Water-soluble cations in shoots of annual crops as affected by lime and phosphogypsum surface application

Surface liming efficiency can be increased through water-soluble organic compounds released by plant residues. However, it is unknown if the contents of these compounds in crop residues can be modified by lime and phosphogypsum surface applications. The objective of this study was to evaluate effects of lime and phosphogypsum surface applications on the content of water-soluble cations in crop residues of rice, common bean and black oat. The experiment was carried out on a Haplorthox located in Botucatu County, Sao Paulo State, Brazil. A randomized complete block design, in a split-plot scheme and four replications was used. The plots consisted of four dolomitic limestone levels (0, 1, 100; 21700; and 4,300 kg ha(-1)). The subplots were represented by the presence or absence of phosphogypsum application (2,100 kg ha(-1)). For the summer crops a split-split-plot scheme was used. The subsubplots were constituted by two upland rice cultivars (Caiapo and IAC 202), in the growing season of 2002103, and two common bean cultivars (Carioca and Perola), in the 2003/2004 growing season. In the winter season of 2003 and 2004, black oat was grown on the entire area, using only one cultivar. The water-soluble cation content in rice shoots, common bean and black oat crops were affected by lime and phosphogypsum surface applications. Surface application of phosphogypsum increased Ca and decreased Mg water-soluble contents in crop shoots, mainly in the first years of cultivation. Surface liming increased the water-soluble cation contents in all crop shoots. The highest water-soluble cation contents in shoot residues were measured in common bean and black oat during flowering.