ABSTRACT
Potassium (K), which limits plant growth and development, is the third most important nutrient after nitrogen and phosphorus. In most of the rice-growing soils, the response of rice to potassium fertilization is not as high as it is to nitrogen and phosphorus (Fageria, Slaton, and Baligar 2003). Figure 4.1 shows the response of upland rice grown on Brazilian Oxisol to nitrogen, phosphorus, and potassium fertilization. Rice growth was most affected by phosphorus, followed by nitrogen and potassium. Therefore, the response of rice to N, P, and K was in the order of phosphorus > nitrogen > potassium. The reason for the low response may be related to the availability of nonexchangeable K when the K level falls below the critical limit. Many soils used for the continuous production of rice or for rice-wheat rotations can be cropped for extended periods without needing supplemental K to maintain production (Dobermann et al. 1996). Rice grown in rotation with legumes, such as soybeans, may require annual K input due to the greater K requirement of the legume crop (Fageria, Stone, and Santos 2003).
