Productivity of clay tailings from phosphate mining. III. Grain crops

Phosphatic clay is a waste product from the Florida phosphate mining industry presently totaling 41000 ha. These clays contain no phytotoxic materials, are high in most plant nutrients, and 226Ra and average about 460 g moisture per kilogram. A split-plot field experiment was conducted to study forage and grain yield, forage quality, plant nutrient concentrations, changes in soil nutrients, and 226Ra contents of four grain crops in various rotations. The crop rotations (i) corn (Zea mays L. 'Jacques 247')-sunflower (Helianthus annuus L. 'Cargill 205'), (ii) sunflower-grain sorghum (Sorghum bicolor L. Moench 'Northrup King Savanna 5'), (iii) soybean (Glycine max L. Merr. 'Williams 80')-grain sorghum, and (iv) grain sorghum-soybean ('University of Florida V-1') were grown on a dry phosphatic clay with and without a 50-mm surface layer of quartz-sand tailings. Nitrogen was the only fertilizer element applied. Results show that corn and grain sorghum produced highest (P < 0.05) forage yields (15.7 and 13.4 Mg ha-1) and highest grain yields (8955 and 4302 kg ha-1 per harvest, respectively. Soybean harvested for forage (Crop 1) contained the highest crude protein (217 g kg-1) and in vitro organic matter digestibility (736 g kg-1). Concentrations of P, K, Ca, Mg, and Fe in most of the forages were adequate for the diets of beef cattle (Bos taurus), while those of Mn, Cu, and Zn were low. Mehlich I-extractable soil P (1601 mg kg-1), Ca (4696 mg kg-1), and Mg (1173 mg kg-1) were considered very high and changed little over the 4-yr production period. Application of 50 mm of sand tailings tended to increase (P < 0.05) Mehlich I-extractable P, Ca, Mn, Cu, Zn, and Fe. Radium-226 concentration in the forage of all grain crops averaged 8.5 Bq kg-1, which was about 17 times higher than that in the grain (0.4958 Bq kg-1) of the same crops. Concentrations of 226Ra in the forage (8.288 Bq kg-1) and grain (0.6771 Bq kg-1 were 1.1% and 0.09% of the concentration in clay (761 Bq kg-1 respectively. These data indicate that phosphatic clays can be a valuable resource for the production of corn and sorghum grain that contain low concentrations of 226Ra.