Saturday, 15 July 2006

Distribution of Non-Exchangeable K in Soil Profile in a Brachiaria-Maize Cropping System.

Rodrigo A. Garcia, Juliano C. Calonego, Carlos A.C. Crusciol, and Ciro A. Rosolem. São Paulo State Univ, College of Agricultural Sciences, C.P.237, Botucatu, AL 18603-970, Brazil

Livestock-crop integration using a forage crop during the winter has been an interesting tool in managing tropical cropping systems under no-till. However, little is known about the nutrient dynamics in these systems, including K. Potassium from the cover crop residues is readily available with the first rains, in considerable amounts, but grasses require a large amount of K, mainly if two grasses, as maize and brachiaria, are grown together. Besides, when Nitrogen fertilizer is added to the system the grasses will respond with an increase in growth and eventually requiring more K. An experiment was conducted in Botucatu, São Paulo, Brazil, to study the distribution of non-exchangeable K under a no-till system using maize and brachiaria (Brachiaria brizanta) growing together and receiving 0.0 or 200 kg ha-1 of Nitrogen. In general, nitrogen fertilization increased K demand irrespectively from the cropping system, i.e., maize alone or maize plus brachiaria. By the time maize was planted, non-exchangeable K contents were lower in the soil profile down to 30 cm in plots where maize and brachiaria were grown together in the previous season as compared with plots where maize was grown alone. By the time maize was harvested, non-exchangeable K depletion in the soil profile was more evident than by the time maize was planted. This was particularly clear in plots receiving 200 kg ha-1 of N. This is an evidence that brachiaria can take up K from the non-exchangeable pool in the soil, a form that is usually considered non available for most crops. In plots receiving 200 kg ha-1 of N, brachiaria accumulated more K than in the 0.0 N plots and non-exchangeable K depletion in soil was higher as compared with plots without N. The high amount of K accumulated by the maize + brachiaria system, including some K from non-exchangeable forms in the soil, increases the amount of K in the straw residues left over the soil surface. This nutrient will be available for the crops to be grown in next season. With the first rains in the spring a considerable amount of this K will be washed down to the soil. Besides, residue decay will also make available some more K. This K will reach the soil as an ion, and will be incorporated into the exchangeable pool of the soil. So, in this system, besides K cycling avoiding leaching below the root zone, non-exchangeable K is made more readily available in the soil profile because it is transformed from non-exchangeable to exchangeable K.

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