Evaluation of the Effect of Calcium Silicate Slag on Trace Contaminants for Rice Production.

Adequate Si supply is necessary for successful rice and sugarcane production in histosolic soils in Florida.  Calcium silicate slag from phosphate mining is potentially a good source of Si.  However, this type of slag contains trace elements that have the potential to contaminate the grains and straw from rice, including Cd, Cr, Pb, ²¹⁰Po, ²²⁶Ra, ²³⁰Th, and U due to their potential toxicity and/or low level radioactivity.  Therefore, before calcium silicate slag can be used as a supplement for crop production, it is necessary to determine its effect on solubility and bioavailability of trace contaminants, as well as its potential to supply Si to the plant.  A greenhouse cultivation study was performed to address this issue.  A factorial design, including 2 typical soils for rice culture from Stoneville, MS, and Everglades, FL, 4 application rates (0, 1000, 2000, 4000 mg kg^-1), and 2 application methods (surface application and incorporated into soil) was used.  Results showed that calcium silicate slag had minimum effect on soil pH.  Addition of calcium silicate slag significantly increased 0.5 M acetic acid extractable Si.  Soil DTPA-extractable Pb and total Cd at harvest were significantly increased in organic soils amended with slag.  Biomass of rice leaf, stem, hull, seed, and root were not significantly improved by calcium silicate slag supplementation.  Total elements in rice leaf, stem, hull, and seed were not significantly increased following calcium silicate slag application.  Total radioactivity in rice seed was 15-times lower than the background.  These results indicate that trace contaminants in calcium silicate slag may not be a concern for rice production.