ABSTRACT
Phosphorus (P) deficiency is one of the major limitations for crop production, particularly in low-input agricultural systems around the world. The problem is widespread for upland and lowland rice grown on acidic soils. In the tropics, the soils used for rice production are mostly Oxisols and Ultisols, which have naturally low levels of P and high immobilization capacity due to Al and Fe oxides (Fageria, Slaton, and Baligar 2003). Because of low natural phosphorus and high immobilization capacity, a heavy dose of P is needed for these soils to achieve high production (Yost et al. 1979; Fageria, Barbosa Filho, and Carvalho 1982). Phosphorus deficiency has been identified as one of the major limiting factors for rice production in highly weathered Oxisols and Ultisols worldwide (Sanchez and Salinas 1981; Fageria, Barbosa Filho, and Carvalho 1982; Haynes 1984; Fageria 2001). For example, in Brazil, upland rice is grown mostly on Oxisols and Ultisols in the centrally located Cerrado region, which makes up about 22% of Brazil’s total land area. These soils are acidic and have low fertility and low P (Lopes and Cox 1977; Fageria, Barbosa Filho, and Carvalho 1982). Brazil also has some 35 million ha of lowlands, known locally as Varzea, that represent one of the world’s largest agricultural lowland regions. However, at present, less than 2% of these lowlands are used for crop production. Lowland rice is the main crop grown during the rainy season, with other crops being grown during the dry season. Varzea soils are acidic, and P fixation is a major problem (Fageria et al. 1991, 1997; Fageria and Baligar 1996). Adequate P application rate is an important factor for rice production on these soils. Table 3.1 shows the influence of applied P on straw yield, grain yield, and panicle density for upland rice grown on central Brazilian Oxisol. All were significantly increased with the addition of P. Figures 3.1 and 3.2 show the dry matter yield of shoots and grain for 20 upland rice genotypes at 25 and 200 mg P kg−1 soil. There were significant differences among genotypes for shoot dry weight and grain yield. However, compared with a low P level, a higher P rate significantly increased shoot and grain yield, thereby indicating the importance of P fertilization for upland rice production.
