Productivity of Clay Tailings from Phosphate Mining: I. Biomass Crops

Abstract

AbstractPhosphate 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—(i) 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’], (v) desmodium (Desmodium cinerascens A. Gray), (vi) sweet sorghum [Sorghum bicolor (L.) Moench ‘USDA M 8IE’], 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 averaged over 4 yr for erianthus, leucaena, and elephantgrass averaged 139.6, 58.5, and 56.5 Mg ha−1 yr−1, respectively. Crude protein and digestibility were low in mature, whole‐plant 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 pCi g−1). Data indicate that these present‐day waste lands can be a valuable resource for biomass production.