide measurements of soil chemistry rapidly, at a fine scale and in real time;
 
Measure chemical/nutrient uptake directly in hydroponic solutions;
 
Measure chemical (e.g. nitrate) depletion to predict chemical uptake by plants directly in soil at one or more depths;
 
Determine the effects of soil moisture gradients on chemical (e.g. nitrate) uptake;
 
Identify which fraction of root system is active in chemical uptake during a plant&#39;s developmental stages; and
 
Determine the movement of nitrate through soil or a solid substrate (e.g. vermiculite) to determine leaching behaviour.
 
       Use of Soil Chemistry Sensor to Measure Soil Chemistry Depletion Zones Around Plant Roots: 
       [0088]    In embodiments, the above-described soil sensor may be used to identify ‘depletion zones’ around the roots of a plant. A nutrient depletion zone develops around a root when nutrients are removed from the soil solution faster than they can be replaced by the movement of nutrients through the solution. For example, ions which have low mobility in the solution may produce a sharp/narrow depletion zone close to the root. 
         [0089]    Turning now to  FIG. 7 , this depicts a schematic of a soil chemistry sensor for measuring chemical depletion (uptake) around plant roots, i.e. to identify a depletion zone around the roots. The depletion zone measuring apparatus  700  comprises a container  710  which contains the soil solution or growth medium. A single plant  714  is planted in the container  710 . To measure nutrient uptake by the roots, the plant roots are contained within visking tubing  712 . In the illustrated embodiment, two sensors are provided within the visking tubing  712 , to measure changes in the nutrient levels close to the plant roots, and two sensors are provided in the container  710  away from the visking tubing  712 , to measure changes in nutrient levels in the soil/growth medium away from the plant roots. 
         [0090]    In an example embodiment, sensor B is placed 2 cm below the soil surface and sensor K is placed 6 cm below the soil surface, where both sensors B and K are provided within the visking tubing  712  (i.e. close to the roots). Further from the visking tubing/roots is sensor H, placed 2 cm below the soil surface, and sensor I, placed 6 cm below the soil surface. The four sensors together measure changes in the soil chemistry at two different depths in the container  710  and at two different distances away from the roots. The apparatus was used to measure soil chemistry changes and the depletion zone around wheat roots. The container  710  was filled with a sand culture with added nutrient solution as the growth medium. 
         [0091]      FIGS. 8   a  and  8   b  show respectively, data collected using this apparatus over a four day period and data collected over a fourteen day period.  FIG. 8   b  also illustrates changes in light over the fourteen day period, which corresponds to the day/night cycle. The data shows that for this particular wheat variety, nitrates are absorbed by the roots at approximately the same rate at 2 cm and 6 cm below the surface of the soil. It also shows that as the nitrates are being depleted close to the roots, nutrients are drawn towards the roots through the visking tubing (i.e. nutrients are depleted around sensors H and I). Over fourteen days, the nitrates were depleted in the volume close to the plant roots. The nitrate concentration in the shallow culture further away from the roots was also depleted over this period. Such data can be used to estimate the depletion zone around plant roots, which may in this case be a few centimetres (e.g. 1 to 2 cm) from the surface of the roots. 
         [0092]    Consequently, the soil chemistry sensor can be used to: 
         [0000]    Measure complete soil chemistry/nutrient depletion at the root surface;
 
Identify soil chemistry/nutrient depletion zones around the root; and
 
Measure diurnal changes in soil chemistry/nutrient depletion (nutrient uptake).
