Establishing environmental soil phosphorus thresholds to mitigate its transfer to water bodies in Mato Grosso State, Brazil

Abstract

Excessive phosphorus (P) applications can increase nutrient levels in the soil, facilitating its transference to aquatic environments and causing contamination. Thus, the environmental P threshold (P-threshold) is a tool to establish a sound level of P in the soil, in which P values below the threshold are harmless to the environment. This study aimed to establish a P-threshold equation for the soils of Mato Grosso State, Brazil. Twenty samples of representative soils from the main swine production regions of the State were collected. These samples were characterized chemically and in terms of P adsorption capacity, then incubated with P rates for 30 days. After incubation, the water-soluble P content (P-water) and the available P content were determined by Mehlich-1 method (P-Mehlich-1). The relationship between P-water and P-Mehlich-1 allowed the determination of the P-Mehlich-1 content from which the amount of nutrient in water increases abruptly, establishing this point as the limit P (P-limit). Subsequently, the P-limit values were plotted against the clay content of the soils, and the resulting equation was used to calculate the P-threshold by adding a safety margin of 20 % to the P-limit versus clay equation. The model was tested on 120 soil samples from Mato Grosso State, and it was able to separate them into two groups with low and high P-water, classified as below or above P-threshold, respectively. Based on that, the simplified equation “P-threshold (mg dm -3 ) = 13 + 0.5 * % clay” was proposed.