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Soil pH Management

Site-specific management of soil pH is a precision agriculture practice that can provide positive economic and environmental impacts on modern crop production. This publication addresses several frequently asked questions related to the meaning of soil pH, lime requirement, and quality of data used to prescribe site-specific management of soil pH.

What is site-specific management of soil pH?

One of the goals of precision agriculture is to manage agricul­tural inputs according to changing local field conditions in order to increase profitability and reduce environmental waste of agricultural inputs. According to many adopters, variable rate liming is one of the profitable and popular practices in site-specific crop management. In addition to acidic field areas, having knowledge of areas with alkaline soil conditions (high pH) can be useful to avoid lime application in these areas and also aid in the selection of crop varieties tolerant to problems associated with high pH (e.g., iron chlorosis).

Currently, variable rate lime prescription maps are gener­ated based on soil samples collected manually and analyzed in laboratory conditions. These samples are usually obtained with a 2.5-acre sampling frequency.

Is 2.5-acre grid sampling an adequate approach?Field image of 2.5-acre grid sampling pattern

The adjacent photo illustrates a common problem with creating a pre­scription map for applying variable rate lime using 2.5-acre grid sampling. In this case, 330- by 330-ft (2.5-acre) grid cells are superimposed on a bare-soil infrared image. The field has terraces which appear as dark lines so it is evident that there is a significant slope in this field. The white areas are eroded Nora soil, with alkaline (high pH) subsoil near the surface. The darker areas are less eroded, and more acid in the upper horizon.

If a few cores are taken near the center of a grid cell (red dot), the sample pH is likely to be greater than 7 since it is within the white area. As a result, the entire grid cell will receive no lime. If several cores are taken randomly throughout the grid cell, such as at the yellow dots, and then combined, the result will be nearer the average pH for the grid cell. However, the variability in this grid cell is likely to be as high as it is across the field, so little is accomplished. Using this method, it is likely that the grid cell will receive too little lime in the non-eroded portion, and too much lime on the eroded spot. Since the grid lines do not coincide with the patterns of variability, the variable rate application is not necessarily more appropriate than a uniform application. In this example, the analysis cost would be eight times as high as when a regular 20-acre composite sampling strategy is used. Overall, grid sampling with 2.5-acre grids increases analysis cost and often fails to adequately measure spatial pH variability, resulting in reduced profitability of variable rate liming.
The quality of prescription maps generated using a grid sam­pling can be improved by decreasing the grid size to 1 acre; how­ever, the cost of the laboratory analysis for pH and buffer pH will increase. Although the procedure will not need to be repeated for five or more years and the cost can be prorated over that time, the profitability of variable rate liming using this sampling strategy remains questionable. Even in a 1-acre grid cell, a 50-ft lime spreader can make four passes with several different applied rates in each pass (more than 16 – 50 by 50-ft squares can be located within 1-acre grid cell). Therefore, the mapping method still does not match the application technique.

If the earlier mentioned spatial structure exists, certain map interpolation methods can be used to better predict lime application rates in unsampled locations. However, even with the best (from a scientific viewpoint) interpolation method, errors will remain. Any type of interpolation is ineffective when substantial soil variability can be found between nearest soil samples.

Could directed sampling be helpful?

Directed (also called guided) sampling according to relatively uniform required lime application zones is a promising approach for many fields. The zones are determined by considering the variations in the field that may affect lime requirement, including soil types, topographic position, past management, aerial images of bare soil and growing crops, spatial variation in historical yields, soil electrical conductivity maps and/or other data layers.

How can the accuracy of soil pH maps be improved?

Since the beginning of precision agriculture approach, several researchers and manufacturers pursued the development of on-the-go soil sensors to accurately map pH (and other soil proper­ties) at a relatively low cost (On-the-Go Vehicle-Based Soil Sensors, EC02-178). Based on research conducted at Purdue University and the University of Nebraska–Lincoln, Veris Technologies, Inc., based in Salina, Kan., launched production of the world’s first automated on-the-go soil pH mapping system in the summer of 2003. This product is called the Mobile Sensor Platform (MSP). It consists of a widely used electrical conductivity (EC) mapping unit and a Soil pH ManagerTM.

