Résumé :
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A study of nitrogen and phosphorus losses from Decatur silt loam soil was conducted over three cropping periods from 1969-1972. Experimental agronomy plots at Alabama A. & M. University were seeded to cotton, corn, soybeans, and millet and compared with uncropped and unfertilized check plots. The effects of these crops on nitrogen losses total nitrogen, nitrate-nitrogen, and ammonium-nitrogen (for 1971-1972 only)- and phosphorus losses were evaluated. Average total rainfall and losses of sediment and runoff were also determined. Losses of total nitrogen and nitrate-nitrogen were generally positively correlated with the amounts of rainfall, runoff, and sediment. Phosphorus losses were positively correlated with sediment losses. More than 95% of phosphorus losses were associated with sediment loss. Corn was generally more effective in reducing total nitrogen and nitrate-nitrogen losses via surface runoff than cotton; whereas nitrate-nitrogen concentrations in subsurface water were lower in samples from cotton plots. The effect of crops on loss of all nutrients was considerably smaller than seasonal effects and sediment losses. The effect of two fertilizers, ammonium nitrate and sulfur-coated urea (a slow?release nitrogen fertilizer), upon nutrient losses showed no evidence of superiority for sulfur-coated urea in reducing the total nitrogen or nitrate-nitrogen losses.
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