NDSU Extension - Mercer County

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Sampling Behind the Combine

soil sampling

Submitted by Craig Askim, Extension Agent/Agriculture and Natural Resources

Soil sampling is one of the best management practices that crop producers and livestock ranches can benefit from. The cost is minimal, but the information gathered can put extra dollars in your pocket. I am asked many times when the best time to sample is and how to sample, the article below explains it.

Harvest has begun for small grains and other early season crops in some parts of North Dakota and this will continue for the next month. I strongly recommend soil sampling as soon as these crops are harvested for a couple reasons:

Harvest has begun for small grains and other early season crops in some parts of North Dakota and this will continue for the next month. I strongly recommend soil sampling as soon as these crops are harvested for a couple reasons:

1. Sampling in August after early crop harvest guarantees that the sampling and soil analysis will be completed, a farmer has soil sample values to optimize fertilizer rates, and there is plenty of time to plan fertilizer application if necessary. Once the calendar hits early October, the chances of being able to sample the field before snowfall decreases rapidly, as we experienced the past couple seasons.

2. A much better 0-6-inch soil core will be obtained compared to sampling following tillage. Achieving a consistent surface core after tillage, particularly after a chisel plow, is very difficult to impossible. If soil test pH, P, K, Zn or any other shallow soil test attribute is wanted, an unworked field is the best possible place to take the sample.

Some farmers/consultants are uneasy about soil sampling in August. At one time, there was a ‘sampling date adjustment’ for the fall nitrate test if the sample was obtained before September 15. This assumed a constant N mineralization rate from August 15 to September 15, and then minimal N mineralization thereafter. Data from a series of NDSU experiments where sites across the state were sampled throughout the year and into the next spring showed that sometimes when a site was sampled in August compared to a spring sample the nitrate value was similar, while sometimes it decreased and sometimes it increased. Neither the increase nor the decrease was large. The change in nitrate was not related to seasonal rainfall at the sites. Therefore, since there was no trend in nitrate value over time, I discontinued the use of a seasonal adjustment in NDSU fertilizer recommendations in the late 1990’s. The August nitrate value is a valid number to use in N rate adjustments to fertilizer rates.

Changes in soil test nitrate do not have to do only with soil N mineralization rates. The greatest N mineralization usually occurs from late April through early June. In some years, this is delayed due to early-season cold, wet or dry weather; however, the least amount of N mineralization is always in the fall. Also, most of our early-season crops, such as small grains or canola, have low-N residue that tends to tie-up N that might mineralize following rain, so large gains of nitrate are uncommon. After field pea, the residue is likewise rather low in N, and there is a previous crop credit for any N release not tied up by the residue or otherwise not identified in the soil test values.

Lastly, soil test K has a seasonal component, particularly in dry years and in soil with dominant smectitic clays. If there is a need to track changes in soil test K over time, sampling for K at about the same time each season will determine trends in K due to K fertilizer additions, or crop removal. Sampling at different times of the year will confound the trend due to the tendency for K to be highest in spring, gradually decrease over summer/early fall, then begin to increase again after crop maturity and rainfall. Also, there is significant K in crop residues, even wheat straw, so the K results in August after early crops, or immediately following corn harvest later, will underestimate the amount of K that is really in the soil. Rainfall leaches the K out of the residues, so soil test K will gradually increase in the fall following harvest just due to residue release.

Source: Dave Franzen – NDSU Soil Science Specialist, david.franzen@ndsu.edu

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