 
       Electrode Design: 
       [0093]      FIG. 9   a  shows a variation of the soil chemistry sensor of  FIG. 1 . In this embodiment, the soil chemistry sensor  100  further comprises one or more holes  926 . The “breather holes”  926  are formed in the nitrate-selective electrode  102  and the double junction reference electrode  104 , to equalise the pressure inside the electrodes  102  and  104  and outside the electrodes, particularly when being transported in a pressurised environment, such as on an aeroplane. The breather holes  926  may be greater than 1 mm in diameter. However, to minimise the risk of external matter (e.g. water/soil water) entering the sensors through the breather holes  926 , the holes are preferably “pin holes” and &lt;1 mm in diameter. Alternatively, breather holes  926  may be replaced with valves or taps to permit pressure equalisation and prevent external matter from entering the sensor.  FIG. 9   a  also shows how the electrodes are sealed to prevent soil water from entering the electrodes through the top of the sensor  100 . A plug  928 , which may be formed of a pliable and/or water-resistant material, is inserted into the top of each electrode  102  and  104  around wires  112 , to form a seal. In this embodiment, the ends of the electrode wires  112  are not encased in the sealant or the plugs  928 , but rather a cable ferrule  930 , such as a bootlace ferrule, is used to form an end termination of each wire  112 . Thus, the cable ferrule  930  is not protected/insulated against any water that may leak through the top of the sensor  100 . The coupling between the electrode wires  112  and a high import impedance volt meter (not shown) is provided by standard wire connections  934 . 
         [0094]    Turning now to  FIG. 9   b , this shows a further variation of the soil chemistry sensor of  FIGS. 1 and 9   a . The sensor  900  comprises a nitrate-selective electrode  902  and a double junction reference electrode  104 , where the double junction reference electrode comprises different outer  918   a  and inner  918   b  filling solutions. Each electrode contains an electrode wire  912 . As mentioned earlier, it is important to make the soil chemistry sensor waterproof (except for the porous plugs). Thus, preferably the ion-sensing and reference electrodes are sealed within a waterproof enclosure. In particular, top sealing the sensor is important to ensure that soil water does not get into contact with the circuitry of the sensor. In embodiments, the conducting wire of each electrode is passed through a plug  928  provided to seal the top of each electrode  902  and  904 . Preferably, the plug  928  is plastic and/or formed from a pliable, water-resistant material, and forms a tight seal to prevent soil water from entering the electrode through the top end and protects the exposed conducting wire  112  from external environmental conditions. In the depicted embodiment, each cable ferrule  930  is partially encased in a plug  928  and an additional sealant  932  is disposed above plug  928  to provide a further seal and to protect cable ferrule  930  against water damage. The sealant  932  may be a putty-like water-repellent adhesive that can be pressed into each electrode to form a seal above the plug  928 . Thus, no metal of each electrode is in contact with the soil/soil water in which the electrode is placed. 
         [0095]    The soil chemistry sensor of  FIG. 9   b  does not comprise “breather holes” as in the embodiment depicted in  FIG. 9   a . Advantageously, the lack of breather holes minimises the chance of the electrode solutions mixing with external water/soil water, which also prevents the chance of the sensor  900  short-circuiting. 
         [0096]    A further sealant (not shown), e.g. a glue or adhesive material, may be used to strengthen the seal/s of the double-junction reference electrode  904  in order to prevent reference solution  918   a, b  from leaking out of their respective chambers. Preferably, rubber sleeving  920  is repositioned relative to the position of sleeving  120  shown in  FIG. 9   a , to further strengthen the double junction electrode  904 . 
         [0097]    As described earlier with respect to  FIG. 1 , each electrode of the soil chemistry sensor  100  is fabricated from a pipette tip with a hole at the tip end to allow liquid to enter into and out of the pipette tip. Thus, the hole enables soil water and nutrients to contact the ion-selective/reference membrane and be sensed by the soil chemistry sensor. However, inserting the soil chemistry sensor vertically down into soil can damage the pipette tip and the membrane. An alternative embodiment is depicted in  FIGS. 9   c  and  9   d , which show a soil chemistry sensor  9000  comprising membranes  942  along a side of each pipette tip. Soil chemistry sensor  9000  is formed of an ion-selective electrode  902  and a single-tip reference electrode  940 , where each electrode is formed of a pipette tip as described earlier. (Although not shown, the reference electrode  940  may be a double-junction reference electrode.) Holes are formed in the side of the pipette tip of each electrode and the ion-selective and reference membranes  942  are formed in the holes. Thus, even if the end of the pipette tip is damaged when the soil chemistry sensor is inserted into soil, the holes and membranes  942  will be undamaged and therefore the electrodes are not prevented from performing their sensing function. 
