YG Yang; ZL He; XE Yang; PJ Stoffella

University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, FL 34945, USA.

The phosphate industry in Central Florida produces a substantial amount of dolomite phosphate rock (DPR), such as reject pebbles, phosphatic clays, and oversize debris. High Ca, Mg, and P contents in these materials make them potentially useful for agricultural production. The objective of this study was to assess the environmental impact of P slow release fertilizers made from DPR materials, as compared with water soluble P fertilizers.

The P slow release fertilizers  were manufactured by blending DPR materials with N-viro soil at the ratio of 30%, 40%, 50%, 60%, and 70%, and applied at the rate of 100 mg available P per kg soil. The total weight of each soil-amendment mixture was 2.50 kg (oven-dry basis). Nitrogen (200 mg N kg^-1 as NH₄NO₃) and potassium (200 mg K kg^-1 as K₂SO₄) were added. For Control, no chemical fertilizers were applied; and for chemical fertilizer treatment (CF), P was supplied as K₂HPO₄ at 100 mg P/kg soil. After mixed and soil moisture adjusted to 70% field holding capacity, the soils were incubated at room temperature for 21 days prior to column leaching experiment.

Seven leaching events were conducted at the intervals of day 1, 3, 7, 14, 28, 56, and 70, respectively, and 258.9 ml of DI water was applied at each leaching events, totaling 1812 ml, which was equivalent to half year’s rainfall in the Indian River area. Deionized water was applied using a peristaltic pump.  Leachate samples were collected from each leaching event, and analyzed for pH and electrical conductivity (EC). Subsamples were filtered through a 0.45 mm membrane prior to analyses for total soluble concentrations of elements using ICP-OES.

Compared with chemical fertilizers or control, DPR fertilizers resulted in higher pH and EC, higher Ca and K concentrations in leachate, but dramatically reduced P concentration in leachate. Cumulative losses of Ca, K, and Mg were greater in DPR than chemical fertilizer or control due to higher concentrations of these elements in the DPR fertilizers. However, this trend declined with increasing DPR ratio in the DPR fertilizers. Use of DPR fertilizers significantly reduced cumulative leaching losses of Fe, P and Mn. DPR fertilizers had a lower cumulative leaching loss of Cu and Zn than chemical fertilizer or control, but higher cumulative losses of Pb, Co, and Ni. However, their concentration in leachate was < 0.50 mg L^-1 (in total 1812 ml leachate), and therefore, its environmental impact should be minimal. These results indicate that use of DPR slow release fertilizers has great potential of reducing P loss from agricultural production systems with minimal likelihood of heavy metal contamination.

Key words: Soil amendment; Leachate; Environmental impact.