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

Agric. Res. and Educ. Ctr., Univ. of Florida, Ona, FL 33865;

W. G. Blue

Soil Sci. Dep., Gainesville, FL 32611.

C. E. Roessler

Dep. Environ. Eng. Sci., Univ. of Florida, Gainesville, FL 32611.

^* Corresponding author.

ABSTRACT

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—(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.

Contribution from the Univ. of Florida, Inst. Food and Agric. Sci. Agric. Exp. Stn. Journal Series no. 8749.

Received for publication February 2, 1988.