Secondary effects of potassium and nitrogen nutrition of rice: Change in microbial activity and iron reduction in the rhizosphere

Abstract

In solution culture experiments with rice the effects of different potassium applications and of various nitrogen sources on some criteria of microbial activity were studied. The following results have been obtained: The number of bacteria in the rhizosphere depends on the form of nitrogen available to the plants and the potassium nutritional state. With ammonium as a nitrogen source a greater number of bacteria is found in the rhizosphere than with nitrate. The interruption of potassium nutrition results in an increase in the total number of bacteria. In accordance with the different bacterial colonisation, the oxygen content in the nutrient solution with nitrate remains higher than with ammonium nutrition. With an increase in bacterial number after K interruption, a corresponding depression of the oxygen content in the nutrient solution can be noted. This is due to the higher respiration rate of the roots and the bacteria adhering to them in K-deficient plants. K deficiency consequently results in an increased concentration of reduced divalent iron in the nutrient solution. From the above results it follows that optimum potassium nutrition makes an essential contribution towards counteracting highly reducing conditions in paddy soils.