Abstract:
A probe for monitoring electrical conductivity of soil solutions is provided. The probe includes an upper hollow tube section stoppered at one end, a lower hollow tube section capped at its bottom with a porous ceramic material for inserting into soil, a T fitting connecting the other end of the upper hollow tube section in vertical alignment with the lower hollow tube section, said T fitting having an aperture which is stoppered, an electric conductivity (EC) sensor electrode inserted through the stopper of the upper hollow tube section into the lower hollow tube section, and a vacuum tube inserted through the stopper of the aperture of the T fitting extending into the lower hollow tube section. Ground solution can be withdrawn from the probe via the vacuum tube by a vacuum generator and fresh ground solution can be drawn into the probe without removing the sensor probe from the probe.

Description:
[0001]    This application is a continuation of international application No. PCT/IL2012/000213 filed on May 31, 2012 and claims the benefit of Israeli application No. 213498 filed on Jun. 12, 2011 and Israeli application No. 218431 filed on Mar. 1, 2012, which are incorporated herein by reference in their entirety and for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to monitoring of chemical and physical properties of soil solutions. More specifically, the present invention relates to a probe and method for direct monitoring electrical conductivity (EC) of soil solution. 
       BACKGROUND OF THE INVENTION 
       [0003]    The activity and productivity of all agricultural crops are strongly influenced by the level of salinity, i.e. of the total dissolved solids, in the soil solution which is easily determined by electric conductivity measurements 
         [0004]    The ability of most crops and plants to draw and extract soil water containing fertilizer by means of the roots depends on the salinity level of the soil solution, i.e., the higher the salinity, or the electrical conductivity, the lower the absorbance of the soil solution by the plant. Therefore, accurate measurement of the level of salinity of the soil solution is decisive in agriculture. Namely, monitoring of the electrical conductivity of the soil solution is highly essential for setting proper treatment scheduling, i.e., for deciding when to irrigate, how much water and fertilizers to use and where they should be applied. 
         [0005]    Typical devices for measuring soil properties are disclosed in the following references: 
         [0000]    U.S. Pat. No. 4,513,608 describes a moisture sensing assembly constructed with different zones of porous material. U.S. Pat. No. 5,418,466 discloses a sensor use for salinity measurement using an oscillating tuned circuit.
 
US Patent Publication 2009/0038390 discloses a vadose zone probe mounted on an inflatable sleeve.
 
US Patent Publication 2009/0166520 discloses an in-situ probe having a plunger inserted in the probe.
 
CN 1362420 relates to a probe with an electrode lead of ion meter connected with ion selective electrode sealed in a porous pottery clay cup in the bottom of probe and connected to an automatic controller via a guide pipe.
 
