Silicon : a new macronutrient deficiency in upland rice | [Silice : una nueva deficiencia de macronutrimentos en arroz de secano]

Silicon (Si) is the second-most abundant element in the earth's crust, yet paradoxically, appears to be massively deficient in upland rice. This new finding from CIAT research could account for over a half billion dollars in lost rice production annually, equivalent to about 15 percent of the region's total rice farm-gate value. The basis of silicon deficiency lies in the low Si-supplying capacity of the highly weathered Oxisol and Ultisol soils which dominate the savannas; and in the absence of Si in rainwater, the only source of water for upland rice. Irrigated rice, in contrast obtains ample Si from the typically less-weathered alluvial soils and Si rich groundwater used to flood the crop. Collaborative research between the Rice and Tropical Lowland Programs of CIAT, with important inputs from the University of Florida and DuPont Company are shedding light on this knowledge gap. Correcting Si deficiency experimentally by applying calcium silicate to typical savanna soils (Matazul and La Florida, 1993) increased the yield of marketable whole rice grains by 61 percent. Most of the effect appears to be caused by a reduction in neck blast disease, a fungus which not only reduces the quantity of grain production, but also reduces the quality of those grains which are produced. Grain discoloration, another widespread cause of low grain quality in upland rice, is also greatly alleviated. Upland rice farmers in the savanna view low grain quality as the major constraint in further development of the industry; so indirectly, low quality could also slow the adoption of upland rice based agropastoral systems, viewed as key to increasing the sustainabilitity of tropical savanna agriculture in South America. The identification and quantification of Si deficiency, and the installation of diagnostic tools and skills at CIAT are the major outputs of this research so far. These can help the region identify and manage the problem to an important extent. Rapid tests for soil, water and plant tissue are now in place, and critical level ranges have become evident. Perhaps the easiest and most powerful indicator is the analysis of rice husk tissue, which is readily available in stored grain samples all over the continent. A mapping of the region's Si status could be quickly achieved in this way. Values of less than 3 percent Si on a dry weight basis suggest deficiency; 3-5 percent, borderline; and more than 5 percent, probable adequacy. Present evidence leads to the conclusion, however that genes alone are not likely to overcome the huge shortfall in Si supplying capacity in the savanna, which is just one-tenth of the amount provided to irrigated rice in the ClAT-Palmira environment. An integrated strategy is clearly called for, to be developed through a continued multi-disciplinary and multi-institutional research effort.