Physiological Effect of Iron on Rice Plant : (I) Effect of iron in relation to soil reduction

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Abstract
To make clear the physiological effect of iron on growth and yield of rice plant, a variety, Norin 29, was grown by pot culture under twelve different treatments (table. 1). Two types, of soil, ie, paddy land soil (alluvial) and upland soil (delluvial) were used. To one half of total number of pots (60) containing each of these soils, a definite amount of filter paper was added in order to induce severe reductive state of soil, and to another half no filter paper was added. The former is referred to reductive soil and the latter control soil. Then iron was added in three different levels, 0, 8 and 16 gm. per pot to all of these four groups. thus giving twelve different plots. The results obtained are as follows; (1) The Eh value of soil was found in the order of upland control soil > paddy land control soil > upland reductive soil > paddy land reductive soil. Content of free iron in the soil was just the reverse of the order (fig. 1). (2) Growth (fig. 2) and yield (fig. 3) of the plants were in the same order of upland control soil > paddy land control soil > upland reductive soil. In the plot no. 5 and 6, where iron was applied to paddy land reductive soil, almost all plants were died due to the severe occurrence of "Akagare" (ferrous iron toxity). Lowering of yield due to iron application was more remarkable with the soil of lower Eh value, and the lowering of yield is attributable to the retardation of ripening process and, to less extent, the decrease in panicle numbers. (3) Nitrogen and potassium content (% to dry weight) in plant was rather increased by iron application. This is merely a direct result of growth retardation. But, phosphate content was significantly reduced by iron application, especially when the soil was more reductive (fig. 4). Dry weight of a plant and the total quantity of mineral nutrients absorbed by a plant were determined and expressed in percentage to these of the control plot without iron application as shown in fig. 5. The figure indicates that not only absorption but also translocation of phosphate in plant was retarded by iron application, particularly in the reductive soil. It was observed that absorption of silica and potassium is also influenced by the presence of excessive iron in soil.

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