Rule of Thumb: Low Soil N = Lost Yield

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The last couple growing seasons have been very wet in many areas and growers have a lot of questions on nitrogen management. Recently a grower called AGVISE and asked if there was a “Rule of Thumb” he could use with the soil nitrate test, to determine if he had applied enough nitrogen fertilizer to his wheat and corn fields. This grower is from an area where the 0-24″ soil nitrate test is done on all his fields after harvest each year.

We looked for university research we could use as a rule of thumb on the soil nitrate test but weren’t able to find that kind of research. While searching for this information, I had a conversation with Ron Gelderman (retired Extension Soil Specialist – SDSU Brookings). I asked Ron about this and he said “If the 0-24” soil nitrate test is less than about 30-40 lb/a following corn or wheat, SDSU research would show that some yield was lost for corn. For wheat, Ron said, “there was probably yield lost and protein as well.” His comments were based on over 30 years of soil fertility research at SDSU.

So far this year AGVISE has tested soil samples from over 27,000 wheat fields in this region. The average soil N following wheat for the past 28 years is shown in the figure. From these 27,000 samples from wheat fields there is a large percentage of the soil samples testing very low in nitrogen this fall. If we used the conservative end of the rule of thumb range, say 30 lb/ac, you can see that there is a larger percentage of fields testing less than 30 lb/ac the past few years. I included 2012 and 1988 so you have some reference to dry years when wheat yields were limited by moisture and not nitrogen. We will have the 2014 data on corn fields later this fall.

So based on the conservative rule of thumb of 30 lb/a, 62% of wheat fields probably suffered yield and or protein loss this year. New high yielding wheat varieties are likely part of the reason. Many growers have not increased their N fertilizer rates to account for the high yield potential and N losses from excessive spring moisture made the problem worse. If your fields consistently test lower than 30 lb/ac of soil nitrate following wheat or corn harvest, you are probably losing yield. Sampling corn fields for soil nitrate after harvest is a good way to evaluate your N fertilizer program, even though these fields may be planted to soybeans next year. Fall soil nitrate testing is a valuable tool for evaluating your nitrogen fertilizer program to make sure you will supply enough N for your corn and wheat crop next year.