Chapter 9

Manure Management on Organic Farms

Timing of Manure Application

Timing of manure application may affect soil nitrogen content, crop nitrogen use efficiency, and crop performance. The best time to apply (low C:N ratio) manures is about one or two weeks prior to planting the crop. This lead time allows for some decomposition of the manure and lessens the possibilities of plant injuries that often occur with freshly applied manures. This lead-time with manure allows for any ammonium that is present in the manure or that is released from the manure after application to be oxidized to nitrates with minimal volatilization. A two-week period of decomposition in the soil is not sufficient for manures with C:N ratios greater than 30:1. If the C:N ratio is high the breakdown of organic material is more likely to temporarily tie up some of the otherwise available nitrogen (process termed immobilization) in the soil, thus creating some short-term nitrogen imbalances.

Spring Manure Application

Manure applied in the spring has less time for organic-N mineralization before crop uptake, especially in the case of manure with a high C:N ratio. If the C:N ratio is too high the rapid breakdown of organic material in the spring is more likely to temporarily tie up some of the otherwise available nitrogen in the soil, creating some short-term nitrogen imbalances.

Fall Manure Application

While spring application of manure provided the best agronomic response in most instances, manures are often applied to the soils in the fall, possibly because of greater availability, lower cost, more time to apply, and busy spring planting schedules for farmers. Fall is a good time to apply a manure with a high C:N ratio (e.g., manure from bedded systems) because it allows more time for organic-N portion of manure to mineralize for plant uptake in the next crop season.

Winter Manure Application

Applying manure in the winter can be risky. Frozen ground that is covered with ice that can quickly melt lends itself more readily to surface runoff, creating a potential for contamination of local waterways. Some states have rules, and others are considering new guidelines, to regulate winter manure application in order to best protect area natural resources and ensure that farmers are maintaining the nutrient value provided by manure application.

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