Abstract:
An apparatus for automatically measuring soil pH at a relatively large number of places in a field, and automatically creating a soil pH map by simultaneously measuring the position of the apparatus and storing the pH data in association with the location from which the corresponding soil sample was taken. The apparatus includes a wheeled chassis, a shank for exposing soil at a controlled depth, a sampling tray for collecting soil, a probe for measuring the pH of the soil, a water supply for cleaning the probe between measurements, a devise for measuring the location of the apparatus, and a computer for controlling the measurement cycle and recording the data.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims benefit under Title 35, United States Code, Section 119(e) of U.S. Provisional Patent Application Ser. No. 60/096,172, filed Aug. 11, 1998. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to detecting the acidity or alkalinity of soil. More particularly, it relates to methods and devises for automatically collecting data on the acidity or alkalinity of soil at a number of places in a field. 
     BACKGROUND OF THE INVENTION 
     Site-specific farming, or precision farming, is used today in order to improve crop yields, lower costs, and to protect the environment, by treating different parts of a given field according to their specific conditions, instead of treating the entire field according to the average conditions throughout. In this way, for example, lime can be applied only to areas of a field which have overly acidic soil, which reduces the costs of materials and labor, and avoids excessively alkaline soil from unnecessary liming. 
     Typically, data on soil properties, including pH, are gathered by collecting a number of soil samples and analyzing them in a laboratory. The standard laboratory pH test for a single soil sample requires mixing equal masses of de-ionized water and soil, stirring vigorously for five to ten seconds, letting the mixture stand for ten to thirty minutes, and measuring the pH of the slurry using a calibrated pH meter. This process must be repeated for each sample. Furthermore, each sample must be individually packaged to keep it from being contaminated during transport from the field to the laboratory, and labeled or otherwise identified with the specific location from which it was collected. 
     Because of the difficulty and expense of collecting and analyzing samples in the field with laboratory analysis, the total number of samples that can feasibly be collected is limited. Consequently, the resolution of the data is limited, so that current methods generally produce values of soil properties for areas on the order of a hectare. 
     As will be appreciated by persons of ordinary skill in the art, soil pH may have significant variation within a field. Some fields have soil pH ranging from 5 to 8, with a coefficient of variation exceeding 10%. Combining soil samples over an area of 1 ha leads to a loss of information about spatial variability, and doubts exist about the accuracy of interpolated maps from grid soil sample data. Therefore, persons of ordinary skill in the art will recognize that the variation in soil pH over the area of a single grid can greatly exceed the experimental error in measuring soil pH. The accuracy with which processes such as liming, which are indicated based at least in part on soil pH, can be prescribed is restricted primarily by sampling density. Consequently, crop productivity as a whole is restricted by sampling density. Thus, there is an ongoing need to develop systems and methods to decrease the cost of soil sampling and to improve the resolution of soil property maps. The present invention is directed toward meeting this need. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method and apparatus for automatically measuring soil pH in the field. An apparatus is disclosed, suitable for a standard tractor to tow, which will automatically measure soil pH at a relatively large number of places throughout a field. Also disclosed is an apparatus for automatically measuring the soil pH at a relatively large number of places throughout a field, including a devise for simultaneously measuring the position of the apparatus, in order to automatically create a soil pH map. 
     In one form of the invention, an apparatus for automatically measuring the pH of soil at a relatively large number of places in the field is disclosed, comprising a chassis suitable for towing by a standard tractor; a plurality of wheels; a tank for holding water; a compressed air tank; at least one shank for exposing the soil at the desired sampling depth; at least one pH sensor, each including at least one probe; a probe assembly for each probe, including a sampling platform for collecting soil from pre-selected soil depths and bringing it into contact with the probe, an actuator for moving the sampling platform between an extended position in which a soil sample is collected and a retracted position in which the soil sample is in contact with the probe, and one or more nozzles connected by hoses to the tanks of water for cleaning the probe between measurements by directing water onto it; and an onboard computer for collecting and storing the data. 
     Another form of the invention includes an apparatus for automatically creating a map of the pH of soil in a field, comprising an apparatus for automatically measuring the pH of soil at a relatively large number of places in the field, and a sensor for continuously detecting the position of the apparatus to allow each pH measurement to be automatically recorded in conjunction with the location of the soil which was measured. 
