Fertilizer Application During and After a Drought

Dave Franzen, NDSU Extension Soil Specialist

Considerations for this year

When soil moisture is low, seeding time approaches and fertilizer has not yet been applied, several strategies might be considered. None is without some risk.

First, application of N, P and K fertilizers may be applied prior to seeding using a broadcast application. If the field is in zero-till and there's rainfall within two to five days, the nitrogen from the urea might volatilize, depending on the temperature and residue cover. Completely dry fields with no morning dew will enable urea to remain intact at the surface for a longer period. If granules are still visible, volatility would be negligible. When the granules disperse due to a light shower or high humidity, conversion of urea to ammonia gas is much more rapid. If the soil is tilled, the urea would be safe from danger of volatility, but the soil will quickly dry, if it has not done so already, to the depth of tillage.

Another strategy is to apply fertilizer at seeding with the seed if the crop can tolerate some fertilizer or ammonia salts. However, most charts are designed with some soil moisture in mind. If the soil is completely dry, then salt injury would be expected at lower fertilizer rates than normal. Rates should be conservative when seed-placed in a drought.

The last strategy is to delay fertilizer application, applying some small amount of phosphate with the seed, if possible, and apply the nitrogen later when the crop has emerged and established some yield potential. This is not a risky strategy for row crops, which are easily fertilized using a side-dress applicator, but it is a riskier strategy for small grains and other solid-seeded crops where knifing in nitrogen later on is not a good option. Top-dressing solid-seeded crops is better conducted using straight-stream nozzle attachments (Chaffer bars) so that leaf burn is reduced and the liquid nitrogen source, usually 28-0-0 (UAN), is more concentrated in a surface band, reducing the rate of urea volatilization. Regardless of the method, top-dressed nitrogen must have adequate rainfall for the nitrogen to enter the soil and be utilized by the crop.

When drought conditions exist, yield potentials are also lower. Crops like wheat and corn can tolerate higher rates of N, and sometimes quality (protein content) is higher in wheat and might translate into higher prices for grain produced, but this is not guaranteed. Crops like barley, especially if malting quality is a consideration, must be fertilized with nitrogen very conservatively to avoid higher protein content.

Next year's strategies

Following a drought, P and K levels are likely similar to what they were the previous year. This is usually the case in most years due to the small amount of these nutrients removed in a single year by even a bumper crop, but especially so in a small, drought-affected yield. However, the amount of residual nitrogen would be expected to be quite high. Soil testing to a 2-foot depth in the fall or early spring is recommended to modify the normal N rates applied to crops following a year of drought.

If the soil is very hard, soil sampling equipment may need to be modified to penetrate some soils. One tool that works well is to rig an electric generator in the back of a pickup, and attach a soil auger to a 3/4-inch drill. Place a plastic bucket, reinforced with metal bars at the base, with an appropriate size hole in the bottom to allow the auger through into the soil. The auger will penetrate even the hardest, driest soil, bringing up the soil into the bucket. This procedure is perhaps not as elegant as a hydraulic piston sampler, but it works well under hard soil conditions.

Procedures and patterns of soil sampling are detailed in Soil Sampling as a Basis for Fertilizer Application, NDSU Extension publication SF-990.