Drought’s impact on crops can be sobering, but more than yields suffer. Every nutrient cycle takes a hit in a drought, increasing the imperative to measure nutrients against the 4R nutrient stewardship goal: Apply the right nutrient source, at the right rate, at the right time and in the right place.
Compared with the soil nitrate levels found during normal-rainfall seasons, residual soil nitrates can be higher after a drought in fields where nitrogen (N) had been applied to a cereal crop. Higher levels arise from decreased downward movement of soil water and from reduced fertilizer N uptake by the drought-stressed plant.
Whether or not residual N will be available for next season’s crop greatly depends on the precipitation that occurs after current harvest. In the Midwest, precipitation occurring early in the following season is associated with higher losses
of nitrate in tile drainage. Soil nitrate tests are the best tool for assessing the quantity of residual soil N available to the next crop. To take up some of the residual nitrate and protect it from environmental losses, farmers can plant cover crops between current harvest and next-season planting.
In many drought-stricken areas, crops originally intended for grain harvest are cut instead for forage. The change from harvesting grain to harvesting most of the aboveground portion of the plant can change each nutrient’s removal rate. If farmers harvest the grain as planned, grain nutrient concentrations of drought-stressed crops may or may not differ from unstressed plants, depending on the crop as well as the timing and severity of water deficits. Tissue testing of harvested forage cuttings in such years is essential to determine changes from planned nutrient removal. This is especially important for phosphorus (P) and potassium (K).
Very little data exists on how N credits are affected for cereals grown after legumes under drought conditions. If drought occurred during the growth of the legumes, soil scientists hypothesize that more residual nitrate will exist in the soil, since legumes often scavenge soil nitrate under normal growth. How drought affects the amount of readily mineralized N from root exudates isn’t well quantified. However, since farmers regularly plant legumes in arid areas to provide N to cereal crops, it seems reasonable to take some or all of the N credit used under normal conditions.
Soil tests after a drought may contain some unexpected variance when compared with tests from more normal years. The immobile nutrient most sensitive to environmental conditions during sampling is K. Lack of rainfall reduces the leaching of K from plant tissues prior to sampling, which can reduce soil test results. Additionally, soil mineralogy can either increase or decrease the amount of available K under drought conditions. Taking soil samples in a drought year and looking at the changes can provide valuable data for improved interpretation in future years when dry conditions strike again.
The impacts of drought on crop nutrition are profound. Soil testing and plant analysis remain our best tools to quantify these impacts to adjust nutrient management for the next crop. Discussing analytical results with a trusted and knowledgeable agronomist ensures informed strategies can be created to meet local conditions and reach the 4R nutrient stewardship goals that keep your fertilizer maintenance program sustainable no matter the weather.
Source: The International Plant Nutrition Institute (IPNI).