Nutrient management related to dry soil conditions
Dry soils have resulted in a high incidence of potassium (K) deficiency symptoms in both corn and soybean. Potassium deficiency occurs with low levels of soil test K. Dry soil reduces K uptake by slowing the movement of K from the soil solution to the plant root. Thus, K deficiency will occur in a dry year at higher soil test levels than will occur in a wet year.
Some of the K deficiency seen this year is in soil that normally has adequate K levels to supply the plant sufficiently, however much of the K deficiency is likely being seen in soil with low K levels. Soil sampling should be used to determine if soil K is adequate for future crops.
Soil Testing Problems in Dry Soils
Unfortunately if dry soil conditions continue, soil samples taken this fall (if the ground is soft enough to get a sample) may provide misleading results for K and pH. Soil test K has been shown to vary substantially with dry conditions. In soils with 2:1 clay minerals soil test K levels vary with moisture content. When field moist soil low in K is dried for analysis, an increase in soil test K occurs. However, when soils high in K are dried, soil test K decreases. If persistent dry conditions continue in the field this season, K availability will likely be overestimated in low testing soils and underestimated in high testing soils. In Indiana topsoils the change in soil test K with drying has been approximately ±15% at the highest and lowest soil test levels examined.
Soil pH measurements are also affected by dry soil conditions. With a dry season and poor plant growth much of the fertilizer added this spring and last fall remains in the 8-inch sampling zone. Higher than normal salt (fertilizer) levels affect the way the pH electrode functions and will produce a pH reading about 0.5-1.0 pH units lower than the actual pH. In addition, soil moisture has been insufficient for normal amounts of limestone reaction in soils limed this spring or last fall. Therefore, soil pH measured this fall will be lower than expected. The lime remains in the soil, however, and when moisture returns it will increase soil pH as expected. Re-testing this fall and adding more lime based on a low soil pH measurement may result in excessively high pH in future years.
Depth of sampling is extremely important especially in fields with reduced tillage. Nutrients and pH are stratified within the upper 8 inches of soil without plowing. Soil test phosphorus (P) and K levels are highest at the soil surface due to the application of fertilizer and plant residues to the soil surface and levels of these immobile nutrients decrease with increasing distance from the soil surface. The stratification of soil pH depends on the placement of N and the relative frequency and quantity of lime application. Typically with subsurface application of N and infrequent liming, soil pH decreases with distance from the soil surface. Differences in sampling depth can substantially alter the results of soil analysis. Hard dry soil tends to reduce sampling depth, resulting in higher nutrient levels and pH than expected. Keep this in mind when comparing results from this year to results from previous years.