Rev. Bras. Ciênc. Solo.2020;44:e0200018.
Establishing environmental soil phosphorus thresholds to decrease the risk of losses to water in soils from Rio Grande do Sul, Brazil
Luciano Colpo Gatiboni 
, Rodrigo da Silveira Nicoloso , Gilmar Luiz Mumbach , Abelino Anacleto de Souza Junior , Daniel João Dall’Orsoletta , Djalma Eugênio Schmitt , Thomas Jot Smyth
17/Aug/2020
DOI: 10.36783/18069657rbcs20200018
Graphical Abstract
Highlights
Phosphorus fertilizer and manure overloading increases soil phosphorus and its potential losses to water bodies.
Regional soil P-thresholds indicate safe soil P levels and fertilizer recommendations.
Clayed soils have higher P retention capacity with lower potential for P losses.
The simplified equation “P-threshold = 20 + clay” is recommended for soils from the state of Rio Grande do Sul.
P levels in sandy soils should be restricted to critical agronomic level with complementary conservation practices
ABSTRACT
The overloading of soil with manure or mineral fertilizers enhances phosphorus (P) availability, promoting its loss to water bodies and increasing the risk of eutrophication. In this sense, the establishment of an environmental P threshold is a simple and useful tool to classify soils regards its risk of P losses. Here we propose a P-threshold for soils from the state of Rio Grande do Sul (RS) in Brazil, with the soil clay content as the principal variable of the model. Samples from 20 representative soils of Rio Grande do Sul State were collected in the 0.00-0.10 and 0.10-0.20 m layers. Samples were physically and chemically characterized and incubated with P rates varying from 0 to 100 % of its maximum adsorption capacity. After 20 days of incubation, the P availability was determined in water and Mehlich-1 extractant. We assessed trough segmented linear functions the value of P-Mehlich in which the levels of P water enhance abruptly (i.e., breaking point) to determine the P limit for each soil. Subsequently, the P limit values were linked to the clay content in the sample, adopting the linear equation arising from it as the P-threshold. Adding a safety margin of 20 % to the P-threshold equation, we propose a simplified one “P-threshold = 20 + clay”. Based on the current critical level of P adopted by the regional recommendation in RG, the equation proposed does not limit the production of the most crop species.
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