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.

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: