Monitoring of phosphorus levels in soils with long-term application of swine manure

Resumo

Continuous applications of liquid swine manure (LSM) in soil can result in high phosphorus (P) contents, especially in surface layers. Excess amounts of this nutrient can be transferred from soil to surface water and contaminate aquatic systems, causing eutrophication problems. To avoid such problems, the objective was to evaluate the environmental critical limit of phosphorus (ECL-P) of the soil of 89 farms with long-term LSM fertilization in Rio Verde-GO, Brazil. In the first stage, the clay and available P contents in the soils were surveyed through the results of soil analysis of pig farms in the region with a history of LSM application for 16 years. Then, the soils were classified according to the P level (very low, low, medium, adequate and high) according to the clay contents. In the second part, ECL-P was applied to the farms. In 32 soils, the P level was considered high, more than 12 mg dm^-3. At this level, it was observed that 14 soils exceeded the ECL-P, of which 12 suggested the immediate suspension of P application, as there is a high risk of P losses to water. Only in two farms was the ECL-P exceeded by less than 20 %; therefore, it is recommended to reduce the application of P sources and the use of techniques to reduce the levels of P in these soils. Although beneficial, organic fertilization can exceed adequate limits, which reinforces the need to monitor areas with long periods of fertilization with LSM.