Plant uptake of legacy phosphorus from soils without P fertilization

The purpose of this study was to evaluate how the different phosphorus fractions in soils contribute to plant uptake, based on the combination between the sequential P fractionation developed by Hedley and path analysis. Successive crops were grown on soil samples with different P legacies, taken from the 0.00–0.10 m layer of an untilled Latossolo Vermelho (Rhodic Hapludox). The experimental treatments consisted of three annual P rates (0 kg P ha −1 –P0; 157 kg P ha −1 –P157; 314 kg P ha −1 –P314). In a greenhouse, 15 successive crops of millet, black oat, corn, soybean, crotalaria and rye were grown in pots, with six replications. Phosphorus fractions in the soil were evaluated before the first crop and after the third and 15th crop. At the beginning of cultivation, the labile P fractions were buffered by the less labile P fractions in soils fertilized with the highest P rate (P314). After the 15 crop cycles, labile P was buffered, mainly by labile and non-labile organic P fractions. In soils without previous P fertilization and intermediate fertilizer rates, there was a significant contribution of organic P fractions to the moderately plant-available inorganic P pool. The organic P fraction only contributed to plant nutrition in soils fertilized with high P rates after depletion of the inorganic P pool. The conclusion was drawn that all soil P fractions can potentially provide plants with P, whereas the contribution of these P fractions depends on the P fertilization history.