Among the cropping systems commonly followed in the Indo-Gangetic plain of South Asia and in China, rice-wheat cropping system occupies more than 26 M ha of cultivated land and removes the highest amount of potassium. To a large percentage of area under rice-wheat cropping system, particularly in the Indo-Gangetic plains, very little or no potassium fertilizers are being applied and thus most of it comes from potassium reserves of the soil. Each harvest leaves the soil poorer with respect to potassium. Imbalance in the use of nitrogen, phosphorus and potassium is further creating situations, which may lead to reduced sustainability of the rice-wheat cropping system. Whereas illite is the dominant potassium bearing clay mineral in soils in the Indo-Gangetic plains, clay minerals in soils under ricewheat systemin China are at a more advanced stage of weathering than illite so that responses of both rice and wheat to applied potassium are substantial in China. Response of sequentially grown rice and wheat to applied potassium is influenced by time and method of application of different sources of potassium and interaction of potassium with other nutrients. Issues pertaining to sustainability of rice-wheat system have been examined in terms of potassium fertility of soils, mineralogy and forms of soil potassium, long-term potassium balances and changes in soil potassium. In spite of potassium incorporation through irrigation, crop residues and fertilizers, the occurrence of negative potassium balance in soils in the Indo-Gangetic plains has serious implications on mineralogy of potassium in soils in terms of advancement of weathering front in illite-vermiculite or illite-vermiculite-smectite phases. In China, most of the soils under rice-wheat systemare already in kaolinite and vermiculte-smectie phases and thus application of potassium leads to increased yields of both rice and wheat. Substantial potassium applications will have to be made to sustain high production levels of the rice-wheat cropping systems and to avoid further advancement of weathering front of potassium bearing minerals in the soil.
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