With respect to nutrient management, timing is everything. If crops have access to nutrients when they are needed, quality and yields are higher. If, however, nutrients are supplied at times when crop need is low, then these nutrients pose a greater environmental risk, especially in regions with higher rainfall. Also, application when the soil is saturated is not recommended or allowed by most state regulations as it may lead to nutrient losses.
Crop growth rates and application conditions are not uniform throughout the year. There are periods during which crop demand for nutrients is very high and other times when nutrient uptake is minimal, and these periods may not be the same for different crops. All nutrient sources should be applied at times that will maximize crop use and minimize loss. Ideally, manure nutrients should be applied to an actively growing crop or within 30 days of planting a crop. If fall application of liquid manure is used to manage the farm work load or reduce soil compaction, then soil temperatures should be below 50 degrees to minimize leaching of nitrogen. If crops for human consumption are grown, manure should not be applied within three weeks of harvest. A cropping system with a variety of crops offers the most flexibility for manure application over many parts of the year. Adding a cover crop into some cropping systems can be a way to increase the application options. Under irrigated conditions, liquid manure nutrients can be injected into the irrigation water to supply nitrogen at rates and timings which closely match anticipated crop utilization.
Local weather conditions also impact the timing of manure application. Manure should not be applied to saturated soils, during rainfall events, or when the soil surface is frozen. Timing is most important for nutrients applied to soils with a high leaching or runoff potential. Applying nitrogen to a sandy soil when there is no crop to remove it will almost certainly result in loss of nitrogen to the shallow groundwater. Manure with the highest nitrogen content should be applied in the spring to take advantage of warmer temperatures and to coincide with the time of greatest crop need. You must also plan for significant wet weather periods if you live in an area that normally receives abundant rainfall in certain months.
Author: Mark Risse, University of Georgia
