Acumulação de nutrientes em solos arenosos adubados com esterco bovino

Abstract

Resumo -Os objetivos deste trabalho foram quantificar as concentrações de carbono e nutrientes em solos de áreas adubadas e não adubadas com esterco; quantificar as concentrações de nutrientes em amostras de esterco bovino utilizado na região e calcular o acúmulo de nutrientes resultantes dessa adubação e o potencial de perdas por lixiviação. Foram amostradas 18 áreas agrícolas, com adição anual de esterco por pelo menos dois anos e, como controle, quatro áreas sob pastagem não adubadas, coletando-se amostras de solo das camadas de 0-20, 20-40 e 40-60 cm, que foram analisadas quanto à granulometria, densidade do solo, pH, C, N e P totais, bases trocáveis, P extraível por água e por Mehlich-1. Amostras de esterco utilizadas em nove áreas também foram analisadas. A aplicação de esterco resultou em acumulações médias ao redor de 20 Mg ha -1 de C, 2 Mg ha -1 de N total e Ca, e de 0,5 a 1 Mg ha -1 de P total, K e Mg (0-60 cm). Acumulações de P solúvel em água e bases trocáveis na camada de 40-60 cm, em relação às testemunhas, indicam grande potencial de perda desses nutrientes.Termos para indexação: agricultura familiar, adubação orgânica, lixiviação de fósforo. Nutrient build up in sandy soils receiving manure additionsAbstract -The objective of this work was to quantify changes on carbon and nutrient contents in soils under continuous manure additions. Eighteen cropped fields that had received annual additions of manure for at least two years and four fields under pasture with no history of manure addition were sampled to assess if this practice results in the accumulation or losses of added nutrients. Soils samples from the 0-20, 20-40 and 40-60 cm layers were taken from each field, and analyzed with regard to their physical (particle size and soil density) and chemical (pH, total C, N and P, extractable P by Mehlich-1 and water, and exchangeable cations) properties. Manure samples used in nine fields were also analyzed for chemical composition. Manure additions resulted in a build up of nutrient stocks near to 20 Mg ha -1 of C, 2 Mg ha -1 of 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, the less mobile in soils. Accumulation of exchangeable cations in the 0-60 cm layer, in comparison with control areas, indicates an even greater potential for losses of these nutrients.

Soil phosphorus fractions in sandy soils amended with cattle manure for long periods

“…This association is justified by the high mean Ca concentration, 14 g kg -1 , in dry FYM (Table 1); in fact, FYM applications in the most fertilized soils elevated the total Ca content almost 10-fold over the unfertilized soils (in Figure 2a the P t values in unfertilized samples are < 90 mg kg -1 ). The increases would have been even higher if it were not for the quadratic effect of the data (Figure 2a), probably caused by the higher leaching of Ca than of P (Galvão et al, 2008).…”

“…This rate was practically 0.1 mg of P res for each mg of increase in P t , that is, 10 % of the P added via FYM was resin-extractable, independently of the P t level. This was unexpected, since, in the soil samples, P res did not include the P w concentration, and these soils are dominated by the sand fraction (Galvão et al, 2008), with low values of labile P in the solid phase. In the case of P w and P bic this rate of increase was approximately 5 %.…”

“…Equal quantities of weighed soil from each simple sample were mixed and homogenized to form a bulk sample for physical and chemical analyses. A characterization of these bulked samples was published by Galvão et al (2008). Summarizing, samples from the control areas (0-20 cm) contained 839, 112 and 49 g kg -1 sand, silt and clay, respectively, pH (H 2 O), 5.5; 3.9 g kg -1 of total organic C (TOC), 86 mg kg -1 of total P, 3.8 mg kg -1 and 63 mg kg -1 of Mehlich-1 extractable P and K, respectively, and 0.55 and 0.18 cmol c kg -1 of Ca and Mg, respectively.…”

“…The FYM used in the cited region is very heterogeneous and Galvão et al (2008) determined some of its properties. Further description of P in this material was sought to justify the variations in the P fractions of the soil; however, due to various methodological difficulties, it was not possible to adapt the sequential extraction used for the soil samples to the manure samples.…”

“…As the N/P ratio of manure is generally lower than the N/P ratio of plant uptake, applications exceed the crop needs, resulting in soil P accumulation P and potential loss. Data regarding this undesirable effect in low input family agriculture in the semi-arid region of Brazil have recently been published (Galvão et al, 2008;Menezes & Oliveira, 2008). Data on P accumulation (Sharpley et al, 2004) and P losses through leaching (Koopmans et al, 2007) related to animal manure applications have also been reported in other countries (Siddique & Robinson, 2004;Anderson & Magdoff, 2005).…”