This past summer, drought affected large areas of ND, SD, MT, and SK and reduced yields greatly. When you have a drought that greatly reduces early season growth and yield, there is usually high nitrate remaining in the soil, like we saw in drought-affected areas this fall (see map on high nitrate areas).
After a drought, we often get the question, “Can I count on the nitrate in my soil test for the 2018 crop?” The simple answer is yes; you can count on the nitrate in the soil test, but there are other factors you need to consider. Even in a drought, areas of each field produce higher yield due to factors like higher clay content and better moisture holding capacity. In those areas with better yield, there will be less nitrate left in the soil profile.
Let’s imagine you have a field severely affected by drought and areas of the field still had about 50% of normal yield (maybe lower landscape position or better water holding capacity). The composite soil test for this field tested 140 lb/acre nitrate-N after harvest, but the better producing areas only tested 80 lb/acre. Based on the composite soil test of 140 lb/acre, you would only need to apply a little starter N for most crops. If you applied only a starter N amount, the parts of the field that produced a moderate crop and only tested 80 lb/acre will be under-fertilized and cost you yield this year.
If you only have a composite soil sample from the whole field, you need to consider variability in soil nitrate across the field caused by the drought. You will want to apply a base fertilizer N rate to cover the parts of the field that have lower nitrate than the field average. The N rate you apply to cover up that variability may range from 30-60 lb/acre and every situation is different.
If you have been doing zone soil testing, you will have a much better idea of the N fertilizer needs in all parts of your fields. With zone soil testing based on productivity, you will have the soil nitrate test level in each zone to use to make your N fertilizer decisions this spring. If you have only been doing 0-6″ or topsoil testing, you do not have enough information on the nitrate in the soil profile to make an educated decision on the N fertilizer your fields will need. You will need to do some soil testing to 24″ this spring to get the information you need to make a good decision on your N fertilizer rate. There is no way to model how much nitrate is left in the soil profile after a drought. Soil testing to 24″ is the best way to determine the amount of nitrate in the soil profile.