         [0006]    The drawback of this probe is the small and narrow inner space of the pottery clay which is not able to hold an EC electrode and to manage electrical conductivity tests. DE 10058416 discloses a probe and method for determining oxygen content of interstitial water in formations containing sediment and clay. The probe has a very small outer diameter of 5-6 mm and even smaller inner diameter of 2-3 mm which allows the insertion of only a sensor. A vacuum is pulled through the side of the tube. The drawback of such a probe is as follows: the extremely narrow inner diameter of 2-3 mm is able to hold only an electrode with very little free space. The very narrow tube may be advantageous for oxygen measurement but not for measuring electrical conductivity. The reason being, that air bubbles enter the tube with soil solution and adhere to the walls of the tube and/or the electrode and in such a narrow tube the bubbles stay there without rising to the surface of the liquid. Such air bubbles distort EC measurement. 
         [0007]    The above prior-art references all describe devices and methods for direct monitoring of ground water properties in the soil. These devices and methods may be suitable for monitoring a large number of soil properties such as pH, phosphorus, ammonium, magnesium, chlorides, nitrates, potassium, water hardness, or other soil properties but can produce inaccurate measurements or no readings at all for Electrical Conductivity (EC) since EC is highly sensitive to continuous disturbances in the ground medium. Every change in the volume and direction of the flow of the water as a result of irrigation or rainfall may cause the creation of a temporary physical state of lack of unity in the ground structure. This, in turn, includes the formation of air pockets which disturb the EC measurements. Such situation gets worse when the water content of the ground falls and as a result the electrodes lose contact with the water in the soil. 
         [0008]    There is also known the Mottes Soil Solution Extractor and the manual portable testing kits which comprise a probe with a vacuum tube inserted therein, a porous ceramic cap at the bottom and a syringe attached to the vacuum tube at the top. Soil solution is drawn into the probe through the porous ceramic cap by vacuum created by the syringe. Once the soil solution is drawn inside the probe, the solution is withdrawn into the syringe outside of the probe to be checked manually by a portable EC meter and also other testing kits. This probe provides precise in-field testing of soil solution as it changes over time due to fresh soil solution that is drawn in from the soil by the manual syringe operation. The Mottes extractor is accompanied by a variety of portable soil solution testing kits for chloride, nitrate, pH and Electric Conductivity (EC). 
         [0009]    Most of the above devices have a porous ceramic section through which ions diffuse in and out of the probe. Such devices may produce correct measurements that are indicative of the true properties of the soil despite the entrapped air in the probe that does not rise to the top surface of the soil solution, and thus, stays in the measuring zone. Air entrapped in the ceramic section space and/or near the electrodes disturbs and restricts the functioning of the EC electrodes and therefore, alters the measurements to be higher than normal resistance readings (lower conductivities, or zero reading). 
         [0010]    As noted above, the various available devices that conduct direct monitoring of soil properties within the ground enable the measurements of numerous soil properties except for EC, as EC is highly sensitive to continuous disturbances in the ground solution medium. The principle of measuring the electrical conductivity is based on the electrolytic electrical conduction between two sensor electrodes, spaced apart approximately 6 mm. The electrodes are generally sheathed in a rigid or semi-rigid tube, approximately up to 12 cm long. To obtain true EC readings, there must be full and complete solution (liquid) surrounding and between the sensor electrodes without any intervals or disconnections resulting from “air pockets”. When the soil is relatively dry, the solution drawn into the porous ceramic tip may lack continuity, and thus, the EC electrodes may not carry out a measurement at all and may show a read error of the value “0”. 
         [0011]    In sensors for measuring soil properties other than EC, the method of sensing is based on the measurement of an ion concentration in the volume surrounding the sensor, and thus correct measurements can be obtained even if there is no perfect continuity of a solution surrounding the sensor. A pH sensor, for instance, examines the ion concentration of hydrogen in the volume surrounding the sensor. 
         [0012]    Thus, the prior art devices and methods are not suitable for conducting EC measurements that are reliable and accurate in all instances, as accurate EC measurements require a perfect continuity of the solution surrounding the sensor electrode. 
         [0013]    It is an object of the present invention to overcome or eliminate the problems associated with the prior art devices. 
         [0014]    More specifically, it is an object of the present invention to provide a probe that measures in situ, accurately and reliably the electrical conductivity (EC) of soil solutions. 
         [0015]    The probe of the present invention is advantageous for obtaining continuous in-situ in-field measurements that are highly precise and reliable. Such measurements provide growers with in-depth information on fluctuations in the electrical conductivity levels which reflect the total dissolved solids in the soil solution. Thus, the growers benefit from water and fertilizer savings, as well as from a good harvest with increased crops together with a reduction in groundwater pollution that keeps the environment clean and green. 
       SUMMARY OF THE INVENTION 
       [0016]    We have discovered that it is possible to introduce a vacuum tube either through the side or through the top opening of the probe and maintain it parallel with the sensor probe. This allows for continuous monitoring of soil solution, making both, the sensor and vacuum tube an integral part of the probe. Moreover, since the sensor is maintained at all times inside the probe, it is not affected by sunlight, keeping the measurements always stable and reliable. 
         [0017]    In accordance with one embodiment of the present invention, there is provided a probe for monitoring electrical conductivity of soil solutions comprising:
       an upper hollow tube section stoppered at one end, a lower hollow tube section capped at its bottom with a porous ceramic material for inserting into soil,   a T fitting connecting the other end of the upper hollow tube sections in vertical alignment with the lower hollow tube section, said T fitting having an aperture stoppered,   a sensor probe with an electric conductivity (EC) sensor electrode inserted through the stopper of the upper hollow tube section into the lower hollow tube section, and   a vacuum tube inserted through the stopper of the aperture of the T fitting extending into the lower hollow tube section.       
 
         [0022]    Whereby when the probe is inserted into the soil and a vacuum is applied to the probe through the vacuum tube, ground solution is drawn into the probe through the porous ceramic material to cover the electric conductivity (EC) sensor electrode enabling measuring the electrical conductivity (EC) of the soil solution and
       whereby the ground solution can be withdrawn from the probe via the vacuum tube and fresh ground solution drawn into the probe without removing the sensor probe from the probe.       
 
