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Issue No. 21, Article 9/September 9, 2011

Pointers on Soil Testing This Fall

We are once again at the end of one growing season and starting to plan for the next. Plans likely include applying phosphorus (P) and potassium (K). Whether you need to apply either of these will depend in part on your soil test levels. Soil P and K tests measure the availability of these nutrients for the crop. Obtaining accurate information through soil testing is critical to ensuring that you do not limit the yield potential of the crop and that you do make profitable management decisions. Soil testing is a very useful tool, but it does have limitations. One of the most valuable uses of test results is tracking changes over time and helping evaluate the impact of fertilizer management on a field.

As in 2010, this year's harvest season got started early. Also similar to last year, it looks like dry soil conditions will prevail in many areas early this fall. Early harvest and dry soil conditions are important factors to keep in mind if you are planning to collect soil samples for P and K analysis. Last year, partly because of these conditions, there was a lot of variability in soil test results, causing some people anxiety.

Taking soil samples early in the fall, especially when soil conditions are dry, can make it challenging to interpret test results. The greatest source of inaccuracy in soil test values is often related to how samples are collected. While there are some things we can do to reduce variability, other factors are not as easy to control.

Consistency is very important. For accurate results, always collect samples in the same location, to the same depth, at the same time of the year, with the same kind of tool. Once samples are collected, process them using the same methodology and send them to the same testing lab.

  • Same location: Collecting samples in repeated years from the same place in a field used to be difficult, but a good GPS now makes it feasible.
  • Same depth: In Illinois the tests for P and K have been correlated and calibrated to determine the level of nutrients a crop needs based on a 7-inch depth sample. Sampling depth is most critical in fields where there is little mixing of the soil with intensive tillage. When soils are not thoroughly mixed, P and K levels tend to be higher in the top inch. Collecting deeper samples dilutes the test concentration, and collecting shallow samples results in higher-than-actual test levels. Dropping a portion of the soil surface during sampling will result in lower-than-actual test levels.
  • Same time: If samples have been collected in the fall, continue to collect them in the fall. If you collect samples in the spring, do not try to compare results to a fall sampling. (See the later discussion.)
  • Same tool: A soil probe is the best tool for collecting samples as it has the same width across the entire sampling depth. With an auger, portions of the top of the sample can easily fall off during collection, creating misleading test results. A shovel is the least desirable tool because it is very difficult to maintain a consistent sample width. Typically shoveled samples have results biased toward the fertility in the surface soil because the samples tend to be wider on the top and narrower on the bottom.
  • Same methodology and testing lab: If soils are mixed and you plan to send a subsample for analysis, make sure the sample is completely mixed before taking the subsample. If you cannot mix the soil, it is best to send the entire sample to the lab. Using the same testing lab each year reduces the inherent and inevitable variability between labs, even reputable and certified labs. For a list of certified testing labs, see www.soiltesting.org. Different labs may use different extraction or analysis procedures, which can produce different results, prohibiting accurate comparisons from year to year.

What about the factors for which we have little or no control? Natural processes can make it difficult to obtain accurate soil test information. There is more uncertainty for K than P in testing and prediction of sufficiency, so I will focus here on K. Research in Illinois showed that soil K test levels are influenced by time of the year and soil moisture (Figure 1). This seasonal variability makes it important to collect samples at the same time of the year across years.

Figure 1. Soil test K levels as affected by time of sampling and soil moisture conditions. (Source: E. Varsa, Southern Illinois University–Carbondale, and S. Ebelhar, University of Illinois.)

Moisture content tends to be more uniform from year to year in the spring, but during the fall there can be substantial differences from one year to the next. As already mentioned, soils were drier than normal last fall, and test results in many cases were lower than expected. This is partially related to clay chemistry in the soil. Clays can be compared to sheets of paper in a book. When soils are moist, some of the clays separate enough to allow K+ ions to move between clay sheets. As the soil dries and the clay sheets collapse, they trap K ions. When soils are rewetted and then freeze and thaw through the winter, K+ ions are once again available for crop uptake. However, when K is fixed between the clay sheets, it is unavailable to plants and undetectable by a soil test that tries to mimic plant K availability.

Another factor that can influence K availability in the fall is the amount of leaching that has occurred from plant materials into the soil. Plants take K as K+ ion, which stays in that chemical form inside the plant. Unlike nitrogen and P, which become part of the plant (DNA, proteins, etc.) and have to be broken down by microbial activity before they are released into the soil, K+ ions can start to leach out of plant materials into the soil once the plant reaches physiological maturity. Last year we had an early harvest, and many soil samples were collected soon afterward. Because little precipitation had fallen before soils were sampled, most of the K taken up by the crop was probably still in the plant residue at the time of sampling. This is not a trivial consideration given that a corn crop of 180 bu/acre can produce enough stover to contain 100 to 200 lb K2O/acre.

The take-home messages are these:

  • Do all you can to reduce year-to-year variability when collecting samples.
  • When interpreting results, keep in mind that moisture conditions can influence test results, especially for K.
  • Remember that soil test values should not be considered absolute figures but are most useful for monitoring trends over time (increasing, decreasing, remaining stable). If one year's results are very different than expected, do not become overly concerned. Instead, either retest the soil or use those values with caution, considering them in the context of the trend of repeated results.--Fabián G. Fernández

    Fabián Fernández

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