Author: Marsha Campbell Mathews, University of California Cooperative Extension Agronomist Nitrate nitrogen moves to the groundwater when water carries it past the root zone of the crop. If water doesn’t move past the roots, neither will the nitrate. So if it is possible to control the amount of water that is applied (e.g., sprinkler irrigation systems) to minimize movement past the roots, timing of manure applications is less critical and nitrogen that isn’t needed immediately by the crop can be “stockpiled” in the soil for short periods within the same season without danger of large losses to groundwater. In many cases such as flood irrigation, it is not possible to adjust the amount of water being applied to a field through the irrigation system. In addition, rainfall is also an uncontrolled source of leaching water. In these instances, leaching losses can be minimized by timing applications to coincide with or just precede crop uptake. This can be accomplished by injecting measured amounts of liquid manure into the irrigation system at rates and timings that match anticipated crop nitrogen in multiple smaller applications over the course of a season. When using this system, it is critical that the crop be supplied with sufficient nitrogen in an available form at all times. However, if total nitrogen from all sources, including organic form nitrogen, exceeds the amount removed in the crop, groundwater contamination may result.

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