NITROGEN AND OR PHOSPHORUS FERTILIZATION EFFECTS ON ORGANIC-CARBON AND MINERAL CONTENTS IN THE RHIZOSPHERE OF FIELD-GROWN SORGHUM/

The effects of nitrogen (N) and/or phosphorus (P) fertilizers on the n utritional status in the rhizosphere were studied by monitoring throug hout the growth period the concentrations of organic carbon (C), inorg anic N, NaHCO3 extractable P, exchangeable K, Ca, and Rig in sorghum ( Sorghum bicolor L. Moench) down in an Alfisol field, and of all these elements except for extractable P, and exchangeable Ca in a Vertisol f ield in semi-arid tropical India. These concentrations were compared b etween the rhizosphere soil and bulk soil of sorghum grown in both fie lds. Organic C content of the rhizosphere soil increased with plant ag e and was significantly higher than that in the bulk soil throughout t he growth of sorghum, but it was not affected by the rates of N or P f ertilizer. Inorganic N concentration in the rhizosphere soil was signi ficantly higher than that in the bulk soil until maturity in sorghum. The content of available P in the rhizosphere soil was significantly h igher than in the bulk soil after the middle of the growth stage. Its average concentration in the rhizosphere soil across growth stages was significantly higher than in the bulk soil, which contradicts the obs ervation in many reports that there is a depletion of P in the rhizosp here soil. The concentration of three exchangeable cations, K, Ca, and Mg, showed different patterns in the rhizosphere and the bulk soils. The concentration of K was almost constantly higher in the rhizosphere soil than in the bulk soil, Ca concentration was not different betwee n the two soils, and Mg concentration was significantly higher in the bulk soil than in the rhizosphere soil. The reasons for these discrepa ncies cannot be explained at present. The concentrations of these cati ons were not affected by the rate of N or P fertilizer except for Mg a t a later growth stage. The differences between rhizosphere and bulk s oils in Alfisol were similar to those in Vertisol with respect to the concentration of organic C, inorganic N, and exchangeable K and Mg.