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Fluid Journal : Fluid Journal 1999-2001
2 Fluid Journal Summer 1999 The more important question may be: how much manure is already being used along with commercial fertilizers? Nobody has an exact answer to that, but we do know it is a significant amount in many areas. For example, chicken litter has been applied to crop and pasture lands in Arkansas, North Carolina and other poultry-producing states, impacting commercial fertilizer use for years. Although dairy manure produced in the Central Valley of California has been applied primarily on cotton and forages (corn silage and small grains), it also has been used on vegetables, citrus, and other crops. Just how practical? One downside of manure that makes it questionable as a practical source for agricultural nutrients is the amount of valuable nutrients lost because of decomposition during storage. Nitrogen losses are greatest because of volatilization. During extended periods of storage, N losses are most common as NH3 volatilization and denitrification. About 50 to 70 percent of N excreted is urea-N, found in urine, most all of which is quickly volatilized. Estimates of field losses by volatilization of NH4-N from surface-applied manure range from 10 to 70 percent. Phosphorus losses occur as runoff or as settling in lagoons. Loss is generally less than that for N. Potassium. Significant K losses can occur when manure is stored outside and is rained on prior to field application. Manure does not need to be decomposed for the K to be leached out by rainfall. While these K losses pose no threat to the environment, they do reduce the value of the manure as a nutrient carrier for K. Figure 1 shows the low percentage of NPK remaining in manure from North Carolina livestock for crop use after accounting for all nutrient losses from storage and transport of manure to the field. Only about two thirds of the nutrient value of the manure produced is collectible, and less than one-third is available for crop growth. Dr. Darst is executive vice president and Dr. Fixen is senior rice president and North American program director for the Potash & Phosphate Institute.
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