Fractionation of soil phosphorus in organic amended farms located on savanna sandy soils of Venezuelan Amazonian

摘要

In Venezuelan Amazonian, some producers have established small agroforestry systems of production on sandy savanna soils by a long-term addition (more than 25 years) of animal manures at a low dressing (2 Mg ha-1) as fertilizer input. As a result of the organic fertilizers regime, the original savanna soil has been changed in terms of soil quality parameters. The main objective of the study was to investigate using sequential fractionation of soil P the impact of organic manures on the amount and partitioning of bioavailable P in soils of the Amazonas. Fractionation was carried out on Typic Ustipsamments amended with three different organic manure sources for extended periods. In general, after fertilization, all Pi and Po fractions increased significantly. The increase was striking in the resin-Pi and HCl-Pi, and among the organic P fractions, the changes were highest for the NaOH-Po sonicated and non-sonicated fractions. The total P increment was more relevant when soils were amended with chicken manure (1,194 mg Pt kg-1) and less relevant for the farm soil treated with compost (500 mg Pt kg-1), where the soil amended with cattle waste presented an intermediate value in total soil P (851 mg Pt kg-1). The importance of this field study was to assess the sustainability of long-term established organic management characterized by the low inputs, and this information is poor in the Amazonas..