     Another form of the invention includes a method for measuring the pH of the soil at a relatively large number of places throughout a field, comprising: providing an apparatus for automatically measuring the pH of soil at a relatively large number of places in the field, including a chassis suitable for towing by a standard tractor, a plurality of wheels, a tank for holding water, a compressed air tank, at least one shank for exposing soil at the desired sampling depth, at least one pH sensor, each including a probe, a probe assembly for each probe including a shank for controlling the location of the probe assembly relative to the surface of the ground, a sampling platform for collecting soil from pre-selected soil depths and bringing it into contact with the probe, an actuator for moving the sampling platform between an extended position in which a soil sample is collected and a retracted position in which the soil sample is in contact with the probe, and one or more nozzles connected to a reservoir of water for cleaning the probe between measurements by directing water onto it, and an onboard computer for collecting and storing the data; selecting a depth at which soil pH measurements will be made; selecting a speed corresponding to the desired distance between locations for pH measurements; and towing the chassis through the field at the selected speed to collect data for a number of places throughout it. 
     Another form of the invention includes a method for creating a relatively high resolution pH map for a field, comprising: providing an apparatus for automatically creating a map of the pH of soil in a field; selecting a depth at which soil pH measurements will be made; selecting a speed corresponding to the desired distance between locations for pH measurements; and towing the chassis through the field at the selected speed to collect data for a number of places throughout it. 
     One object of the present invention is to provide a unique apparatus for creating relatively high resolution pH maps. Other objects and advantages of the present invention will be apparent from the following description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of an apparatus for automatic measurement of soil pH according to the present invention. 
     FIG. 2 is a side elevational view of further details of the probe assembly shown in FIG.  1 . 
     FIG. 3 is a side elevational view of the sampling platform shown in FIG.  1 . 
     FIG. 4 is a schematic diagram of an apparatus for automatic measurement of soil pH having multiple probe assemblies. 
     FIG. 5 is a plan view of a field showing the relative positions of pH measurements made according to the standard method and the present invention. 
     FIG. 6 is a plan view showing the relative positions of certain parts of the apparatus shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described processes, systems, or devices, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring to FIG. 1, an apparatus  100  for automatically measuring the soil pH according to the present invention is shown. Apparatus  100  includes a probe assembly  110  affixed to a chassis  120 . Chassis  120  has a forward side  121  and a rear side  122 , corresponding to the direction in which apparatus  100  moves when collecting data. Chassis  120  is supported by one or more wheels  130 , which are adjustably mounted thereto, so as to allow chassis  120  to be raised or lowered relative to the surface of the ground  131 . A tow hitch  140  is affixed on the forward side  121  of chassis  120 , so that the apparatus can be towed by a standard tractor  132  or other appropriate vehicle. A shank  150  is affixed to chassis  120  directing forward of probe assembly  110 , and with the bottom edge of shank  150  positioned lower than the bottom of probe assembly  110  when the apparatus is not collecting data. A removable plate  180  is positioned parallel to the direction of travel, adjacent to the trailing edge of shank  150 , and to the side of probe assembly  110 . A top plan view of the shank  150 /removable plate  180  arrangement is illustrated in FIG.  6 . 
     A water tank  160  is connected by water hoses (shown as  165  in FIG. 2) to a water pump  167 , which is in turn connected to nozzles (shown as  250  in FIG. 2) in probe assembly  110 . In one embodiment, water pump  167  is a standard 12V water pump (such water pumps being commonly known in the art). A compressed air tank  170  is connected to an air cylinder (shown as  230  in FIG. 2) though air hoses  175 . Preferably, water hoses  165  and air hoses  175  are affixed to chassis  120  at a number of places along chassis  120 . 
     A location sensor  190  is affixed to chassis  120  or other convenient location. In one embodiment, location sensor  190  is a standard global positioning satellite (GPS) antenna. 
     A computer  199  is affixed to chassis  120 , and is telemetrically connected to water pump  167 , air cylinder  230 , location sensor  190 , and a probe (shown as  240  in FIG.  2 ), and is programmed automatically to record pH measurements made by probe  240  and location measurements made by location sensor  190 , to correlate pH measurements with the location at which they are made, and to control water pump  190  and air cylinder  230 , as further described below. In an alternative embodiment, water pump  190  and air cylinder  230  are controlled by a separate computer. 
