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Productivity of Clay Tailings from Phosphate Mining: II. Forage Crops

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

W. G. Blue

Soil Sci. Dep., Univ. of Florida, Gainesville, FL 32611;

C. E. Roessler

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

^* Corresponding author.

ABSTRACT

Phosphate mining in Florida produces phosphatic clay and quartz-sand tailings waste products. Presently there are about 41 000 ha of phosphatic clay (slime ponds) in Florida. These clays contain no phytotoxic materials, are high in most plant nutrients, and average about 460 g moisture kg^–1. A split-plot field experiment was conducted to study forage yield, quality, plant nutrient concentrations, changes in soil nutrients, and ²²⁶Ra contents of 12 grass-N source treatments. The treatment combinations (i) bahiagrass (Paspalum notatum Fluegge var. saurae Parodi ‘Pensacola’) + N, (ii) bahiagrass + white clover (WC) (Trifolium repens L. ‘Arcadia’), (iii) stargrass (Cynodon nlemfuensis Vanderyst var. nlemfuensis ‘Florico’) + N, (iv) Florico stargrass + WC, (v) Florico stargrass + alfalfa (Medicago sativa L. ‘Florida 77’), (vi) Florico stargrass + carpon desmodium (Desmodium heterocarpon (L.) D.C. ‘Florida’), (vii) Limpograss [Hemarthria altissima (Poir) Stapf et. C.E. Hubb. ‘Floralta’] + N, (viii) Limpograss + WC, (ix) Stargrass (C. nlemfuensis Vanderyst var. nlemfuensis ‘Florona’) + N., (x) Florona stargrass + WC, (xi) Florona stargrass + red clover (Trifolium pratense L. ‘Nolins’), and (xii) Florona stargrass + alfalfa, were grown on dry phosphatic clay with and without a 50 mm surface layer of quartz-sand tailings. Nitrogen (500 kg ha^–1 yr^–1) was the only fertilizer element applied on grasses. No fertilizer was applied on legumes during the 4-yr period. Results indicated that Floralta hemarthria + N, Florona stargrass + N, and Florona stargrass + alfalfa produced highest dry-forage yields averaging 20.8, 18.1, and 17.7 Mg ha^–1. Florico stargrass + WC and Floralta hemarthria + WC had annual averages of 233 and 211 g CP kg^–1 and 769 and 751 g IVOMD kg^–1, respectively. Grass + N treatments were about 5 to 10 percentage units lower in CP than grass + legumes. Forage mineral concentrations for P, K, Ca, Mg, Mn, Cu, Zn, and Fe, ranged from 1.3 to 39 times higher than required for growing and finishing cattle (Bos sp.). Mehlich I-extractable soil P (1650 mg kg^–1), Ca (4588 mg kg^–1), and Mg (1130 mg kg^–1) were very high and changed little after 4 yr of forage production. Forages had a differential uptake of ²²⁶Ra with Florona stargrass + N (1.28 pCi g^–1) having the highest uptake and Pensacola bahiagrass + N (0.32 pCi g^–1) having the lowest uptake. Radium 226 in the phosphatic clay was 22 pCi g^–1, which was 80 times higher than levels in an unmined Spodosol; however, plant uptake was three times higher than plants grown on unmined soil. Data indicated that phosphatic clays are a fertile resource that can be used for forage production.

Contribution from the Univ. of Florida, Inst. Food and Agric. Sci., Florida Agric. Exp. Stn. Journal Series no. R-00170.

Received for publication September 21, 1989.

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