DinÃmica de nutrientes em solos arenosos adubados com esterco bovino por longos perÃodos / Dynamics of nutrients in sandy soils amended with cattle manure for long periods

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

Additions of farmyard manure (FYM) to the soil are a widely adopted alternative to supply nutrients, particularly nitrogen and phosphorus, in areas of subsistence agriculture in the semi-arid and agreste of Paraiba State. Additions of this residue in quantities that far exceed crop uptake, for long periods, can cause accumulation of nutrients in the soil. These accumulations occur initially on the surface layer, but on the onset of the rainy season, with most of the soil surface without plant cover, it can migrate to deeper layers and possibly be lost from the soil profile, especially in sandy soils. Although relevant from a socioeconomic and environmental perspective, information regarding these issues are almost nonexistent in the region. Therefore, the objective of this study was to quantify the accumulation of nutrients and the dynamics of phosphorus in sandy soils fertilized with FYM for long periods, on 22 areas of subsistence agriculture in the municipality of EsperanÃa/PB. Eighteen of these areas received annual additions of manure for at least two years while four of them were under pasture and had not been fertilized in the recent past. Soils samples from the 0-20, 20-40 and 40-60 cm layers were taken from each field, and analyzed for particle size, bulk density, pH, total C, N and P, exchangeable bases, water and Mehlich-1 extractable P. In addition, P in the samples was sequentially fractionated with water (Pw), resin (Pres), NaHCO3 (Pibic and Pobic), NaOH (Pihid and Pohid), H2SO4 (PÃcido), and the soil residue finally digested with H2SO4/H2O2 (Presd). FYM samples used in nine fields were also analyzed for total N, P, K, Ca and Mg, and additionally analyzed for P extractable with water, resin, Mehlich-1, H2SO4, and NaOH (batch extractions). Manure additions resulted in a build up of nutrient stocks, 2 Mg ha-1 for N and Ca, and 0,5 to 1 Mg ha-1 of P, K and Mg (0-60 cm). Water-soluble P concentrations of 7 mg kg-1 in the 40-60 cm layer, indicated losses of this nutrient, eventhough it is considered a nutrient of limited mobility in soils. Thus, accumulation of exchangeable bases in the 0-60 layer, in comparison with control areas, indicated a potential for even greater losses. The Pacid fraction, related to Ca-P forms, showed the greatest increases (P <0.01) as a result of FYM additions, raising from 8,4 mg kg-1 in non-fertilized samples to 43,8 mg kg-1 in samples showing greater Pt contents. The sum of Pw, Pr and Pibic, considered as labile P forms, showed comparable increases with Pacid, while Pihid showed the smaller increases. Concentrations of organic P forms also increased, more so the fraction considered less labile, Pohid, than the more labile one Pobic. The residual P fraction was practically half of Pt, independently of changes in total P content. Increases observed in the various fractions were justified by the large average concentrations of Pw (36%), Pacid (34%), and Pohid (30%) present in the FYM. Significant changes in the proportion of P forms among soil layers indicated the downward movement of organic P forms. The relative constancy of a high proportion of Presd (50-60%), independently of the Pt content in the samples, suggests the need to improve the fractionation of soil P. Increases in the labile fractions indicate the need of studies to optimize the rates of FYM additions actually being used, as well as to assess the residual effect of nutrients, and more specifically of the P-labile and Pacid fractions added with the FYM

ASSUNTO(S)

fracionamento de fÃsforo agricultura familiar lixiviaÃÃo de fÃsforo leaching of phosphorus leaching of nutrients migraÃÃo vertical de nutrientes subsistence agriculture organic amendments adubaÃÃo orgÃnica fertilidade do solo e adubacao fractionation of phosphorus

ACESSO AO ARTIGO

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