Abstract
The release rates of Ca, Mg and K from six green manure (clotararia, pigeon pea, sesbania, guinia grass, sorghum and oats) and three composts prepared from poultry manure, cattle feces and sugar-cane leaves were measured. Green manure and composts wrapped in glass-fiber filter paper were placed in Ap horizon of a Dark Red soil and analyzed for Ca, Mg and K after 1, 3, 6 and 12 months. In addition, the chemical forms of Ca, Mg and K in the original organic materials were characterized by successive extraction with water, 1 tool L^<-1> NaCl, 2% acetic acid and 0.6 mol L^<-1>HCl. For all the materials, more than 90% of K and Mg was water soluble and water plus 1 mol L^<-1> NaCl soluble, respectively, whereas the water-soluble fraction was variable for Ca, ranging from 4 to 68%. The release rate followed the order of K>Mg>Ca for all the materials, reflecting the proportion of water-soluble fraction. For three bases, the release rate was higher for the green manure than for the composts. The amount of K released from the materials in the first 1 month was highly correlated with the water-soluble K, giving the regression of y=1.04χ-0.04 (r=0.99^<**>), where χ and y stand for the amounts of water-soluble K and released K, respectively. Similar relations with lower correlation coefficients were found for Mg, y=1.02χ+0.03 (r=0.84^<**>), and Ca, y-0.67χ+0.08 (r=0.78^*).
