Rhizosphere effect on phosphorus availability to soybean in Vertisols

Abstract

ABSTRACT Vertisols of India are calcareous and have a heavy texture with high clay content but low to medium content of Olsen estimated phosphorus (P). The on-farm and on-station trials revealed that Vertisols with various levels of P2O5 responded negligibly to fertilizer application on soybean crops. In Vertisols, the inorganic P is primarily associated with Ca (Ca-P), which gradually solubilizes under acidic conditions surrounding rhizosphere. Hence, in order to correlate both the findings, the current study investigated the rhizosphere/rhizoplane pH. The study used agar plate technique to observe that the pH of the rhizosphere and rhizoplane soils, which was observed to be acidic although the soil pH was 7.6. The correlation between P uptake and soil-available P measured using the four test methods (Bray 2, Olsen, Truog, and Mehlich 3) revealed that Bray 2 had high correlation with the amount of P absorbed by soybean. Bray 2 method extracts approximately 10 times more P from Vertisols than does the Olsen extractant. This could be a plausible reason for insignificant difference in crop post fertilizer application due to sufficient P fertility of the Vertisols. Hence, Bray 2 method was established as a suitable method for the extraction of P from Vertisols, in which the pH of the extraction solution was the most acidic, among the selected four extraction methods and had high correlation with the amount of P absorbed by soybeans. Hence, this method is recommended for P estimation for Vertisols present in Central India. Essentially, this study provided information that an accurate measurement of P availability is required to explain the non-responsive nature of soybean to P fertilizers in Vertisols. Additionally, the current study emphasized the need for optimum recommendations of P from the view point of rhizosphere; it may facilitate the reduction of the application rates of P to the crops, without reducing the crop yields.