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Production of High-Value Cash Crops on Mixtures of Sand Tailings and Waste Phosphatic Clays


Mandatory reclamation has placed increased pressure on the phosphate mining industry to utilize mined lands for agriculture. Due to the unique physical and chemical properties of phosphatic clays, farming pure clay settling areas has been limited primarily to pasture systems. Recent research efforts have been orientated towards devising new, sometimes unconventional technologies, which will allow production of more diverse agricultural commodities including vegetables, row crops, turfgrass, and high-value forages.

Sand/clay mix technology, whereby sand tailings are mixed with thickened clays prior to deposition into settling areas, is an alternative disposal technology which has been successfully adopted by CF Industries in Hardee County. The research presented herein describes an attempt to evaluate the effect of adding sand to phosphatic clay on the production and quality of vegetables and high-quality forages.

The research was divided into three different phases. The sand:clay ratio (SCR) phase evaluated the production and quality of vegetables grown in varying SCR. The SP-1 alfalfa phase was conducted to evaluate the production and quality of alfalfa grown on sand/clay mix amended with several different organic materials. A third phase was conducted to investigate the capacity of phosphatic clay to reduce Cd uptake by plants grown in sludge-amended soils.

For the SCR experimental phase, three seasons of vegetable production research were conducted on four different sand/clay mixes with sand:clay ratios (SCR) of 2, 4, 6, and 8. Vegetables grown included turnip, cabbage, banana pepper, yellow squash and mustard. Peat and phosphogypsum were evaluated as soil amendments; peat to “lighten” the soil and provide a source of organic matter, and phosphogypsum to increase soil Ca and thereby competitively inhibit plant uptake of 226Ra. In addition to yield, soil and plant nutrient levels were monitored as well as Cd and Ra226 contaminants.

We found that yields were often l0-30% higher in the SCR = 2 mix than in the higher SCR mixes. However, from a production perspective, yield increases in the SCR = 2 mix were offset by the considerable increase in difficulty required to carry out conventional farming operations in this mix versus the SCR = 4 and higher sand/clay mixes. Soil chemical properties remained dominated by the phosphatic clay component so that variations in soil and plant nutrient (also Cd and 226Ra) levels due to varying SCR were minor and not agronomically significant. Boron and Mn deficiencies were identified for several crops for all SCR.

Peat had little effect on production and quality (including 226Ra) of vegetables. Phosphogypsum had no effect on vegetable yield, quality, and 226Ra concentration at both the 22 Mg/ha rate (10 ton/acre) and the very high application rate of 134 Mg/ha (60 ton/acre). Plant uptake of 226Ra was more affected by vegetable tissues than by soil parameters and soil  amendments. Results of over 600 226Ra analyses revealed that tissue 226Ra was positively correlated with tissue Ca. Maximum ratios between 226Ra and Ca in tissues indicated that 226Ra did not excessively accumulate in vegetable tissues due to indiscriminate uptake.

Alfalfa was grown on CF Industries’ sand/clay mix reclamation area, SP-1, to evaluate alfalfa yield and quality as affected by several organic amendments. The organic materials  investigated included sawdust, humate, composted garbage, peat, sewage sludge, and composted sewage sludge. Due to the sensitivity of alfalfa to waterlogged soil conditions, these materials were added at two rates, 22 and 45 Mg/ha (10 and 20 ton/acre) in order to lower soil bulk density and improve internal surface drainage. Alfalfa yields were increased by the application of sewage sludge but not sawdust, humate, peat, sludge compost and garbage compost. Annual dry-weight yields (7 cuttings) were 11.0 and 13.6 Mg/ha (4.9 and 6.1 ton/acre) for control and sludge-amended plots, respectively. The benefit of sewage sludge additions appeared to be due to micronutrient contributions, especially Mn. Organic materials had little effect on alfalfa nutrient composition, 226Ra conc., and in-vitro-organic-matter digestibility.

At the University of Florida, glasshouse experiments were conducted in which sand tailings and 4:l sand:clay mix were amended with several different waste organic materials in order to determine the influence of phosphatic clay on the growth and Cd uptake by legume forages. Materials evaluated included sawdust, peat, raw shredded garbage, garbage compost, and a high- Cd sewage sludge. Dry-matter yields of Alyce clover and alfalfa were greater in sand than in sand/clay mix and, with the exception of high sludge rates which decreased or killed plants, were generally unaffected by organic amendments. Increasing application rates of high-Cd sludge to sand and sand/clay mix resulted in increasing extractable Cd levels in both soils. However, increases in extractable soil Cd for a particular application rate of the high-Cd sludge were greater for sand than for sand/clay mix.