During field operation, the Soil pH ManagerTM automatically collects and measures a soil sample without stopping. While mapping a field, row cleaners remove crop residue. A hydraulic cylinder on a parallel linkage retracts to lower the cutting shoe assembly into the soil, and the cutting shoe creates a soil core which flows into the sampling trough. The previous core sample is discharged at the rear of the trough as it is replaced by the new sample core entering in the front. The hydraulic cylinder extends to raise the sampling trough containing the soil core out of the soil while bringing the new sample in contact with two ion-selective pH electrodes. During sampling, the electrodes are washed with two flat fan nozzles. Covering disks fill the soil trench and cover the track. Measurement depth is adjustable from 1.5 to 6 inches, typically with a 3-inch average effective measurement depth. Soil cores are brought into direct contact with the electrodes and held in place for 7-25 seconds (depending on the electrode response). Every measurement represents an average of the outputs produced by the two electrodes. Two independent mea­surements allow cross-validation of electrode performances and filtration of erroneous readings. The recorded electrode output is converted to pH values according to the selected electrode calibration parameters. Every measurement is geo-referenced using a Global Positioning System (GPS) receiver.

Comparison between soil pH on-the-go maps and 2.5-acre grid sampling

Comparison between soil pH maps obtained through on-the-go mapping and conventional 2.5-acre grid sampling.

This increase in sampling density frequently results in more accurate soil pH maps. For example, the figure above illustrates a 60-acre Kansas field. The neutral soil band near the northwest field boundary (caused by an adjacent gravel road) and a fuzzy pattern of acidic soil in the middle of the field were hidden when the 2.5-acre grid sampling approach was applied. Laboratory analysis of 10 validation samples confirmed that the map based on on-the-go sensing was more accurate than the interpolated map based on 2.5-acre grid sampling.

Can on-the-go soil sensing be used directly to prescribe lime application rates?

Soil pH maps based on on-the-go measurements indicate the variability of soil acidity/alkalinity but need to be translated to lime application maps prior to variable rate liming. This is somewhat challenging as soil buffering capacity typically varies across the field, and the amount of lime needed to change soil pH by one unit is not constant. Therefore, the Veris® MSP combines soil pH and electrical conductivity mapping capabilities as electrical conductivity maps often reflect changes in soil texture (percentage of clay, silt, and sand), the major factor affecting soil buffering capability. Therefore, lime prescription maps can be calculated from the simultaneously obtained electrical conductivity and soil pH measurements.

Does variable rate liming pay?

As with other site-specific crop management strategies, the profitability of variable-rate liming depends on: 1) quality of information, 2) additional application cost and data collection and processing costs, and 3) the variability in lime requirement for the particular field.

For instance, variable-rate liming will not be profitable if lime requirement is uniform or soil acidity is not limiting the yield. Also, liming may require several years to impact the yield and should be considered a long-term investment. Finally, poor quality of information used to prescribe variable-rate liming may result in inappropriate changes of lime application rates and therefore increase (rather than reduce) soil pH variability at the farmer’s expense.

In a recent University of Nebraska–Lincoln study of the value of soil pH maps, it has been shown that the expected net return (crop sale revenue) over cost of lime (NRCL) during a four-year corn-soybean growing cycle is affected by the errors associated with different mapping approaches. Based on the model devel­oped, higher errors mean lower potential benefit.

For the selected field conditions with slightly acidic (5.8 average pH) soil and 9% variability, “low accuracy map” means either a map obtained using 2.5-acre grid sampling or simply assuming that soil pH is constant across the field (composite field sampling). A “high accuracy map” can be obtained through 1-acre grid sampling, on-the-go mapping, or properly conducted directed sampling. The difference between expected NRCL corresponding to high and low accuracy maps represents the expected economic benefit that typically ranges between $5 and $15 per acre. Of course, this benefit should cover the difference in costs associated with both methods, which ranges between $0 and $20/acre. For example, the cost of on-the-go mapping can be similar to the 2.5-acre grid sampling, and the 1-acre grid sampling costs $20/acre more than the whole-field composite sampling.

Summary

When implementing different precision agriculture practices, site-specific management of soil pH has been shown to be one of the most promising strategies in fields with substantial variability in soil pH. Justification of variable-rate liming is complicated by the following: liming is a long-term investment; lime requirements across fields are not always highly variable; and the conventionally implemented 2.5-acre grid soil sampling does not provide the sampling density needed to accurately determine the variability of soil pH in many fields. The recently commercialized technology of on-the-go soil mapping provides a better basis of information about spatial variability of soil pH and other properties related to buffering characteristics (i.e., electrical conductivity). With proper consideration of all the information available, an optimized strategy for site-specific pH management can be developed and positive economic and environmental impacts can be achieved.

(Source – http://cropwatch.unl.edu/ssm/ssmphfaq)

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