     Referring to FIG. 2, further details of probe assembly  110  for making periodic measurements of the pH of soil at a pre-selected depth is shown. The probe assembly  110  includes a sampling platform  210  affixed to a first shaft  211 , which extends through a spring  214  and a mounting  218 , discussed hereinafter. At least the top portion of first shaft  211  is threaded, and is engaged by a threaded nut  216 . The bottom end of spring  214  abuts mounting  218 , and the top end abuts a washer  219 , which in turn abuts threaded nut  216 . An adjustment piece  220  for adjusting the distance between the threaded nut  216  and the air cylinder  230  is provided, comprising two parallel portions, each having a threaded through-hole  221 , and a perpendicular portion. Through-holes  221  are oppositely threaded, with the bottom hole threaded appropriately to engage the threads on the top end of first shaft  211 . First shaft  211  and an second shaft  212  engage oppositely threaded through-holes  221 , so that the distance between shafts  211  and  212  can be increased or decreased by rotating adjustment piece  220  about the axis of shafts  211  and  212 . Second shaft  212  passes through mounting  228 , discussed hereinafter. The end of shaft  212  opposite to adjustment piece  220  is affixed to air cylinder  230 . 
     Air cylinder  230  is adapted to actuate second shaft  212  (i.e. move the shaft  212  away from air cylinder  230 ) when air pressure is applied by compressed air tank  170  through air hoses  175 . The range of actuation is at least about 5 mm, and preferably does not exceed about 40 mm. Air cylinder  230  is affixed to mounting  228 . In one embodiment, the air pressure required to cause air cylinder  230  to actuate shaft  212  is 250 kPa. 
     A probe  240  is affixed to the bottom end of a third shaft  241 . At least the top portion of third shaft  241  is threaded. Third shaft  241  passes through a spring  242 , mounting  218 , and mounting  248 . A threaded nut  246  engages the top portion of shaft  241 , and abuts mounting  248 . The top end of spring  242  abuts mounting  218 , and the bottom end abuts mounting  218 . When correctly positioned, probe  240  is just above sampling platform  210  in its retracted position. 
     Nozzles  250  are connected to hoses  165 , and are directed toward probe  240 , so that when water is pumped from tank  160 , streams of water contact probe  240 . 
     Probe assembly  110  is affixed to chassis  120  by mountings  218 ,  228 , and  248 . Mountings  218 ,  228  are affixed on chassis  120  sufficiently closely to place spring  214  in compression when sampling platform  210  is positioned at least below the bottom edge of shank  150  by turning adjustment piece  220 . Mountings  218  and  248  are affixed to chassis  120  sufficiently far apart so as to place spring  242  in compression when probe  240  has been positioned at least as low as the lowest position in which it might be used to make measurements. This allows spring  242  to hold probe  240  in position against jostling of probe assembly  110  during motion. The position of probe  240  can be adjusted by turning threaded nut  246 . In one embodiment, when spring  214  is in tension, it will return sampling platform  210  to the retracted position when air cylinder  230  is at rest (deactivated). When air pressure is applied to air cylinder  230 , the compression in spring  214  is overcome, and sampling platform  210  is moved to the extended position. In another embodiment, spring  214  retains sampling platform  210  in an intermediate position when no air pressure is applied to air cylinder  230 , wherein the bottom of sampling platform  210  is above the bottom edge of shank  150 . 
     Referring now to FIG. 3, further details of the preferred embodiment of sampling platform  210  are illustrated. Sampling platform  210  comprises a tray portion  310 , and support portions  320 . In the preferred embodiment, tray portion  310  is lower in the middle than on the sides. In another embodiment, tray portion  310  is flat. In one embodiment, tray portion  310  is about 50 mm wide, and less than 66 mm long, relative to the direction of motion. Support portions  320  are affixed to bar  330 , which is affixed in turn to shaft  211 , so that sampling platform  210  is in mechanical communication with air cylinder  230 . In one embodiment, sampling platform  210  is about 150 mm from the top of bar  330  to the bottom of tray portion  310 . 
     In the preferred embodiment, probe  240  is the sensor of an Accumet Model 25 pH meter, which is operable to measure the pH of a soil sample when placed in contact therewith. In order to allow its measurement of the pH of the soil samples to stabilize, the probe must remain in contact with the sample for at least 5 seconds, and preferably for 6 seconds. Collection of the sample and cleaning of the probe between measurements can be accomplished in about 1.5 seconds. Therefore, the preferred period of a measurement cycle is about 8 seconds. As will be readily apparent to those skilled in the art, this permits a range of sampling densities to be used, by varying the speed at which the apparatus  100  is moved. 
     When measuring soil pH with apparatus  100 , adjustably mounted wheels  130  are positioned so that the bottom edge of shank  150  is a distance below the point where wheels  130  contact the ground  131  corresponding to the desired depth of measurement, so that, when apparatus  100  is towed, shank  150  will remove soil above the selected depth forward of probe assembly  110 , creating a trench of the selected depth. Removable plate  180  prevents upturned soil from falling back against probe assembly  110 , potentially contaminating measurements or otherwise disrupting performance. FIG. 6 is a plan view, illustrating the relative position of shank  150 , sampling platform  210 , and removable plate  180 . 