         [0098]    As shown in  FIG. 9   d , preferably, the membrane  942  of each electrode is positioned such that the membranes face each other. This may require the electrodes  902  and  940  to be rotated from a side-by-side orientation, as shown in  FIG. 9   c , to a face-to-face orientation as shown in  FIG. 9   d . It is preferable that the electrodes  902  and  940  are close to each other (so that they each measure soil chemistry at approximately the same position in the soil structure) but are also insulated from each other to prevent shorting. Thus, as described above, the electrode wires  912  and any (metal) connectors (e.g. a cable ferrule) holding the wires in place are insulated by plug  928  and sealant  932 , which also protects the wire and connectors from exposure to external conditions (e.g. soil water). In particular, the separation distance between the membranes of the electrodes  902  and  940  should be minimal, particularly when taking measurements in situ, as the soil structure and/or porosity may modify the water available for sensing by each of the electrodes. Thus, preferably the distance between the membranes is between 1-2 mm. More preferably, the distance (A) between the tips of each electrode is greater than the distance (B) between the membranes  942  when the membranes are side-by-side (as in  FIG. 9   c ), and distance B is greater than the distance (C) when the membranes  942  are face-to-face (as in  FIG. 9   d ), i.e. A&gt;B&gt;C. 
         [0099]    Turning now to  FIGS. 9   e  and  9   f , these depict the plastic lumen  944  of the side membrane soil chemistry sensor  9000  of  FIG. 9   d . As mentioned above, typically, a pipette tip  944  has a hole  946  at one end to allow liquid to enter into and out of the pipette tip. To form the side membranes of the soil chemistry sensor  9000 , the hole  946  of each pipette tip is sealed using a soldering iron and one or more holes  942  are formed along the length of the pipette tip instead. The holes may be formed using a hot needle that is inserted into the pipette tip  944  at desired locations. The membrane is formed as described earlier. 
         [0100]    In embodiments, the soil chemistry sensor  9000  of  FIGS. 9   c  and  9   d  may be covered in a plastic or water-resistant material to provide an insulating and water-proof layer over the sensor  9000 . Techniques such as plasti-dip may be used to provide the layer. Preferably, the layer covers all but the membranes of the electrodes. 
         [0101]    In embodiments, the soil chemistry sensor  9000  may be formed with an additional protective cover over the wires extending from out of the electrodes. As shown in  FIGS. 9   g  and  9   h , a pipette tip  950  is attached to the electrodes such that the wide end contacts the sealant  932  and the connecting wire  934  is passed through the narrow end/tip of the pipette tip  950 . The pipette tip  950  prevents the connections between the electrode wire and the connecting wire  934  from damage. A protective rubber sleeving  948  is provided over the pipette tip  950  to secure the pipette in position and to provide a degree of flexibility to the connection between the electrode and wire  934 . A plastic coating, as shown in  FIG. 9   h , may be provided over the pipette tip  950  to provide an insulating and water-proof layer over the connection. The plastic coating may be formed by the plasti-dip technique or otherwise. 
         [0102]    As described above, each of the ion-selective and reference electrodes is fabricated from a plastic lumen, which in embodiments is formed of a disposable pipette tip for low cost. Another advantage to using a disposable pipette tip over a plastic syringe is that the small pipette tip ensures a smaller surface area of the ion-selective membrane is in contact with the soil water, and thus, the risk of damage to the membrane is reduced. 
