Effect of organic matter and moisture regime on the availability and uptake of sulfur and zinc in rainfed lowland rice

Laboratory, greenhouse, and field experiments were conducted to determine sulfur (S) and zinc (Zn) availability and uptake by rainfed lowland rice as influenced by rice straw and moisture in S and Zn deficient soils from Tarlac and Batangas [Philippines]. A laboratory experiment was carried out to determine the SO4 adsorption capacity of the soils. The Freundlich adsorption isotherm developed showed that the Tarlac soil had a higher SO4 adsorption capacity than the Batangas soil under aerobic condition. The chemical properties of the former soil influenced its adsorption capacity. A greenhouse experiment was conducted to study the electrochemical properties of the two soils with added rice straw, as well as the nutrient content of S and Zn in plant tissues and growth and yield of IR66 under varying moisture regimes. Rice straw incorporation adversely affected soil solution Eh as well as the contents of solution SO4 and Zn. Rice straw incorporation during the vegetative phase adversely affected S and Zn uptake by IR66 but not at flowering and maturity. Straw incorporation, however, enhanced grain yield. A field experiment was undertaken on the Batangas soil to study the effect of rice straw with added S and Zn on IR66 response. Rice straw application of 10 t/ha adversely affected plant growth as well as the plant contents and uptake of S and Zn. Zinc applied at 20 kg/ha depressed S content and uptake. Similarly, 40 kg/ha applied S caused a decline in plant Zn content. Variable response to nutrient application is reflected, in part, on soil properties and level of organic matter. Organic matter influences the availability of nutrients in excess of that fixed by the soil. In soils with notable S- and Zn- fixing capacities, the study revealed that incorporation of high quantities of organic matter adversely affects the availability of both nutrients.