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Estimation of the pH of soybean rhizoplane, rhizosphere and bulk soil and its effect on availability and uptake of phosphorus in calcareous Vertisols

Vertisols are spread over central and western parts in Madhya Pradesh in India.As the Vertisolsare calcareous and/or alkaline in nature, mobility of P from soil to root surface is carried by diffusion process, and this diffusion rate is quite low i.e. 0.13mm day-1 (Jungk 1991). One of the major limitation is thatmany rhizosphere chemical interactions that can be involved in the changes ofP ion concentration in the soil solution and in the replenishment of the depleted soil solution (P buffering capacity)do not taken into account (Darrah, 1993).This prompted us to re-evaluate the P-fertility of Vertisols. In the study an attempt has been made to evaluate the most suitable method for P availability in calcareous Vertisols for crops considering the pH of rhizosphere. By agar plate technique, the pH of rhizoplane and rhizoplane soil was found acidic even though soil pH was7.6. The major portion of inorganic P in Vertisols is associated with Ca (Ca-P), which can be soluble more under acid condition than pH 8.5 of Olsen’s condition. The pH of bulk soil, that is unplanted soil which is treated in same way of applied nutrient and water as the planted pots, is 7.9. Soybean crop decreased the pH of rhizosphere and rhizoplane by 7.5and 6.0 respectively. Following the various crops the pH of rhizosphere decreased. Among various crops tested the lowest pH (5.8) of the rhizosphere and rhizoplane -attached soil was noticed in care of Chickpea. In case of pea, maize, sorghum and wheat the pH of rhizosphere and rhizoplane were 7.4 and 6.1, 7.6 and 6.4, 7.5 and 6.4, 7.5 and 6.3, respectively. Decreased pH due to rhizosphere can dissolve the phosphorus from the Calcium and increase the availability of P in Calcareous/ Alkaline soil.