         [0024]    In accordance with the present invention, the aperture in the T fitting is perpendicular to the vertically aligned hollow tube sections. 
         [0025]    In accordance with the present invention, the aperture in the T fitting is at an angle to the vertically aligned hollow tube sections. 
         [0026]    In accordance with this embodiment of the present invention, the upper and lower hollow tube sections of the probe have outside diameters of between 18 to 25 mm and inside diameters of between 14 to 21 mm to accommodate both the sensor and the vacuum tube and to enable bubbles to rise and not adhere to the inner walls of the tube section, the vacuum tube, or the sensor electrode. 
         [0027]    In accordance with another embodiment of the present invention, there is also provided a probe for monitoring electrical conductivity of soil solutions. The probe comprises:
       a hollow tube stoppered at its top end and capped at its bottom end with a porous ceramic material for inserting into soil;   a sensor probe with an electric conductivity (EC) sensor electrode inserted into the hollow tube through the stopper at the top end of the tube, and   a vacuum tube inserted into the hollow tube through the stopper at the top end of the tube;   whereby when the probe is inserted into the soil and a vacuum is applied to the hollow tube through the vacuum tube, ground solution is drawn into the hollow tube through the porous ceramic material to cover the electric conductivity (EC) sensor electrode enabling measuring the electrical conductivity (EC) of the soil solution and   whereby the ground solution can be withdrawn from the hollow tube via the vacuum tube and fresh ground solution drawn into the hollow tube without removing the sensor from the probe.       
 
         [0033]    In accordance with the other embodiment of the present invention, the hollow tube of the probe has an outside diameter of between 18 to 25 mm and an inside diameter of between 14 to 21 mm to accommodate both the sensor and the vacuum tube and to enable bubbles to rise and not adhere to the inner walls of the tube, the vacuum tube, or the sensor electrode. 
         [0034]    In accordance with the present invention, the probe is between 15 cm to 20 cm long. 
         [0035]    In accordance with the present invention, the vacuum tube reaches down near to the ceramic tip. 
         [0036]    In accordance with the present invention, the vacuum tube in the hollow tube is substantially parallel to the sensor probe. 
         [0037]    In accordance with the present invention, the vacuum tube is comprised of a semi-rigid section inserted into the probe and a flexible section attached to a vacuum generator. 
         [0038]    In accordance with the present invention, the semi rigid section has an outside diameter of about 3 to 4 mm and the flexible section has an inside diameter of about 3 to 4 mm. 
         [0039]    In accordance with the present invention, the outside diameter of the sensor tube is 6 mm. 
         [0040]    In accordance with the present invention, the porous ceramic material permits free movement of all ions in the ground solution into the tube. 
         [0041]    In accordance with the present invention, the vacuum tube is connected to a vacuum generator. 
         [0042]    In accordance with the present invention, the vacuum generator is a syringe. 
         [0043]    In accordance with the present invention, the syringe is connected to the vacuum tube via a valve. 
         [0044]    In accordance with the present invention, the sensor probe is connected to a processor to monitor and transfer of measurement data. 
         [0045]    In accordance with the present invention, measurement data can be transferred from the sensor to the processor via wireless connection. 
         [0046]    In accordance with the present invention, there is provided a method for monitoring of soil solution properties. The method comprises the following steps: 
         [0000]    (1) Providing a probe as defined above,
 
(2) Inserting the probe into the ground within the range of the irrigation means in proximity of a plant or tree roots system,
 
(3) Applying vacuum to draw ground solution into the probe to cover the EC sensor electrodes,
 
(4) Conducting measurements via the EC sensor on said solution, and
 
(5) Reading and analyzing the resulting data.
 