         [0000]    Measuring Soil Chemistry In Situ at Different Depths with Vertical Sensors 
         [0103]    The soil chemistry sensors described thus far are capable of measuring the soil chemistry in situ at a certain distance below the surface of the soil, where the distance itself (i.e. depth) is limited by the length of sensor/sensor electrodes. The soil column system shown in  FIG. 4   a  is suitable for measuring soil chemistry within the soil column, i.e. for a plant grown within the column, but is not suitable for in situ measurements (e.g. in a field). It may be desirable to measure the soil chemistry deeper in a soil structure in a field, and/or to measure the soil chemistry at multiple different depths in a soil structure simultaneously, and/or to obtain real-time soil chemistry data in situ. If real-time measurements indicate that fertilizer needs to be added to the top of the soil, but that the soil is prone to leaching, then a farmer may use the information to decide when to irrigate after applying fertilizer to his crops to minimise the effect of the water washing away the nitrates from near the top of the soil surface. 
         [0104]      FIG. 10   a  shows a variable length probe  1000  comprising a vertical soil chemistry sensor for measuring soil chemistry within a soil structure, according to an embodiment of the invention. The soil chemistry sensor  1016  is substantially similar to that shown in  FIG. 9   b , and comprises an ion-selective electrode, a double-junction reference electrode, a plug  1020  and sealant  1022  to seal each electrode. The probe  1000  is formed of three main parts: a non-porous tube part  1008 , an adaptor part  1010 , and a porous cap part  1014 . The non-porous tube part  1008  is provided by a tube or pipe of a length required to place the soil chemistry sensor at a particular depth below the soil surface. Thus, advantageously, the probe  1000  has an adjustable length, which is adjusted by constructing the probe from a non-porous tube part  1008  of the length required to place the sensor at the required depth. The tube part  1008  may be provided by an insulating and water-proof material, e.g. a plastic tube/pipe, to protect the top of the soil sensor  1016  and electrical wiring  1006  from contact with the external environment (e.g. water/soil water). Porous cap part  1014  is formed of a porous material, e.g. ceramic, to enable soil water to enter the probe  1000  for sensing by the soil chemistry sensor  1016  contained with the probe  1000 . As shown in more detail in the zoomed-in view of the sensor-end of probe  1000 , the porous cap part  1014  contains a liquid (water) reservoir  1018 . Electrolytes/ions in the soil outside of the porous cap part  1014  flow into the reservoir  1018 , enabling the soil chemistry sensor  1016  to measure the soil chemistry via the reservoir  1018  rather than by direct contact with the soil. 
         [0105]    Adaptor part  1010  couples together the non-porous tube part  1008  and the porous cap part  1014 . In addition, adaptor  1010  secures the soil chemistry sensor  1016  in a vertical position within the cap part  1014 . A circular gasket  1012  within the adaptor  1010  both holds the soil chemistry sensor  1016  in place within the cap part  1014  and forms a water-tight seal to prevent fluid in the reservoir  1018  from leaking out of cap part  1014  and into contact with the wiring  1006  in the tube part  1008 . Preferably, gasket  1012  is formed of a water-resistant material such as rubber. 
         [0106]    As shown in  FIG. 10   a , the probe  1000  is inserted into the soil structure so that the sensor end of the probe is at the desired depth at which the measurements are to be taken. The probe  1000  is sealed at the top end with a rubber stopper  1004  and a plastic cap  1002  to prevent water/external matter from entering the tube part  1008  and contacting the wiring  1006 . Wiring  1006  is fed through the stopper  1004  and cap  1002  to connect the electrodes to a data logger  1024 . 
         [0107]      FIGS. 10   b  and  10   c  show external views of the probe of  FIG. 10   a , and in particular show the three main parts of probe  1000 . The porous part  1014  may be detachably coupled to adaptor part  1010  by, for example, a threaded connection (not shown). Similarly, adaptor part  1010  may be detachably coupled to tube part  1008 . Consequently, as mentioned above, the length of the probe  1000  may be altered by selecting a tube part  1008  of a particular length (or by changing the length of the adaptor part  1010  or porous cap part  1014 ). A further advantage of probe  1000  is that the soil chemistry sensor  1016  can be readily removed from the probe  1000  if the sensor is not functioning and/or to change the ion-selective electrode to measure the presence of different chemicals in the soil. 
