Importance of contact between rice roots and co-situs applied fertilizer granules on iron absorption by paddy rice in a calcareous paddy soil

Abstract

The term “co-situs application” refers to a technology that uses controlled-release fertilizer (CRF) attached with the seeds or seedlings as the single application for the entire growing season without causing salt injury. Using a co-situs application technique, a pot experiment was carried out to evaluate the importance of contact between rice roots and co-situs applied fertilizer granules on Fe absorption by paddy rice in a calcareous paddy soil. Oriza sativa cv. Hatajirushi was used as the test plant. The treatments were as follows: (1) co-situs application of CRF containing only NPK (CRFNPK), (2) co-situs application of CRF containing only NPK and broadcast application of micronutrients (CRFNPK + M), (3) co-situs application of CRF containing NPK and micronutrients without a barrier between the roots and the fertilizer (CRFM), (4) co-situs application of the same fertilizer as on the CRFM treatment with a barrier between the roots and the fertilizer (CRFM + barrier). In the final treatment, a barrier was formed by placing the fertilizer into a sealed nylon mesh (30 µm) bag to prevent contact with the rice roots. For all treatments, the soil was kept flooded during the experimental period. Plants in the CRFNPK treatment showed severe chlorosis in the young leaves and all died within 40 days after transplanting (DAT). The barrier decreased the concentrations of Fe, Zn and Mn in the rice leaves by 22.5, 32.7 and 48.5% respectively, relative to plants grown without a barrier (CRFM treatment); the barrier also decreased the total number of grains per hill, number of ripened grains, dry matter yield and grain yield by 34.5, 44.1, 41.5 and 73.2%, respectively. Plants in the CRFNPK + M treatment grew poorly and were shorter, had less tillers and showed yellowish leaf color than those in the CRFM and CRFM + barrier treatments. We conclude that contact between rice roots and fertilizer granules is of considerable importance because the use of a barrier to prevent such contact decreased the concentration of Fe and some other micronutrients in the leaves to levels close to those known to be critical for rice growth, as well as markedly reducing yield.