The relationships between soil N mineralization, soil water content, and matric suction were studied with nine soils of widely differing chemical and physical properties. Highest N mineralization rates occurred between matric suctions of ⅓ to 0.1 bar, in which range 80 to 90% of the total pore space was filled with water. In the range from optimum soil water content (⅓ to 0.1 bar) to 15 bars, a near‐linear relation generally existed between amounts of mineral N accumulated and soil water contents (percent of oven‐dry soil). With increasing dryness, N mineralization continued to decline. Water levels above optimum often reduced mineral N accumulations, presumably because of denitrification.Upon expressing the values for N mineralization (Y) and soil water content (X) for each soil on a relative basis with respect to maximum N mineralized (Y = 100) and associated optimum soil water content (X = 100), the regression of Y on X did not differ among soils. Regression coefficients for common (based on covariance) and total regressions, respectively, were 1.07 and 1.02. Corresponding Y‐intercepts were −2 and −4. A reasonable approximation of the relationships is expressed by Y = X. Possible application of the findings in predicting N mineralization under fluctuating soil water conditions is discussed.
The effects of different application rates of sewage sludge and sludge compost (0, 40, 80, 120, and 240 metric tons/ha) on soil physical and chemical properties were studied at two pH levels.Sludge application rates exceeding 120 metric tons/ha were difficult to apply and produced a cloddy seedbed, while the same rate of compost was easily applied and produced a friable seedbed. Sludge and compost increased the water content and the water retention of a silt loam soil. Both the sludge and compost increased the salinity and chloride levels of the soil to a level which may affect salt‐sensitive plants. The soil's cation exchange capacity increased as much as threefold as a result of the addition of sludge and compost. Nitrate‐nitrogen levels were highest at the 15–20 cm soil depth but decreased sharply below this level. Available phosphorus was high during the 2‐year study and appeared to be in excess of that needed for good crop growth.
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