ISSUE 15 August 14, 2003
SOIL TESTING SEASON
Small grains are coming out of the field and that means that it is time for soil sampling to begin for next years nitrogen program. There are several reasons why early sampling is not very popular, and probably the biggest reason is tradition. It seems that many growers believe that large differences in soil test nitrate are possible between Aug. and Oct., and it is true that some fluctuations, usually plus or minus 10 lb/acre of nitrate-N are possible during this period, usually after a shower or a tillage operation. But as a rule, stubble fields are pretty dull when it comes to changes in nitrate-N levels. A large study during the summers and falls of 1970-1971 showed that changes in soil nitrate-N averaged under 10 lb/a between the first week in Aug. and the end of Sept.
Changes that can occur usually happen when sampling is delayed during years with significant volunteer grain growth in the fall. Volunteer growth (as shown in a study several years ago by Moraghan at NDSU-see 1994 sugarbeet reports) can contain substantial N taken up from the soil. Unlike wheat straw, during whose decomposition will extract additional N from the soil, green, growing wheat plants contain enough N that when they are tilled or winter-killed, decomposition easily released N back into the soil. Sampling when there is volunteer grain growth, or recently tilled volunteer grain growth will underestimate the amount of available N for the upcoming growing season. A recent journal article from Nebraska looked at rye cover crop growth and estimated a 40 lb N/ton credit be given for the rye cover tilled into the soil. Although NDSU is not ready to make a similar adjustment to small grain volunteers, it is clear that it has some immediate value for the next crop not factored into current recommendations. Early sampling eliminates the guessing caused by volunteer grain growth.
Another often overlooked advantage of early sampling, particularly right behind the combine and before tillage is the quality and consistency of the soil core taken from undisturbed soil compared to the quality of the core taken after tillage. This is particularly a problem for the 0-6 inch core depth which is the depth needed for P, K, pH, and zinc analysis. It is difficult if not impossible to do a consistent job of sampling a shallow depth with an automatic soil probe, since clods and loose soil often are moved out of the way during the procedure. Some samplers wisely take the sample from a wheel track of their vehicle, which helps greatly, but the sampling would be most consistent if taken right after harvest.
Samplers need to be aware of safety concerns when sampling. If sampling in stubble, the vehicle should be high enough so that there is no fire danger. When sampling below 18 inches, which practically everyone is, it is necessary to call the North Dakota one-call number 1-800-795-0555 several days before beginning.
NDSU Extension Soil Specialist