Effect of Long-Term Phosphate Fertilization on Cation Exchange Capacity (CEC) and Cation Movement in Variable-Charge Soils

Abstract

Soil profile samples collected in 10 and 15 cm increments to a depth of 60 cm from residual phosphate experiments on two soil families were used to investigate the effect of long-term P fertilization on CEC and cation movement in variable-charge soils. Increases in NH4 0Ac - CEC and ECEC of 0.37 and 0.16 me/100 g per 100 ppm of applied P, respectively, were observed in the 0-10 cm layer in the Hydric Dystrandept from Hawaii. In the Tropeptic Eutrustox at the Waialua and Waipio sites, however, where relatively low rates of phosphate were applied, increases in CEC were not statistically significant at the 10% level. Leaching of cationic nutrients was decreased as a result of increased CEC in the Hydric Dystrandept. The level of divalent cations extracted was in the order NH4 Cl > NH4 0Ac > EUF. For Kand Na the order was NH 4 0Ac > NH4 Cl > EUF. A single extraction with 1 N NH4 0Ac removed from 70% to 95% of the total amount of extracted divalent cations, suggesting that conventional extraction with 1 N NH4 0Ac does not give complete removal of divalent cations in these variable charge soils. The results further indicated that the first P increment was the most effective in increasing maize grain yield. The concentration of N, P, K, and Mg in maize grain increased with increasing rates of applied phosphorus. P application, however, had no effect on the concentration of Ca in maize grain. A balance sheet approach indicated that the amount of cationic nutrients unaccounted for decreased in the order K >Mg> Ca. Correlation studies suggest that the initial levels of Kin the soil determined by the NH 4 0Ac and the EUF111–K represent the form of K taken up by maize, whereas Ca and Mg extracted by NH40Ac and NH 4Cl were more closely correlated with Ca and Mg uptake than EUF extracted Ca and Mg. EUF – P was more closely related to P uptake than P found by the modified Truog method.