Don't Guess Soil Test


With record high fertilizer prices, it is more important than ever to efficiently use our fertilizer resources. The WSU Small Grains website has a number of useful tools and calculators for assisting in developing nitrogen rate recommendations for both small grain and oilseed crops. Nitrogen fertilizer recommendations rely on estimating crop nutrient demand and an assessment of the nutrients supplied by the soil. The crop demand is based on the yield goal and the amount of fertilizer required to produce a unit of the crop of interest. The nutrients supplied by the soil is the sum of nitrogen from soil organic matter, the soil test nitrogen, and a debit or a credit from the previous crop. Soil organic matter changes very slowly over time and historical soil test results can be used to estimate the release of nitrogen from soil organic matter. However, the soil test nitrogen changes dramatically from year to year and it is important to soil test annually prior to developing fertilizer recommendations. Hence the saying popular among soil fertility specialists, “Don’t guess soil test”.

In addition to the record high fertilizer prices for 2022, high commodity prices, and low yields and low precipitation in 2021 makes soil testing more important than ever for this crop year. Low crop yields generally result in less nutrient removal than high yielding crops leaving a greater amount of residual nitrogen in the soil. The amount of nitrogen used by a wheat crop can be estimated using the WSU Post Harvest Nitrogen Efficiency Calculator tool. While tools such as this can estimate the residual Nitrogen following harvest, they cannot predict the amount of Nitrogen available in the following year. Nitrogen remaining in the soil following harvest can be lost through leaching or denitrification. Soil testing is the only way to accurately assess the amount of nitrogen available to the plant in the soil. Once you have soil sampled, the WSU Nitrogen Fertilizer Recommendation Calculator can help you more accurately predict your spring and winter wheat nitrogen needs for the upcoming season. As you develop your fertilizer management plans for 2022, please make sure to soil test and use the tools provided on the small grains website. A little extra effort in soil sampling could save a lot on the fertilizer bill this year and improve yield and farm revenue.

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For questions or comments, contact Dale Whaley via email at or phone at 509-745-8531.