LONG-TERM EFFECTS OF FERTILIZERS AND MANURE APPLICATION ON THE FORMS AND AVAILABILITY OF SOIL-PHOSPHORUS

Long-term application of cattle manure and fertilizer can affect the forms and availability of soil phosphorus. This cumulative effect was evaluated on Le Bras silt loam (Humic Gleysol) cultivated with silage corn (Zea mays L.). In this long-term trial, treatments were arranged in a split-plot design, with dairy cattle manure applied at 0 and 20 Mg ha(-1) as the main factor. The subplots consisted of six fertilizer treatments (NK, PK, NP, NPK, NPKMg and the unfertilized check). Fertilizer rates for silage corn were 150, 100, 150 and 40 kg ha(-1) N, P2O5, K2O and Mg, respectively. The N fertilizer rate was reduced to 100 kg N ha(-1) in manured plots. Soil inorganic P (P-i) and organic P (P-o) fractions were sequentially extracted by resin, NaHCO3, NaOH, HCl and a final H2SO4 wet digestion of the residue. On average, labile P extracted by resin and NaHCO3 represented 17% of the total P (P-t); moderately labile NaOH-P-i and -P-o more than 40%; and stable P 36%. Application of manure and fertilizers increased significantly resin-, NaHCO3-, NaOH-P-i and P-t. However, NaOH-P-o was decreased by P fertilizer application in NPK and NPKMg treatments, while long-term manure application maintained this P-o pool in the soil. Stable P fractions were not affected by fertilization or by manuring. In all 6 yr of the study, P uptake by silage corn was significantly increased both by long-term N and P fertilizer application and also by manure incorporation. Phosphorus uptake by corn was highly related to all labile and moderately labile P-i fractions and P-t. Long-term application of dairy manure at a rate of 20 t ha(-1) increased soil P-i forms and maintained P-o fractions.