Nutrient effects of modification of shifting cultivation in west Africa

Recycling of plant residues, fertilizer use and inclusion of trees and perennials in crop fields are key components in systems for improved food crop production on infertile soils in the forest regions of West Africa. 2 On Alfisols of pH 5.5 or higher in the forest and forest-savanna transitional zones, systems including food crop-cover crop rotation, minimum tillage and the judicial use of fertilizer can sustain yields on small farms for 10 years or more without reverting land to bush fallow. Inter-planting leguminous trees such as Leucaena leucocephala with annual food crops is shown to improve food crop yields. The leguminous trees fix atmospheric nitrogen, recycle mineral nutrients from the subsoil, and prevent soil erosion and run-off water on sloping land. 3 On strongly leached acid soils (i.e. Ultisols and Oxisols) in high-rainfall regions, nutrient cycling and green manuring are less effective because of subsoil infertility. Intensive food crop production on such soils requires costly inputs of fertilizer and lime. However, experiments have shown that periodic application of small dosages of lime can sustain a moderate crop yield. Gradual saturation of subsoil horizons with calcium is a prerequisite for effective recycling of nutrients in agricultural systems in the high-rainfall region

Resíduos vegetais na superfície do solo afetam a acidez do solo e a eficiência do herbicida flumetsulam Soil surface residues affect soil acidity and flumetsulam herbicid efficiency

Com o objetivo de avaliar o potencial de resíduos de duas espécies vegetais em alterar a acidez do solo e a atividade do herbicida flumetsulam, foi conduzido um experimento em casa de vegetação no Departamento de Solos da Universidade Federal do Rio Grande do Sul, em novembro de 1998. Amostras da camada superficial (0-20cm) de um cambissolo húmico distrófico foram colocadas em vasos, com capacidade de 1,0kg, onde se aplicou na superfície: 0, 5 e 10t ha-1 de resíduos de aveia-preta e de ervilhaca comum, com e sem aplicação do herbicida. Utilizou-se aveia-branca como cultura reagente. O delineamento experimental foi o inteiramente casualizado com três repetições. O resíduo da ervilhaca aumentou o pH do solo e ambos os resíduos diminuíram o alumínio trocável na camada de 0-3cm de profundidade, sendo o efeito da ervilhaca mais pronunciado. O decréscimo do alumínio trocável foi relacionado a dois mecanismos: complexação pela matéria orgânica e aumento do pH. Os resíduos vegetais aplicados sobre a superfície do solo aumentaram a eficiência do herbicida flumetsulam, especialmente quando o herbicida foi aplicado sobre os resíduos da leguminosa, que teve maior efeito na elevação do pH do solo.<br>An experiment was conducted in the greenhouse of the Soils Department of the Federal University of Rio Grande do Sul, in November of 1998, to evaluate the potential of two crop residues to modify the acidity of the soil and the activity of the flumetsulam herbicide. The experimental units consisted of pots containing an Haplumbrept soil from the surface layer (0-20cm) in with it was applyed 0, 5 and 10t ha-1 of black oat and of common vicia residues, with and without the herbicide application in the soil surface, and arranged in a complete randomized design, with three replications. White oat was used as test culture. Vicia residues increase soil pH and both residues decreased exchangeable Al in the 0-3cm layer, with the effect of vicia being greater. The decrease of the exchangeable aluminum was related to both mechanisms: organic complexation and soil pH increase. Soil surface crop residues increased the herbicide efficiency, specially when this was applied over vicia residues, wich had greater increased in the soil pH

Adsorção de fósforo em solos de argila de atividade baixa Phosphate sorption in low activity clay soils

Neste estudo foram avaliadas possíveis correlações entre atributos físicos e químicos do solo e a capacidade máxima de adsorção de fósforo (CMAP) em solos com argila de atividade baixa de diferentes regiões do Brasil, sendo utilizadas amostras de horizontes superficiais (A) e subsuperficiais (B) de 16 perfis de solos. As doses para a determinação da CMAP foram calculadas com base no teor de fósforo remanescente e variaram de 0 a 135 mg.L-1. A partir dos dados obtidos foram então construídas isotermas de adsorção de Langmuir. Foram feitas correlações entre CMAP e os teores de argila, de carbono orgânico, ki, kr, superfície específica, teor de alumínio determinado por ataque sulfúrico e teores das diferentes formas de ferro extraível. Foi observada grande variação nos valores de CMAP para o horizonte superficial (48 a 1429 mg.kg-1) e para o horizonte subsuperficial (455 e 1.667 mg.kg-1). Os coeficientes de correlação demonstraram haver uma associação significativa entre a CMAP e o teor de argila dos solos, ferro e alumínio, determinados pelo ataque sulfúrico e ferro extraído com ditionito-citrato-bicarbonato (DCB). A análise de regressão múltipla mostrou bom ajuste aos valores da CMAP com base em atributos dos solos estudados.<br>In this study possible correlations between physical and chemical properties and the maximum phosphorus adsorption capacity (MAPC) were evaluated in soils with low activity clay from different regions of Brazil, using samples from the surface (A) and subsurface (B) horizons of 16 soil profiles. The dosages for the determination of MAPC were calculated from the level of remaining phosphorus and they ranged from 0 to 135 mg L-1. From this data isotherms of P adsorption were built. Correlations were tested between MAPC and the clay content, organic carbon, ki, kr, specific surface, Al content determined by sulfuric acid attack, and content of different forms of extracted iron. A high variation in the values of MAPC for the surface horizon (48 to 1429 mg kg-1) and for the subsurface horizon (455 to 1667 mg kg-1) was observed. The correlation coefficients showed a significant association between MAPC and clay content, iron and aluminium determined by sulfuric acid attack and iron extracted by dithionite-citrate-bicarbonate (DCB). The multiple regression analysis showed a good fit to MAPC values based in soil attributes