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 lig ht-textured Tropaqualf. Two rice Crops were grown each year: the first dire ct seeded (DS) in moist soil that was later flooded by monsoon rain; the se cond 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 fertiliz er and manure combinations studied, removal of K in the crop exceeded K add itions and the soil K balance was negative unless crop residues were incorp orated. For DS rice the relations between grain yield and K uptake fell wit hin the limits of maximum K dilution and maximum K accumulation expected fo r well-managed irrigated rice. But those for TP rice tended to fall below t he limit of maximum accumulation, yield being constrained by factors other than mineral nutrition, especially water deficit. In the DS rice, grain yie lds 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 m easured by ammonium acetate extraction. The mass balances of K inputs, K up take and exchangeable K in the soil however, showed that a large part of th e uptake was from non-exchangeable pools. The mobilization of non-exchangea ble K was apparently plant-induced and was greater in treatments with great er growth. A mechanism for root-induced solubilization of non-exchangeable K, peculiar to rice growing in flooded soil, is proposed. (C) 1999 Elsevier Science B.V. All rights reserved.