Effects of liming, green manuring, and phosphate addition on electrochemical attributes of an Oxisol from central Brazil

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Abstract
Highly weathered tropical soils are characterized by having a predominantly 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. Available information on this subject comes mainly from short‐term laboratory and greenhouse experiments. There is a lack of data regarding the cumulative and long‐term effects of those practices used at farm‐scale levels and on the dynamics and availability of nutrients to the plants under field conditions. In the present work, changes in some electrochemical 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 during a six‐year period. In this period, rice, bean, wheat, or corn, were grown in seventeen successive crops. Phosphate (total 334 ppm P) and phosphate + lime (total 5.5 t ha‐1) were shown to increase net electric 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 exchange capacity (AEC) of the soil at the field pH. The effects of phosphate + 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 properties.