Potassium dynamics and availability from composted and uncomposted rice straw

Managing rice straw is essential for a sustainable supply of K to be maintained with small amounts of inorganic K fertilizer. About 80% of the K taken up by rice (Oryza sativa L.) resides in the straw. To increase the utilization of the K from the rice straw, it is essential to know the K supplying characteristics and availability from this material. This information may also facilitate a better unerstanding of K cycling in rice-based cropping systems. Two experiments were conducted in the glasshouse, firstly, to study changes in the tensile load of rice straw during composting, and the possibility of using it as an indicator of the state of decomposition and the quantity of K released. Secondly, a pot experiment using a corn (Zea mays L.) variety PJ-58 as test crop on soil from the Bungor series (Typic Paleudult), was done to evaluate K availability from composted (CRS) and un-composted (UCRS) rice straw compared to Muriate of Potash (MOP) as the standard K fertilizer. Also to evaluate the use of tensile load of rice straw to predict K release from UCRS incorporated in soil, crop K uptake and leaf K concentration. The results from the composting experiment indicated that the individual relationship between the percentage pf organic matter remaining, the contents of total and water soluble K of decomposting rice straw with the tensile load of indicator rice straw were highly significant (p=0.001) with linear correlation coefficients of 0.97, -0.96 and -0.94, respectively. Therefore, tensile load of indicator rice straw can be used as an alternative indicator of the state of decomposition of rice straw and to predict K release. Application of MOP, CRS and UCRS to the soil increased K accumulation in the corn crop, the total and exchangeable K contents of the soil, but these were dependent on the growth period, the rate and availability of applied K. Compared to MOP and UCRS, the use of CRS was more beneficial in increasing the crop K, P and Ca uptake and resulted in better crop growth. Potassium was more readily available from CRS than UCRS and MOP. However, at 56 days after planting (tasseling stage) and an application rate of 360 mg K pot-1 (90 kg K ha-1), the crop available (uptake) K from the three fertilizer sources was comparable, and ranged from 90.48 to 109.25%. Based on the correlation between tensile load and K content of the decomposing rice straw, tensile load of indicator rice straw can also be used as an indicator to predict K release from UCRS incorporated into the soil. However, this was unsuitable for predicting K uptake and leaf K concentration in corn.