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Rates of decomposition and inorganic N release from composted manures should be known and predictable on the basis of their composition for their proper use as a source of available N. Four composted cattle manures were analyzed for their inorganic N, soluble organic C and N, and total N and organic matter contents. The composts were incubated in soil for 32 wk at 30°C and 60% water‐holding capacity. Inorganic N and CO2 evolution were determined periodically. Inorganic N released from the composts after 32 wk ranged from 11 to 29% of their total N content, 2 to 12% of total N were initially inorganic, and 1 to 5% were soluble organic N. The rates of decomposition of the composts were computed by the simulation model NCSOIL by minimizing the deviation of the model output from the periodically measured CO2 and inorganic N release and by using the soluble and insoluble organic C and N contents of the composts as input. The soluble components decomposed before the end of the first week. The decomposition rate constant obtained for the insoluble components of three composts was 4 to 5 × 10−4 d−1, whereas for one compost it was 1 × 10−8 d−1, indicating that this compost was much more stable. To obtain universal rate constants that will fit any compost, components of the insoluble material must be better defined.

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Simultaneous K exchange with Ca, Mg, and Na occurs in soils, but most of the published information is obtained from binary systems. The theoretical question of whether selectivity coefficients obtained in binary systems are valid for ternary systems has not yet been clarified; however, no significant difference has been found between selectivity coefficients in binary and ternary systems. Potassium exchange with Mg plus Ca was studied in two calcareous soils with variable concentrations of Na, using batch and miscible-displacement methods. Preference for K over the divalent cations was found in both soils, regardless of Na concentration and the experimental methods. Modified Gapon (k c) and Vanselow (ft v) selectivity coefficients were calculated for each cation pair in binary and ternary systems. In both soils, the k v and the k c decreased with increasing K fraction in the solution, but the k G value was less affected than the k v. A simple mathematical solution of the Gapon binary equations, K-(Mg + Ca) and Na-(Mg + Ca), in combination with an assumption of constant cation-exchange capacity (CEC), was used to predict the amount of exchangeable K as a function of solution composition in a ternary system. The high correlation of predicted exchangeable K with the directly measured values shows that binary selectivity coefficients can be used in multicationic systems on calcareous montmorillonitic soils. "DOTASSIUM EXCHANGE REACTIONS with each of the A cations Ca, Mg, or Na on clay minerals and soils have been investigated intensively (Bolt, 1979; Sparks and Huang, 1985). Irrigation water, especially sewage effluents and brackish waters, may contain considerable quantities of Na, K, Ca, and Mg. These cations undergo simultaneous exchange reactions with each other on soil surfaces. The possible reactions involving K-Ca-Mg, K-Na-Ca, and K-Na-Mg in soils and the study of their effect on K distribution between the solution and the solid phases is extremely important in plant nutrition. Theoretical understanding of the relationship between ternary and binary exchange systems in clays and soils has been enhanced in the last decade. Chemical and geochemical models were adapted to soils for predicting cation exchange in a ternary system using binary-exchange data (Elprince and Babcock, 1975; Elprince et al, 1980). Chu and Sposito (1981), using data obtained from experiments with pure clays and with careful analytical methods, found that the effect of ternary systems on binary-exchange coefficients was not greater than experimental error. In subsequent studies on ternary cation exchange on clay minerals and soils, Sposito and others (

The variation of nitrification rates among soils and within soil profiles has received little attention compared with the well‐established relations with soil moisture and temperature. In this study, the rate of nitrification was determined in profiles of various soil types and under different agricultural management. Verhulst's equation was used to express the accumulation of nitrate (NO‐3) with time and modified to describe the rate of ammonium (NH+4) decrease. The maximal rate of nitrification (Kmx) and the delay period (t′) were derived from the equation and used to characterize quantitatively the nitrification process in various soil samples. The Kmx and t′ of surface soil samples were from 5 to 70 mg kg−1 d−1 and from 0.2 to 8 d, respectively. The Kmx decreased and t′ increased with soil depth. Soil factors mostly affecting the rate parameters were: pH decreasing from 7.8 to 6.6 or HCO‐3 decreasing below 1 mol m−3, previous agricultural management, and soil depth. Data obtained from 15N studies indicated that mineralization rate of soil organic N in NH+4‐treated soils was not always negligible relative to nitrification.