         [0047]    In accordance with the present invention, the resulting data received from the EC electrode is sampled, collected and transmitted continuously. 
         [0048]    Further aims, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the following figure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0049]      FIG. 1  illustrates one embodiment of a probe for direct monitoring of soil solution properties in accordance with the present invention. 
           [0050]      FIG. 2  illustrates another embodiment of a probe in accordance with the present invention. 
           [0051]      FIG. 3  illustrates yet another embodiment of a probe in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0052]    Referring to  FIG. 1  there is shown a probe  10  comprised of a divided hollow tube consisting of a lower tube section  12  and an upper tube section  14 . The two sections  12  and  14  are connected in series through a T fitting  16  by gluing them into the T fitting  16  hermetically. The lower tube  12  section is capped at the bottom with a porous ceramic tip  18  for inserting into soil. The upper tube section  14  is sealed at the top with a stopper  19  which has an opening through which a sensor probe  24  having a sensor electrode  26  at its tip is inserted down towards the bottom of tube section  12 . 
         [0053]    The T fitting  16  has a side aperture from which extends a hollow tube section  28  sealed (i.e., glued) therein hermetically. Tube section  28  is sealed with a stopper  30 . A vacuum tube  32  is inserted through stopper  30  and is connected hermetically to a semi-rigid inner vacuum tube  34  that continues into the lower tube section  12 . The other end  35  of the vacuum tube  32  is connected to a valve  36  that is connected to a syringe  38  through its conical tube section  40 . 
         [0054]    The semi-rigid inner vacuum tube  34  preferably has an outside diameter of about 3 mm-4 mm, and the flexible outer vacuum tube  32  has preferably an inside diameter of about 3 mm to 4 mm. 
         [0055]    Such tubing arrangement is suitable since the inner vacuum tube  34  can neither be too flexible nor too rigid in order to maneuver the angle through the T fitting  16  if the inner vacuum tube  34  is to be inserted substantially down into the lower tube  12 . The outer vacuum tube  32  is preferable flexible for easy manipulating and positioning of the syringe. 
         [0056]    A plunger  42  in the syringe  38  can pull a vacuum in the probe  10 . It is preferable that the inner vacuum tube  34  be inserted till the bottom of tube section  12 . This will ensure that the entire ground solution drawn into probe  10  can be withdrawn and that fresh ground solution can be sucked in. 
         [0057]    The sensor probe  24  has the tip of the sensor electrode  26  located parallel to the inner vacuum tube  34  in tube section  12  preferably extends its entire length (generally 12 cm) down the tube section  12 , (but may be shorter as long as it can be immersed in the soil solution). In this embodiment, the probe  10  is relatively short, about 15 cm, and the sensor electrode  26  extends close to the top of the ceramic tip  18 . 
         [0058]    The sensor electrode  26 , in accordance with this invention, is an electrical conductivity (EC) sensor. Electrical conductivity measurement is very sensitive, and unlike measurements such as pH, chlorides, oxygen, nitrates, phosphorus, ammonium, water hardness, potassium, magnesium, or other soil solution properties that are not sensitive to the presence of air bubbles in the liquid media. EC measurements could not be carried out if the liquid media contains air bubbles that may accumulate around the sensor. Such air bubbles are introduced by drawing in the ground water through the ceramic cap  18 . Therefore, to provide a soil solution free of air bubbles, tube section  12  preferably has an inner diameter wide enough to enable bubbles to rise to the top surface of the solution and not adhere to the inner walls of the tube section  12 , the vacuum inner tube  34  and around or between the sensor electrode  26  itself, which would interfere with the EC measurement. Moreover, the tube section  12  should be sufficiently wide to accommodate both the sensor and the vacuum tube. Therefore, in accordance with the present invention, tube sections  12  and  14  preferably have an outside diameter of between 18-25 mm and an inside diameter of between 14-21 mm in order to accommodate the sensor probe  24  and inner vacuum tube  34  side by side. In a preferred embodiment the sensor probe  24  is connected to a controller (not shown) via cable  44 . 
         [0059]    The porous ceramic cap  18  is made of inert material, which is not affected by soil components such as fertilizers and allows free transfer of the ions present in the ground solution into the probe  10 . 
         [0060]    The stoppers  19  and  30  can be made of rubber, plastic, cork, glass or other material able to form a tight seal. The T fitting  16  must be made form a tight fit material to match hermetic sealing (gluing) with tube sections  12 ,  14  and  28 . The syringe  38  should preferably be of sufficient volume (not less than 60 ml.) in order be able to create a vacuum of up to 0.9 bars easily. 
         [0061]    The role of the syringe  38 , in accordance with the present invention, is two-fold: (1) to create a vacuum and draw soil solution into probe  10  to at least cover the sensor electrode  26  by opening the valve  36  and pulling the plunger  42  fully all the way out and closing the valve  36 , then after measuring the electrical conductivity (EC) of the solution, and (2) to transfer the soil solution from tube section  12  to the syringe  38  by opening the valve  36  and drawing the solution from the tube into the syringe  38  while drawing in new fresh soil solution into probe  10  for the next measurement, then closing the valve  36  and expelling the solution from the syringe  38 . Thus, filling and emptying of probe  10  with a refreshed ground solution can be achieved quickly and without requiring a significant period of time for the soil solution to come to equilibrium. 
         [0062]    It should be noted that the sensor probe  24  is inserted through the stopper  19  of upper tube section  14  into tube section  12  sufficiently to have the sensor electrode  26  completely immersed in the soil solution drawn into the probe  10 . It is desired to keep the diameter of the tube section  12  within the limits as defined, in order to draw in soil solution and avoid air bubbles that interfere with EC measurements. 
         [0063]      FIG. 2  illustrates another embodiment of a probe  20  in accordance with this invention. In this case the electric conductivity (EC) sensor probe  24  does not go to the bottom of the tube section  12 , but still can measure the electric conductivity of the soil solution since the vacuum pulled via the inner vacuum tube  34  will bring the solution up in the tube section  12  and cover the sensor electrode  26 , enabling correct measurement. The vacuum created by the plunger  42  via the syringe  38  is preferably up to about 0.9 bar as the probe  10  is long and the soil solution is to be drawn up high.  FIG. 3  shows yet another embodiment of a probe  50 , wherein both, the inner vacuum tube  34  and electric conductivity (EC) sensor probe  24 , are introduced through a stopper  54  into a tube  56 . As can be seen in  FIG. 3 , the probe  50  itself can vary in length depending on the particular soil to be measured, but since the (EC) sensor probe  24  is generally only about 12 cm long, the soil solution is drawn up high enough in the probe to completely cover the sensor electrode  26 . The inner vacuum tube  34 , however, can be as long as desired; the closer it is to the bottom of the tube the easier to remove all of the soil solution from the tube  56  by vacuum. 
       Operating Procedures 
       [0064]    The operation will be discussed with reference to  FIG. 1 . The probe  10  operates as follows: the probe  10 , having a suitable length for the specific soil depth is inserted into the soil to the depth desired. Initially, the syringe  38  is connected to valve  36  at its connector  40  while the plunger  42  is pushed in fully to the end. The valve  36  is then opened to provide a free air path from the syringe  38  through the vacuum tubes  32 ,  34  to the inner space of the probe  10 . The plunger  42  is then fully withdrawn to create a vacuum through tubes  32 ,  34  in the probe  10  and the valve  36  is then closed to block the path between the syringe  38  and the vacuum tube  32 , in order to maintain the vacuum in probe  10 . The syringe  38  may be disconnected from the valve  36  at this time. The soil solution is thus drawn into tube section  12  through the porous ceramic tip  18 . 
         [0065]    Since the soil solution rises up at the tube section  12 , it covers the sensor electrode  26  and measurement can be made continuously. 
         [0066]    It should be noted that the syringe  38  as such, is not part of probe  10 , but a practical means of creating a vacuum, and can be replaced by other vacuum generators. 
         [0067]    After rain or irrigation, the relative concentrations of nutrients, fertilizers, etc. in the ground solution may vary significantly, therefore, one way of measuring electrical conductivity (EC) manually on site is as follows: 
         [0000]    (a) The syringe  38  is connected to the valve  36  with the plunger  42  pushed in fully to the end.
 
