Productivity of clay tailings from phosphate mining. I. Biomass crops

Phosphate mining in Florida yields waste products of phosphatic clay and quartz sand tailings, each making up about one-third of the original matrix (PO4 ore, sand, and clay). Phosphatic clay ponds typically occupy about 50% of the mined sites and normally require 10 to 15 yr before 40 to 50% solids are obtained. These clays contain no phytotoxic materials and are high in most plant nutrients. When surface water has disappeared, these clays are classified as clayey Haplaquents. A split-plot field experiment was conducted to study biomass yield, quality, plant nutrient concentrations, changes in soil nutrients, and 226Ra. Seven biomass crops--(ii) elephantgrass (Pennisetum purpureum L. 'PI 300086'), (ii) leucaena [Leucaena leucocephala (Lam.) De Wet], (iii) alemangrass [Echinochloa polystachya (H.B.K) Hitche], (iv) erianthus [Erianthus arundinaceum (Retz) Jesw 'IK 76-63'], desmodium (Desmodium cinerascens A. Gray), (vi) sweet sorghum [Sorghum bicolor (L.) Moench 'USDA M 81E'], and (vii) forage sorghum] Sorghum bicolor (L.) Moench 'Pioneer 931']-were grown on the phosphatic clay with and without a 5-cm surface layer of quartz sand tailings. Nitrogen was the only fertilizer element applied for grass species and no fertilizer was applied for legumes during the 4-yr period. Dry biomass yield average over 4 yr for erianthus, leucaena, and elephantgrass averaged 139.6, 58.5, and 56.5 Mg ha-1yr-1, respectively. Crude protein and digestibility were low in mature, whole-pla nt samples except for leucaena (122.0 g kg-1). Generally, all whole plants contained adequate concentrations of nutrients averaging (g kg-1) P, 2.05; K, 11.5; Ca, 4.3; Mg, 2.77 and (mg kg-1) Cu, 4.0; Zn, 26; Fe, 68; and Mn, 35. Mehlich-I-extractable soil nutrient concentrations changed little over the 4-yr period. Radium-226 concentration in plant tissue (0.23 pCi g-1) was nearly six times higher than the concentration measured in plants from an unmined surface Spodosol (0.04 pCig-1). Data indicate that these present-day waste lands can be a valuable resource for biomass production.