Changes in Soil P Availability as Affected by Plant Extracts.
Paulo S. Pavinato and Ciro A. Rosolem. College of Agricultural Sciences - São Paulo State Univ, Produção Vegetal Fazenda Experimental Lageado, CEP: 18603-970, Botucatu - SP, Brazil
Phosphorus (P) fractions in soils and P availability are affected by the kind and energy of the interactions with the soil solid phase (organic: diester-monoester; inorganic: Fe-P, Al-P, Ca-P, adsorbed). Some authors have classified P fractions in soil as available, less available and not available, but this classification is rather academical and does not reflect all the possible field situations. As to soil fertility, phosphorus is divided as easily or hardly available, with the assumption that all fractions can be available, depending on the time and specific reactions. The inorganic P is the fraction available in soil solution or interacting with the minerals of the soil, being less available when the interaction is stronger in energy, and vice-versa. On the other hand, the organic fractions of P have attracted more attention in the last years because of the growth of no till practices, and have been a way to maintain this nutrient available in the soil. This is possible when the soil is continuously cropped throughout the year and has always some kind of plant or residue on the surface. This work aimed at explaining the effects of soil surface plant decomposition in the availability of P to the following crops in field areas. The experiment was carried out in Botucatu, São Paulo State, Brazil, in an Oxisol with not high fertility index. Each plot was mounted using a column of PVC tube with 50 mm of diameter and 300 mm long, full of air-dried soil sieved through 2 mm screen. The plant extracts of black oat, millet, corn, radish, soybean and sorghum were obtained by shaking dry matter in water (1:25 w:v) for 1 hour, after being filtered and applied in the soil columns. The soil was maintained near the field capacity for all experimental time (7 days). After that, the columns were destroyed and soil samples were collected in the layers of 0-5, 5-10, 10-15 and 15-30 cm from the surface. P, K, Ca and Mg were determined in these soil samples. Soil P was fractionated following Hedley et al. (1982) with some adaptations. The data from P fractions in the surface layer (0-5 cm) showed that the application of plant extracts increased not only the inorganic available P (resin + NaHCO3) but also some fractions of less available P, like the inorganic P extracted by NaOH 0,1 M and 0,5 M. Soybean, radish and millet showed higher levels of available P in the surface layer. The organic fractions were not significantly affected by the application of plant extracts. These results seem to be related with the action of some organic acids present in the plant extracts that released P from fractions less available. However, the quantity of P present in the plant extract is responsible for part of the P increase in this layer. Deeper layers were not affected by the applications of plant extracts. The available potassium showed that all plant extracts increased this nutrient in all layers, but higher levels were obtained with radish. The availability of calcium was not significantly affected by the plant extracts. For available magnesium, the effect was detected only in the surface layer (0-5 cm), soybean and millet were the plants that showed more increase this nutrient in the soil. With the results obtained in this work it is possible to conclude that, plant residues are responsible for increasing or decreasing the availability of some nutrients in soils, and that the presence of some organic acids in plant tissue is the main way to explain the changes in availability of nutrients in soils, because these organic acids could act as ion exchangers.