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 f orms and availability of soil phosphorus. This cumulative effect was e valuated on Le Bras silt loam (Humic Gleysol) cultivated with silage c orn (Zea mays L.). In this long-term trial, treatments were arranged i n 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 t reatments (NK, PK, NP, NPK, NPKMg and the unfertilized check). Fertili zer 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 extr acted by resin and NaHCO3 represented 17% of the total P (P-t); modera tely labile NaOH-P-i and -P-o more than 40%; and stable P 36%. Applica tion of manure and fertilizers increased significantly resin-, NaHCO3- , NaOH-P-i and P-t. However, NaOH-P-o was decreased by P fertilizer ap plication in NPK and NPKMg treatments, while long-term manure applicat ion 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. Phospho rus uptake by corn was highly related to all labile and moderately lab ile 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 fracti ons.