     Air cylinder  230  is adapted to actuate sampling platform  210  between at least two positions, including an extended position and a retracted position. In one embodiment, when air pressure is applied to air cylinder  230 , sampling platform is moved to the extended position, in which sampling platform  210  is brought into contact with the ground  131 , and when air pressure is removed from air cylinder  230 , compression in spring  214  returns sampling platform  210  to the retracted position, in which the soil sample is brought into contact with probe  240 . In another embodiment, air pressure can be applied in two directions to air cylinder  230 , one causing air cylinder  230  to move sampling platform  210  to its extended position, and the other causing air cylinder  230  to move sampling platform  210  to its retracted position. In this embodiment, spring  214  retains sampling platform  210  in an intermediate position when no air pressure is applied. In yet another embodiment, a variable air pressure can be applied to air cylinder  230  so as to cause sampling platform  210  to move to any position intermediate to the extended and retracted positions. Furthermore, those having ordinary skill in the art will recognize that any suitable linear actuator device may be used in place of air cylinder  230 , such as a solenoid, stepper motor/lead screw, etc. 
     Prior to beginning measurement, sampling platform  210  is positioned by turning adjustment piece  220  so that when air cylinder  230  moves sampling platform  210  to its extended position it is 5 mm below the bottom edge of shank  150 . The tension in spring  214  is optionally adjusted by turning threaded nut  216 . When the apparatus is moving, this will cause soil to be collected on sampling platform  210 . Probe  240  is positioned by turning threaded nut  246  so that when sampling platform  210  is moved to its retracted position soil samples contained on the sampling platform  210  are brought into contact with probe  240 . 
     While sampling platform  210  is extended, water is pumped by water pump  167  from tank  160  through hoses  165 , and projected through nozzles  250  onto probe  240 , so as to remove remnants from the previous measurement which might otherwise contaminate the new sample. Preferably, water is projected under pressure of at least about 100 kPa. By projecting water onto probe  240  while sampling platform  210  is extended, samples are collected simultaneously to cleaning probe  240 , minimizing measurement cycle time. The operation of water pump  167  and air cylinder  230  is synchronized by computer  199 , which controls the operations of both. 
     Computer  199  also records the position of apparatus  100  as measured by position measuring devise  190  while sampling platform  210  is in the extended position. The pH measurement of this sample is recorded approximately 6 seconds later, just before the measurement cycle is completed, and is associated in the data storage device with the position of apparatus  100  when the sample was collected. In this way, the pH data is correctly identified with the position in the field from which the soil sample was taken, and not with the position of the apparatus  100  when the measurement is made, which is later in time, when it will have moved some distance from the sample collection location. 
     Referring to FIG. 4, an alternative embodiment of an apparatus for automatically measuring soil pH having a plurality of probe assemblies  110  and corresponding shanks  150  and removable plates  180  is shown. As will be readily apparent to those skilled in the art, by placing multiple probes on a single apparatus, a wider strip of field can be measured at a given sampling density, reducing the number of passes needed to measure a given area. Optionally, probe assemblies  110  and shanks  150  are slideably mounted on chassis  120  to permit a variable sampling density. In one embodiment, shanks  150  are independently adjustable, to allow each probe assembly to measure the pH of soil of an independently selected depth. In another embodiment, shanks  150  and probe assemblies  110  are positioned in at least two rows perpendicular to the direction of motion, so that at least two probe assemblies will measure soil from the same strip of soil. In this embodiment, shanks  150  can be adjusted so that shanks in more rearward rows run deeper than those in more forward rows, so that pH measurements can be simultaneously collected for a three dimensional map. In the preferred embodiment, the more rearward rows of probe assemblies can be positioned a distance behind the more forward rows of probe assemblies corresponding to an integral multiple of the sampling interval, such that deeper measurements are made at nearly the same position at which the shallower measurements are made. 
     Referring now to FIG. 5, typical sampling densities which can be achieved according to the standard soil mapping methods and the present invention are compared. A field  500  is shown, in which the pH was measured using both the prior art method and the present invention. Manually collected measurements  520  show the positions of data points in a pH map created by the prior art method, wherein the sampling density is one measurement per 0.5 acres. Automatically collected measurements  540  show the positions of measurements made with the apparatus  100  and methods of the present invention, using a single-probe apparatus  100 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.