Dynamic Studies on Nutrients Uptake by Crop Plants (Part 10) : Contact Solutional Uptake of Fused Magnesium Phosphate (P^<32>) by Acidoidal Plant Root and Unsaturated Soil Colloid

Abstract

In a series of the study on the nutrients uptake by crop plants, a dynamic Plant root-Fertilizer particle-Soil colloid relationship was observed in the uptake of phosphoric acid from the fused magnesium phosphate, a kind of hardly water soluble phosphoric fertilizers. Fused magnesium phosphate and superphosphate of lime labelled with P^<32> corresponding to 30 mg water soluble or 2% citric soluble P_2O_5 per beaker, were mixed with 170g of quartz sand and with 0,0.04,0.2,1.0,5.0,25.0g of acid soil (Mikatagahara mineral acid soil, the hydrolytic acidity y_1 of which was 22.5) respectively. In the third series labelled superphosphate was added with 71.3mg precipitated CaCO_3,equivalent to the alkalinity of the fused magnesium phosphate. In each beaker seeds of wheat, pop-corn, small bean and soy-bean were sown and seedlings were grown for 17 days under the condition of artificially controlled light and temperature. The tops were cut off close at the ground, ashed and the intensity of P^<32> radiation was measured by G-M-counter. The results are shown in Table 2 (a), (b). The outstanding difference between the two phosphates was, that by the hardly water soluble fused magnesium phosphate, the P^<32> absorbed by crop plant remarkably increased as the amount of acid soil and inceased up to a point where sudden decline set in, while by the water soluble superphosphate, only gradual decrease in P^<32> absorption occurred with increasing amount of added soil as expected. This latter trend could slightly be modified by the addtion of calcium carbonate, but was unsimilar to the fused manegsium phosphate, where the maximum peak of uptaken phosphoric acid was extraordinary high. The phenomena above mentioned were explained on the basis of a hypothetical consideration of the dynamic relationships among Plant root-Fertilizer particle-Soil colloid. A contact solutional uptake of phosphoric acid, i. e. a cationic contact exchange followed by solution and uptake of phosphoric acid by crop plants was proposed as schematically shown in Fig. 1. This hypothesis was supported by several model expriments in the laboratory as shown in Table 3,4 and a few further experiments not tabled.