Precision pH pays off

High resolution pH maps improve management of this key soil property

Soil pH is an important factor in crop production.  Nutrient usage, crop growth, and herbicide activity are all affected by the pH of the soil.  Grid samples are simply not dense enough to do the job.  Within many 2.5 acre (1ha) grids, there is a wide range of pH values, often ranging from soils that call for lime to soils that are already extremely high in pH.


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1. How accurate is on-the-go pH mapping?

Results from field and lab tests show that this method produces pH measurements that are highly correlated with laboratory analyses of soil pH. The main advantage to on-the-go mapping is the increased sample density. While lab samples are more accurate at the point where the sample is pulled, the interpolation process that fills in the gaps in the map has significant errors due to small-scale spatial variability. The only way to reduce the errors from this variability is to take more samples--the only affordable way to do that is with on-the-go mapping. Under controlled conditions, with lab-analysis of the identical soil used for direct sensing with the Veris system, the correlation is above .95 R².

In extensive field trials, where soil samples were collected within 20’ of the sensor point, the correlation is still quite high, considering the lab and the Veris system are analyzing different soil samples.

2002-2004 Field Trials: 328 validation samples... 15 fields... 4 states...

2. Does it pay to have this mapping done?

Each field will have a different economic return, based on initial pH levels, lime costs, and other yield-limiting factors, etc. On field tests in IL, KS, NE, and WI that compared on-the-go pH mapping to 2.5 acre grid sampling, on-the-go pH mapping generated an improved accuracy of lime usage of over 1100 lbs./acre (1200kg/ha).

5th ECPA Paper [PDF] 1.5MB

An important consideration in evaluating this economic question, is to examine the alternatives. The two most common alternatives to on-the-go pH mapping are grid sampling and zone sampling using soil surveys and layers such as soil EC. Here are maps of sensor pH data overlaid on 2.5 acre grids and over a USDA soil survey. It is quickly evident that the pH pattern does not follow the lines of either overlay. Soil pH varies widely within each soil type and within each grid cell. Neither of the alternative sampling strategies would come close to prescribing the correct lime rate for the field. This field, and most fields which require lime, can only be mapped accurately with several samples per acre—and that density is only affordable with on-the-go sensing. The accuracy problem isn’t with lab sampling—the lab analyses are typically very reliable. The inaccuracy occurs between the sample points when data is interpolated. Lime requirements simply vary too much within a field to allow the computer to fill in the gaps between a handful of samples.

What about using soil EC zones? It depends on the field—here are two examples of pH sensor data overlaid on an EC map. The first field to the right shows a clear correlation between EC and pH. Using EC-defined zones to sample pH would be a good fit for fields like this.

The second field to the right shows a different story—the pH pattern doesn’t follow soil EC as well. On that map, most of the EC zones show pH variability within them. In general, EC zones are a good start, and have greater precision than soil survey lines. Unlike grid sampling, EC management zones are based on actual soil changes. But only on-the-go pH mapping can provide a clear picture of the actual pH variability.

There are numerous scientific journal articles reporting on the topic of within-field spatial variability, and the need for increased sample density. Here are a few:

Bianchini, A.A. and A.P.Mallarino. Soil-Sampling Alternatives and Variable-Rate Liming for a Soybean—Corn Rotation. Agronomy Journal November-December 2002 Vol 94: No. 6 pp. 1355-1366

Brouder, S.M., B.S. Hofmann, and D.K. Morris Mapping Soil pH: Accuracy of Common Soil Sampling Strategies and Estimation Techniques Soil Science Society of America Journal March-April 2005 Vol 69: pp. 427-441

Lauzon, J.D., I.P. O'Halloran, D. J. Fallow, A. P. von Bertoldi and D. Aspinall. Spatial Variability of Soil Test Phosphorus, Potassium, and pH of Ontario Soils Agronomy Journal March-April 2005 Vol 97: pp. 524-532

3. How much does this cost?

Commercial rates will vary by ag supplier, and will depend on services included, number of acres, field size, and other factors. Typical rates are in the $8-12/acre range.

4. Can I convert a Veris 2000XA or 3100 EC Mapping System to the new MSP system?

Some components, including coulter electrodes and electronics can be used when upgrading older Veris units.

5. How fast can I drive? How many acres can I cover per hour?

The table below shows the number of sample points collected per acre at various speeds and transect widths. We recommend that the sample/acre density be a minimum of 5/acre, up to 10/acre, if possible. At 10 samples/acre, the transitions between pH ranges is typically evident, which confirms the spatial structure of the pH variability.

Speed (mph)

Acres Mapped Per Hour

Transect Width-Ft

Speed (mph)

Aprox. pH Samples/Acre

Transect Width-Ft

6. How does the system operate?

Here's the process: (1) row cleaners clear crop residue, (2) firming wheel compacts loose soil, (3) hydraulic cylinder lowers cutting shoe, cutting shoe creates a soil core which flows into sampling trough, hydraulic cylinder raises the trough with the soil core against (4) two pH electrodes. During each cycle the cutting shoe is cleaned by a scraper, and pH electrodes are washed with two 150 psi nozzles. Wash water for cleaning soil off the electrodes is held in a 100 gallon tank, and (5) covering disks cover the track. The sampling process is controlled with an external electronic control module (6), and the pH data is recorded on a Veris recording instrument.

7. Why do you use two electrodes?

As a method of insuring quality measurements, the pH values from each of the two electrodes are compared, and any point where the two readings differ by more than .50 pH are eliminated. The Veris instrument warns the operator with an audible alarm whenever that condition exists.

8. Are the pH electrodes expensive? How long do they last?

These are metal-tipped ion-selective electrodes and they typically can collect several thousand field measurements. That translates into a per-acre electrode cost of $.20-30/acre.

9. What field conditions does the system require?

By creating a soil core with the horizontal sampling device (US patent # 7,216,555), the system has the versatility to handle a wide range of soil texture and moisture conditions. If the soil is dry enough to allow a vehicle to traverse the field, the sampler will work as well. On the other end of the moisture spectrum, if fields are so hard and dry that a conventional hand probe could not penetrate the soil, the Veris system will also have difficulty acquiring a sample. The row cleaner clears a 4-6” wide path through field residue. Trouble-free operation can be expected in seedbed-ready fields, in cotton, soybean, wheat and most corn residues. Recently-harvested ridge-till corn stalks and incorporated residue are more challenging, and may require decomposition before mapping.

10. What depth is this unit sampling?

Depth is adjustable-from 2" to 4". It's critical to keep depth consistent across the field. This is accomplished by gauge wheels that are in close proximity to the sampling shoe. The adjustable depth and rapid mapping creates the opportunity of sampling a field at two depths to investigate stratification.

11. What calibration procedures are there?

A simple calibration routine makes calibrating the pH electrodes convenient, and helps insure the quality of the pH data that will be collected. It takes about 2 minutes to complete the process and it is recommended that it be done at least daily.

12. How do you deal with the buffer factor in creating lime prescriptions?

There are two methods recommended by Veris Technologies: Using the first method, you would submit a small number of samples to a lab for field calibration of pH and buffer pH. The Veris software routine called LimeCalc creates a lime rec for each pH sensor point. This Visual Basic routine uses the lab samples, along with the pH and EC sensor data in a multi-variate regression equation. Another approach is to use the on-the-go pH sensor map to guide targeted sampling for lime recs, and then build the lime prescription from the directed samples.

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