Long-term phosphorus fertilization of wheat, soybean and maize on Mollisols: Soil test trends, critical levels and balances

Abstract

Few reports have compared the P critical level between different crops under equivalent growing conditions and the impact of P balance and P fertilization practices on the long term dynamics of soil available P. The objectives of this study were: i) to determine and compare, under similar field conditions, the P critical values for soybean, maize and wheat; and ii) to evaluate the effect of long-term application of P fertilizer on P balance and soil-test P. Results from a long-term experiment (2000/01 and 2013/14) involving soybean, maize and wheat crops in five experimental sites located at the Pampean Region (Argentina) were analyzed. Phosphorus levels included a −P treatment without P application and a +P treatment with continuous P fertilization (annual average 37 kg P ha−1). The critical Bray-P thresholds were 14.3, 12.5 and 19 mg kg−1 for soybean, maize and wheat, respectively. The rate of decline of the Bray-P pool in the −P treatments was described by an exponential decay function common to the five study sites. Obtained results indicated that a net extraction of 327 kg P per hectare is needed to reduce their initial Bray-P values ​​by half, regardless of the initial soil Bray-P value. The soils fertilized with P showed a significant and linear increase in Bray-P. It was possible to fit a single function after pooling the data of the five sites. This combined function indicated that 3.2 kg P ha−1 were necessary to increase Bray-P in 1 mg kg−1. Obtained data on crop P critical levels and rates at which soil-test P declines or increases according to the P balance constitutes a useful tool for sustainable use of P resources in Mollisols and related soil units. They can help to monitor future changes of soil P levels and to estimate the P demand of croplands.