         [0108]    Turning now to  FIG. 10   d , this shows a probe  1000   a  which comprises multiple vertical soil chemistry sensors, for measuring soil chemistry at multiple depths within a soil structure, according to an embodiment of the invention. In this embodiment, probe  1000   a  comprises one or more sensor sections  1028  and a tip sensor section  1030 . Each sensor section  1028  comprises a non-porous tube part  1008 , an adaptor part  1010  and a porous part  1026 . The tip sensor section  1030  comprises a non-porous tube part  1008 , an adaptor part  1010  and a porous cap part  1014 . The length of each sensor section  1028  and the tip sensor section  1030  is alterable (by, for example, altering the length of tube part  1008  as described earlier), to take soil chemistry measurements at two or more desired depths in the soil structure below the soil surface. The porous part  1026  may be detachably coupled to adaptor part  1010  by, for example, a threaded connection (not shown). Similarly, adaptor part  1010  may be detachably coupled to tube part  1008 . Consequently, as mentioned above, the length of the probe  1000  may be altered by selecting a tube part  1008  of a particular length (or by changing the length of the adaptor part  1010  or porous cap part  1026 ). 
         [0109]    One or more sensor sections  1028  may be detachably coupled together to form probe  1000   a , such that the porous part  1026  of one sensor section  1028  is coupled to the tube part  1008  of an adjacent sensor section  1028 . For example, the probe  1000   a  illustrated in  FIG. 10   d  is capable of measuring soil chemistry at three different depths below the soil surface. Thus, the probe is formed of two sensor sections  1028   a, b  and a tip sensor section  1030 . The first sensor section  1028   a  is used to sense soil chemistry at a depth A in the soil structure. As shown, the tube portion  1008  of the first sensor section  1028   a  is sealed using a rubber stopper  1004  and plastic cap  1002  to prevent water/soil water from entering probe  1000   a  from the top of the probe. The porous part  1026  of the first sensor section  1028   a  is coupled to the tube part  1008  of the second sensor section  1028   b . The second sensor section  1028   b  is capable of measuring soil chemistry at a depth B in the soil structure. The porous part  1026  of second sensor section  1028   b  is coupled to the tube part  1008  of tip sensor section  1030 . The tip sensor section  1030  is capable of measuring soil chemistry at a depth C in the soil structure. The porous cap part  1014  of the tip sensor section  1030  forms the tip of probe  1000   a  (i.e. no sensor sections can be coupled to the porous cap part  1014 ). 
         [0110]    As described earlier with reference to  FIG. 10   a , each soil chemistry sensor  1016  of each sensor section  1028 ,  1030  is held by adaptor  1010  such that the ion-selective and reference membranes are immersed in a liquid reservoir contained within each porous part  1026  and  1014 . Each porous part  1026  is fabricated such that electrical wires from each sensor  1016  can pass through the porous part  1026  and extend out through the top of sensor  1000   a . In addition, each porous part  1026  is fabricated to minimise the risk of the liquid reservoir within each porous part  1026  leaking out into adjacent sensor sections. Consequently, the electrical wires may be covered with an insulating and water-proof material to prevent shorting. 
         [0000]    Measuring Soil Chemistry In Situ at Different Depths with Horizontal Sensors 
         [0111]      FIG. 11   a  shows a probe  1100  comprising multiple horizontal soil chemistry sensors for measuring soil chemistry in situ at multiple depths within a soil structure, according to an alternative embodiment of the invention. 
         [0112]    In this embodiment, probe  1100  comprises one or more sensor sections  1152  and a tip sensor section  1154 . Each sensor section  1152  comprises a non-porous tube part  1108  and a sensor part  1148  or  1150 . The tip sensor section  1154  comprises a non-porous tube part  1008 , a sensor part  1110  and a pointed tip  1122 . The length of each sensor section  1152  and the tip sensor section  1154  is adjustable (by, for example, selecting tube part  1108  with different lengths as described earlier), to take soil chemistry measurements at two or more desired depths in the soil structure below the soil surface. The sensor part  1148  or  1150  may be detachably coupled to tube part  1108  by, for example, a threaded connection (not shown). 
