EFFECTS OF LIMING, GREEN MANURING, AND PHOSPHATE ADDITION ON ELECTROCHEMICAL ATTRIBUTES OF AN OXISOL FROM CENTRAL BRAZIL

Highly weathered tropical soils are characterized by having a predomin antly variable charge. Many management practices commonly used in the exploitation of these soils (e.g., liming, phosphate application, and manuring) are known to modify their electrical charge and the sorption /desorption behavior of cations and anions. This process is, at least, partially governed by the charges existing in the soil system. Availa ble information on this subject comes mainly from short-term laborator y and greenhouse experiments. There is a lack of data regarding the cu mulative and long-term effects of those practices used at farm-scale l evels and on the dynamics and availability of nutrients to the plants under field conditions. In the present work, changes in some electroch emical attributes of a variable charge soil (Oxisol) were studied, as influenced by treatments with phosphate + green manure (Cajanus cajan) , phosphate + lime, and phosphate + lime + green manure, applied durin g a six-year period. In this period, rice, bean, wheat, or corn, were grown in seventeen successive crops. Phosphate (total 334 ppm P) and p hosphate + lime (total 5.5 t ha(-1)) were shown to increase net electr ic charge and soil cation exchange capacity (CEC) at the field pH, and not to affect zero point of charge (ZPC), CEC at pH 7.0, or anion exc hange capacity (AEC) of the soil at the field pH. The effects of phosp hate + lime were more pronounced than those of phosphate alone. Green manure (total 16 t ha(-1) dry matter), associated to crop residues and phosphate or phosphate + lime, did not influence electrochemical prop erties.