(b) The valve  36  is set to an open position between the syringe  38  and the vacuum tubes  32 ,  34 .
 
(c) The plunger  42  is pulled back all the way to create a vacuum, drawing in soil solution into the tube section  12 .
 
(d) The valve  36  is then closed to block the path between the syringe  38  and the vacuum tube  32 , in order to maintain the vacuum in probe  10 .
 
(e) The syringe  38  may be disconnected from the valve  36  at this time, and the plunger  42  pushed back into the syringe  38 .
 
(f) The above procedure may be repeated in order to increase the vacuum in the probe  10  up to about 0.9 bars.
 
         [0068]    The solution drawn into the tube section  12  is a representative sample of the ground solution and the first measurement may be conducted once the soil solution reaches above the sensor electrode  26 . 
         [0069]    As time passes, the amount of nutrients, fertilizers and amounts of water in the ground vary and the ions will flow in and out the tube  12  till equilibrium is established between the ions in the ground and the ions in the tube  12 . Continuous monitoring of the solution in the probe  10  will provide a curve of the actual fluctuation of ion concentration in the ground solution over time. 
         [0070]    In order to improve the equilibrium process, it is recommended to refresh the solution in probe  10  by repeating the above manual procedure, preferably once a week. 
         [0071]    The probe  10  is suitable and will work properly in field capacity state and less so in dry soil conditions. 
         [0072]    The in-situ measurements may be transmitted continuously from the EC sensor probe  24 , via a cellular data-logger to an internet database server for a continuous viewing of the data which may be presented at the website in the form of a graph or table, and for generating analyses of the obtained data based on which the farmer may regulate the delivery and supply of fertilizers, nutrients and irrigation.