         [0113]    One or more sensor sections  1152  may be detachably coupled together to form probe  1100 , such that the porous part of one sensor section  1152  is coupled to the tube part  1108  of an adjacent sensor section  1152 . For example, the probe  1100  illustrated in  FIG. 11   a  is capable of measuring soil chemistry at three different depths below the soil surface. Thus, the probe  1100  is formed of two sensor sections  1152   a, b  and a tip sensor section  1154 . The first sensor section  1152   a  is used to sense soil chemistry at a depth A in the soil structure. As shown, the tube portion  1108  of the first sensor section  1154   a  is sealed by a rubber stopper (not shown) and a plastic cap  1102  to prevent water/soil water from entering probe  1100  from the top. The sensor part  1148  or  1150  (described in more detail below) of the first sensor section  1152   a  is coupled to the tube part  1108  of the second sensor section  1152   b . The second sensor section  1152   b  is capable of measuring soil chemistry at a depth B in the soil structure. The sensor part  1150  or  1148  of second sensor section  1152   b  is coupled to the tube part  1108  of tip sensor section  1154 . The tip sensor section  1154  is capable of measuring soil chemistry at a depth C in the soil structure. The pointed tip  1122  of the tip sensor section  1154  forms the tip of probe  1100 . The pointed tip  1122  is preferably a sharp metal tip which helps insertion of the probe  1100  into a soil structure, and minimises damage to the end of probe  1100  and the sensors inside the probe. 
         [0114]    As shown in  FIG. 11   a , a soil chemistry sensor  1124  is located within each sensor part in a substantially horizontal/lateral orientation. Turning now to  FIGS. 11   b  and  11   c , these depict in more detail two different arrangements for the horizontal soil chemistry sensors  1124 . Probe  1100  may be formed using either or each of the sensor parts  1148 ,  1150  described below. 
         [0115]      FIG. 11   b  shows sensor part  1148 , which comprises a porous membrane  1136  and a soil chemistry sensor  1124 . The sensor part  1148  contains a liquid (water) reservoir  1138 . Electrolytes/ions in the soil directly outside of the membrane  1136  flow into the reservoir  1138  though porous membrane  1136 , enabling the soil chemistry sensor  1124  to measure the soil chemistry via the reservoir rather than by direct contact with the soil. The sensor part  1148  is tightly sealed to prevent the liquid in the reservoir  1138  from leaking into an adjacent sensor section. Wiring  1106  from each ion-selective electrode  1126  and reference electrode  1128  exits each sensor part  1148  through a tight seal (not shown) to prevent leakage. In embodiments, the sensor part  1148  (and  1150 ) may not extend across the full width or cross-section of the probe  1100 , such that wiring  1106  from each electrode may run up through the probe  1100  in a space not occupied by the sensor parts. 
         [0116]      FIG. 11   c  shows sensor part  1150  in more detail, which comprises a ‘button-shaped’ membrane  1142  on reference electrode  1128  and a ‘button-shaped’ membrane  1144  on ion-selective electrode  1126 . Here, electrodes  1128  and  1126  may not be formed fabricated using pipette tips as described above. The membranes  1142  and  1144  are flush or substantially flush with the surface of probe  1100 . In this arrangement, sensor part  1150  does not require a liquid reservoir, as the membranes of the electrodes are in direct contact with the external environment (e.g. soil or water). Advantageously therefore, the sensing membranes  1142  and  1144  sense chemical concentrations in the soil adjacent each membrane, rather than merely the chemicals which flow into a liquid reservoir. This may be particularly preferable when the probe  1100  is used to measure the chemistry of soil water, which flows slowly through a soil structure. The embodiment shown in  FIG. 11   b  may be preferred when measuring chemical concentrations in water (e.g. hydroponic growth media or rivers). As above, the sensor part  1150  may not extend across the full width or cross-section of probe  1100 , such that wiring  1106  which connects the electrodes to a data logger (not shown) may run up through the probe  1100  in a space not occupied by the sensor parts. 
