The crop nutrients needed in the largest amounts by a corn silage crop are nitrogen and potassium. While the amount of crop nutrients in dairy lagoon water is quite variable, concentrations of nutrients in pond water that has been used for flushing during the winter months can be quite high. Typical amounts of ammonia from nitrogen are from 55 to 65 lbs. per acre inch of pond water (winter time, used for flushing) and from 110 to 200 lbs. per acre inch of water of fertilizer equivalent potassium. Additional nitrogen is supplied in the organic form. In most cases, pond water nutrients are likely to be most concentrated in the winter time, when only minimal releases are made, and the pond water is recycled in the flush system. Once the pond has been drained during spring drawdown and possibly refilled with district water, concentrations will be much less.
The key to using pond water as a fertilizer source for crops is timing the application to match crop needs. Corn takes up most of its nitrogen during the period from about 8 leaves to tasseling (see chart below). The period of uptake of potassium is similar. In this area, this corresponds to the time that commercial water run ammonia is commonly applied during irrigation. Pond water should be applied in the same way if it is to be used as a nutrient source.
On sandy loam or loamy sand soils, nitrogen from pond water that was applied during preirrigation is likely to be leached out in the first irrigation(s) and not be available to the corn during the time that the crop needs it most.
For this reason, only a minimal drawdown of the pond should be done during preirrigation. The amount of pondwater released should be limited to the amount needed to accommodate storage needs until the crop is knee high or taller. This amount should be divided equally over all the available acreage so that each field receives a small amount. This not only minimizes nitrogen losses, it also minimizes the amount of salt applied during the most sensitive seedling stage.
In many instances, there should be adequate nitrogen and potassium in the pond water to supply all or most of the needs of the corn crop, depending on storage capacity, the number of cows, and number of acres pond water can be applied to. However, if most of the nutrients are put out in the winter or in the preirrigation, they won't be available to the corn when the crop needs them. If a complete drawdown and flushing of the pond is desired, drawdown should occur prior to tasseling, and flushing should take place after tasseling. Commercial fertilizer applications can be minimized or eliminated based on the measured amount of nitrogen applied to the field from the pond water.
November 5, 1999