While much of the Great Lake’s region was fortunate to get some rain in the last couple of weeks, we still remain well below average for the growing season. These dry conditions are having an impact on the results of some soil and plant tissue tests.
One of the most noticeable trends is very low potassium levels in corn tissue samples. When testing the most recently mature leaves of corn in the V5 to V7 growth stages, the normal potassium level ranges from 2.0% to 2.5%. This summer most corn samples are below this range with a surprising number of samples showing severely low levels below 1.0% potassium. While potassium soil test levels have been steadily declining for several years, that is not the likely cause of these low tissue tests. The plants simply cannot take up the potassium that is there. Plants take up potassium through two main mechanisms, mass flow and diffusion. Both require adequate soil moisture to occur. This means that adding more potassium to these fields is not likely to correct these deficiencies until we get some more rain.
Another common trend is high testing soil nitrate levels. This is the result of there being just enough moisture for the soil microbes to mineralize and nitrify the nitrogen from soil organic matter and manure, but not enough moisture for efficient plant uptake or to leach the nitrate through the soil profile. This has made it very difficult to decide how much additional nitrogen should be used in a sidedress application. Traditional PSNT interpretations will say that no additional nitrogen is needed when soil test levels are greater than about 25 ppm. In a season with adequate moisture, soil nitrate levels are often 20-40 ppm. This season nitrate levels have commonly been between 50-100 ppm. Some will take a conservative approach and not apply any more nitrogen with the expectation yields are likely to be reduced with the dry season. Some will still apply some additional nitrogen while there is an opportunity to make the application in hopes we will get more moisture as the season progresses. Either approach is justified. Unfortunately, we might not know if we made the right decision until the combine goes through the field this fall.
Routine soil tests so far this season do not seem to be impacted by the dry conditions. However, should the droughty conditions continue, this could potentially change as samples are collected following wheat harvest and into regular fall sampling season. The two most common measurements impacted by drought conditions are potassium and pH. The potassium levels will be lower due to collapsing of clay particles trapping potassium inside. The pH can possibly come back lower than it should be due to excess salts in the sample that interfere with pH electrode readings. Fortunately, it takes a severe drought to have extreme impacts on routine soil tests. We have not seen this level of drought since 2012, and hopefully won’t anytime soon.