         [0000]    Measuring Soil Chemistry In Situ at Different Depths with ‘Button-Style’ Sensors 
         [0117]    The above-described probes use soil chemistry sensors in either a vertical or lateral arrangement within the probe. In a further alternative embodiment of the probe, the soil sensors resemble a button along a length of the probe.  FIG. 12   a  illustrates a probe  1200  comprising multiple ‘button-style’ soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure. 
         [0118]    The probe  1200  comprises a tube part  1204  and multiple ‘button-style’ sensors  1206  disposed along the length of the probe. Membranes of each sensor  1206  are flush or substantially flush with the external surface of probe  1200 . The sensors  1206  are positioned at fixed points along the probe, such that the first sensor measures soil chemistry at a depth A below the soil surface, the second sensor measures soil chemistry at a depth B and so on. Thus, the probe is configured to measure the soil chemistry at these fixed depths below the soil surface. Probes of different lengths and/or different sensor positions may be fabricated to take measurements at alternative depths. At the bottom end of probe  1200  is a pointed tip part  1212 . The pointed tip  1212  is preferably a sharp metal tip which helps insertion of the probe  1200  into a soil structure, and minimises damage to the end of probe  1200  and the sensors inside. The top end of probe  1200  is sealed by a plastic cap  1202  to prevent water/soil water from entering probe  1200  from the top. 
         [0119]    As shown in  FIG. 12   a  and in the zoomed-in view of  FIG. 12   b , the sensor  1206  is a double button style sensor, as each membrane is provided by a separate button-shaped sensor. Reference membrane  1208  and ion-selective membrane  1210  both have a button shape. The membranes  1208  and  1210  are positioned side-by-side so that they are as close as possible and are measuring the soil chemistry at substantially the same position in the soil structure. While in  FIG. 12   a  the membranes  1208  and  1210  are positioned one above the other, they may in embodiments be positioned side-by-side. Each membrane  1208 ,  1210  is coupled to a conductive (e.g. silver) contact element  1216  via a conductive gel  1214 . Each contact  1216  is coupled to wires  1220  via a soldering point  1218 . Wires  1220  may be wrapped in an insulating material  1222  for connection to a data logger (not shown). 
         [0120]    Turning now to  FIGS. 12   c  and  12   d , these illustrate an alternative arrangement of probe  1200 , also comprising multiple ‘button-style’ soil chemistry sensors for measuring soil chemistry at multiple depths within a soil structure. Here, probe  1200  comprises a tube part  1204  and multiple single button style sensors  1224 . A single button shaped sensor  1224  provides both the reference and ion-selective membranes. Both membranes are cast on a single button shaped sensor. As shown in more detail in  FIG. 12   d , the sensor  1224  comprises two portions, where one portion provides a reference membrane  1226  and the second portion provides an ion-selective membrane  1228 . While the two portions are shown one above the other, they may in embodiments be positioned side-by-side (i.e. the button sensor  1224  may be rotated by 90°). Each membrane  1226 ,  1228  is coupled to a conductive (e.g. silver) contact element  1216  via a conductive gel  1214 . Each contact  1216  is coupled to wires  1220  via a soldering point  1218 . Wires  1220  may be wrapped in an insulating material  1222  for connection to a data logger (not shown). 
         [0121]    A single or double button-shaped sensor is detachably ‘plugged’ into an aperture provided along the length of the tube part  1204 . Advantageously, this enables the sensors to be detached and replaced if faulty or if a different ion is to be measured. Furthermore, each membrane of the double button-shaped sensor can be removed separately. 
         [0122]    No doubt many other effective alternatives will occur to the skilled person. For example an embodiment of the sensor can be used not in situ in the soil but to measure nitrate levels in soil on-site. In this case a core sample may be dug up and shaken up with water to obtain a measurement on the spot. Similarly an embodiment of the sensor can be used may be employed to measure nitrate levels in leaves (again shaken up with water), which has value in research and breeding as well as in farm crop testing. 
         [0123]    It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art and lying within the spirit and scope of the claims appended hereto.