Potassium balances in rainfed lowland rice on a light-textured soil

An experiment, over 3 years, studied the effects of mineral fertilizers and organic manures on potassium (K) balances in rainfed lowland rice on a light-textured Tropaqualf. Two rice crops were grown each year: the first direct seeded (DS) in moist soil that was later flooded by monsoon rain; the second transplanted (TP) into flooded soil towards the end of the wet season. A soybean crop followed the TP rice in the first year. In all the fertilizer and manure combinations studied, removal of K in the crop exceeded K additions and the soil K balance was negative unless crop residues were incorporated. For DS rice the relations between grain yield and K uptake fell within the limits of maximum K dilution and maximum K accumulation expected for well-managed irrigated rice. But those for TP rice tended to fall below the limit of maximum accumulation, yield being constrained by factors other than mineral nutrition, especially water deficit. In the DS rice, grain yields per unit K uptake were close to maximal in the treatments that received no K, but they were well below that in the K-fertilized treatments. Uptake was well correlated with exchangeable K in the soil at maximum tillering measured by ammonium acetate extraction. The mass balances of K inputs, K uptake and exchangeable K in the soil however, showed that a large part of the uptake was from non-exchangeable pools. The mobilization of non-exchangeable K was apparently plant-induced and was greater in treatments with greater growth. A mechanism for root-induced solubilization of non-exchangeable K, peculiar to rice growing in flooded soil, is proposed.