Sorption of NO 3 by different horizons of a highly weathered, acid tropical soil was measured in laboratory batch experiments. Sorption was found to increase with depth, ranging from small amounts in the 0-15 cm layer to amounts that would be roughly equivalent to 25 to 50% of the NO 3 in the 90-120 cm layer at water and NO 3 contents commonly found under field conditions. Calculations, based on sorption isotherms, demonstrated how sorption may be important for managing N in a tropical acid soil. Sorption of CI-was also found in the range of 0.1 and 2.0molm -3. In this range of concentrations sorption of NO 3 and chloride were found to be independent, suggesting that anion exchange sites were far from saturated.
RESUMOUma das alternativas para diminuir o volume de lixo urbano a ser descartado é a compostagem, cujo produto -o composto -é utilizado na agricultura. Esses compostos podem apresentar substâncias tóxicas ao ser humano, a exemplo dos metais pesados. O objetivo deste trabalho foi avaliar as características químicas de compostos de lixo urbano de seis capitais brasileiras. Analisaram-se, também, amostras de esterco, calcário, superfosfato simples, KCl e uréia, para comparações com os compostos de lixo. Foram feitas determinações de M.O., C, N, P, K, Ca, Mg, Cu, Mn, Mo, Zn, Fe, Al, Ba, Cd, Co, Cr, Ni, Pb, Sr, Ti e V. As análises foram realizadas, em 1995, nos Laboratórios do Centro de Energia Nuclear na Agricultura -CENA/ USP. Houve diferenças nos teores de nutrientes entre os compostos, provavelmente devidas aos diferentes processos de compostagem, estando alguns deles acima dos níveis toleráveis em compostos na Alemanha. Os demais elementos analisados, incluindo os metais pesados, estão dentro da faixa observada em lodo de esgoto na Inglaterra. De acordo com a legislação alemã, o Pb, Cr, Ni e o Cd estão acima dos valores toleráveis para compostos de lixo. Na maioria dos compostos analisados, os teores totais de nutrientes foram menores do que no esterco, mas a concentração dos outros elementos, incluindo metais pesados, foi mais elevada do que no esterco. Embora os teores dos elementos avaliados nos compostos sejam os totais, é necessário cautela para seu uso, até que sejam obtidos resultados seguros sobre a "disponibilidade" desses elementos para as plantas.Termos de Indexação: análise de composto de lixo urbano, nutrientes em composto de lixo, metais pesados.
High rates of N'fertilizers are often nec
Acidity constraints in Oxisols of the Amazon may entail both Al toxicity and Ca deficiency. A field study was conducted in a Xanthic Hapludox near Manaus, Brazil, to evaluate lime and Ca requirements for corn (Zea mays L.), cowpea (Vigna unguiculata L.), soybean [Glycine max (L.) Merr.], and peanut (Arachis hypogea L.) during five consecutive years. Calcitic lime was applied once at rates of 0, 0.6, 1.2, 2.3, and 4.6 t of CaCO3 equivalent per hectare. The effects of 1 t ha−1 of gypsum, applied to four crops, were evaluated with lime rates of 0, 1.2 and 2.3 t ha−1. During the two initial years, maximum yields of corn and soybean occurred with 1.2 t of lime ha−1. In subsequent years, maximum corn yields occurred to 4.6 t of lime ha−1. Liming only increased cowpea yields in the third year. Peanut yields increased with liming to the highest lime rate in the fourth and fifth year. Gypsum increased yields for all crops to which it was applied. When averaged across time gypsum increased exchangeable Ca and reduced exchangeable Al by 0.41 and 0.20 cmolc kg−1, respectively. Increased peanut shelling percent and peanut and cowpea leaf Ca concentrations with lime, gypsum, or both were indicative of improved soil Ca supply. Maximum corn and soybean yields occurred at 27% Al saturation, whereas the critical level for peanut was 54%. Regressions of relative yield for all lime and gypsum treatments on the exchangeable Ca:Al ratio gave R2 values equal to those achieved with Al or Ca saturation. Critical Ca:Al ratio values would provide a useful index to ensure that lime recommendations based on Al saturation provide an adequate Ca supply.
Phosphorus soil test interpretations in the Brazilian Amazon currently do not account for differences in P requirements among crops and lack information on the changes in available soil P per unit of applied fertilizer P. A long‐term P experiment in a Xanthic Hapludox near Manaus, Brazil was used to determine soil and leaf P critical levels for corn (Zea mays L.) and cowpea (Vigna unguiculata L.). A total of six corn crops were planted in annual rotation with five crops of cowpea during five consecutive years. Critical levels were established by a segmented linear regression, a linear plateau, of relative crop yields on soil test or leaf P concentrations for each crop species. Mehlich 1 (1:10) critical P levels were 6 and 8 mg kg−1 for corn and cowpea, respectively. Relationships between soil test P and crop yields were similar for fertilizer P placement as either broadcast and/or frequent bands. Fertilizer P required to raise the initial Mehlich 1 soil P to the critical levels were 41 and 60 kg P ha−1 for corn and cowpea, respectively. Higher amounts of P were extracted by Bray 1 than by Mehlich 1, but both extractants were effective in relating available soil P to yield and applied fertilizer P. Critical foliar P concentrations for corn and cowpea were 1.6 and 1.8 g kg−1, respectively. Higher soil and leaf P critical levels for cowpea relative to corn were attributed to greater P requirements for plants depending on symbiotic N2 fixation for their N supply.
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