NUTRIENT STATUS OF RHIZOSPHERE AND PHOSPHORUS RESPONSE OF RADISH

Radish (Raphanus sativus L.) exhibits a high efficiency in the utiliza tion of sparingly-soluble phosphates. A greenhouse experiment was desi gned to investigate the growth response of radish to different phospho rus (P) sources and the nutrient status of the rhizosphere associated with radish growth and nutrient absorption. Radish plants were grown i n pots with the roots confined in rhizobags, in such a manner that the concentration of roots was very high within the rhizobag. The rhizosp here soils and non-rhizosphere soils were analyzed separately for acti ve silicon (Si), aluminum (Al), iron (Fe), and manganese (Mn) using Ta mm's solution and for ''available'' P using the Bray P1 extraction rea gent. The radish growth response was mostly attributable to phosphate amount and availability, and the lime level used in the experiment. Co ncentrations of active Fe, Si, Al, and Mn were reduced in the rhizosph ere, especially when lime and rock phosphate (Ps) were added. Availabl e soil P was accumulated in the rhizosphere under lime and Ps addition , whereas its concentration was reduced with the zero lime treatment. Phosphorus utilization, characterized by P accumulation in shoots, was in accordance with the concentration pattern for ''available'' P in t he rhizosphere, bur not with the growth response of radish itself. The calcium (Ca) concentration of the shoot followed the same trend as th e radish growth. There was an antagonism between potassium (K) and Ca absorption as well as between Ca and magnesium (Mg) absorption. With t he addition of P, shoot Mn concentration increased, while shoot Fe and Al concentrations increased with no lime addition but decreased with lime addition. The high P efficiency of radish is discussed from the v iew of rhizosphere chemistry. The high Mn efficiency of radish may be influenced by the same rhizosphere processes that are involved in its high P efficiency. It was concluded that rhizosphere processes and the status of nutrients determined the nutrient efficiency of radish and thus influenced its growth response and nutrient uptake.