AVAILABILITY OF PHOSPHORUS IN CALCIUM SILICATE SLAG

Significant yield increases of sugarcane and rice have been observed in the Everglades Agricultural Area of Florida from application of soluble silicon as calcium silicate slag. Although calcium silicate slag contains trace quantities of P, significant quantities of P can be applied using commercial rates of slag. This may be of concern to both the agronomists responsible for determining P fertilization rates for these crops and to land managers attempting to limit the application of P in this region. The objective of our research was to determine the biological availability of P applied as calcium silicate slag. Two sugarcane studies and two rice studies were conducted on a Terra Ceia muck (Euic, hyperthermic Typic Medisaprist), Pahokee muck (Euic, hyperthermic Lithic Medisaprist), and a Okeelanta muck (sandy or sandy-skeletal, siliceous, euic hyperthermic Terric Medisaprist) at five locations for one to three years each. Calcium silicate slag was applied at rates ranging from 0 to 20 Mg ha-1. For each crop, plant tissues were sampled for analysis of Si and P: sugarcane leaves were sampled during the "grand-growth" period, rice Y-leaves were collected at panicle initiation, and rice straw was collected at harvest. Soils used for sugarcane production were also sampled and analyzed for water-soluble P and acetic acid-soluble P. Leaf Si concentration increased with slag application, however, neither the P concentration in leaf nor soil were influenced by the application of slag. Although the total quantities of P applied as slag can be significant, P was shown to remain biologically unavailable to rice and sugarcane. Because of the low solubility of P in calcium silicate slag, it appears unlikely that P enrichment of drainage waters can occur by application of calcium silicate slag.