Spatial relationships among different forms of soil nutrients in a paddy field

Within-field soil variability has often been reported in Japanese paddy fields. In one of such fields, we examined the relationships of spatial variations among different forms of soil nutrients and their direct influence on rice growth. Samples of surface soils collected from 100 plots (5 x 10 m) in the field were analyzed for the contents of N, P, and K as well as for the total C and clay contents. The values of the coefficient of variation (CV) for the soil properties were relatively high, ranging from 7% (total K) to 33% (inorganic N). More than 60% of the variation was spatially dependent, with ranges of less than 30 m for the contents of inorganic N and clay, 30-40 m for the contents of N (mineralizable and total), P (water-soluble, available, and total), and total C, and more than 60 m for the content of K (water-soluble, exchangeable, and nonexchangeable). These properties were grouped into two types; those significantly correlated either with the total C content (contents of mineralizable N, total N, all P forms, water-soluble K, exchangeable K, and 0.01 M HCl-extractable nonexchangeable K) or with the clay content (contents of fixed Nand NaBPh₄-extractable nonexchangeable K). In a pot experiment in which rice plants were grown in soil from each plot for 50 d, the dry matter weight of shoots exhibited a large variation (CV 12%) and increased significantly with the increase of the level of the components related to the total C content, especially the content of mineralizable N. Significantly positive correlations were also found between the contents of N, P, and K in shoots and the corresponding soil nutrients occurring in a moderately labile form. These soil-plant relationships suggest that site-specific application of organic fertilizers for altering the level of the components related to the total C content in the topsoil could be effective to homogenize rice growth in the paddy field examined.