Field, greenhouse and laboratory research was begun in 1990 to develop low cost, yet effective methods for establishing vegetation on by-product phosphogypsum stacks in Florida. The approach of using a combination of adapted plant species and minimal amendments was contrasted with the use of soil or overburden cover over phosphogypsum. Amendments such as dolomitic limestone, phosphatic clay, composted sewage sludge or garbage, and fertilizers were tested for their effectiveness in ameliorating the acidity, infertility and tendency for crusting or caking of phosphogypsum. Field experiments were conducted on two “inactive” stacks (the U.S. Agri-Chemicals Bartow facility and Estech’s Silver City stack) and one active stack (the IMC-Agrico New Wales stack). Plots have been established on flat surfaces and on 3:1 and 2:1 slopes.
In the greenhouse and laboratory studies, plants were able to grow on phosphogypsum if the pH was 4.0 or greater. Leaching with water was effective in raising the pH of very acidic phosphogypsum (pH 2.8) to 4.0, and this is probably the mechanism responsible for the higher pH values (often 4.5 to 5.0) found on older, inactive stacks or on a portion of an active stack with subsurface drainage installed. Limestone, lime, phosphatic clay, and composted garbage amendments were effective in raising the pH of phosphogypsum. Other experiments showed that it was necessary to add nitrogen, potassium and magnesium nutrients for good plant growth. Phosphatic clay was a good source of potassium and magnesium, and dolomitic limestone was a good source of magnesium.
In field experiments, one ton per acre of dolomitic limestone, or a two inch layer of sand tailings or overburden, applied to the surface of phosphogypsum and rototilled, enhanced plant establishment. Subsequently, when the rate of magnesium fertilizer was increased, plant growth on the unamended phosphogypsum increased to levels similar to growth on the amended gypsum. The composted garbage amendment (and similarly, sewage sludge in another experiment) initially inhibited germination and growth (possible cause still under study) but enhanced plant growth in the second growing season. Bermudagrass (Cynodon dactylon) was able to grow well directly on phosphogypsum with fertilizer only, when the pH was 4.5 to 5, but bahiagrass (Paspalam notatum) and weeping lovegrass (Eragrostis curvula) grew much better with the overburden and composted garbage amendments. Phosphatic clay also enhanced plant growth.
Plant establishment and growth were compared on phosphogypsum and on phosphogypsum with an overburden cap up to six inches in thickness. Plant cover was equally good with or without the overburden cap. The main effect of the overburden cap on vegetation was an increase in the number of plant species that became established. The presence of bermudagrass on phosphogypsum reduced the efflux of radon by half, while six inches of overburden, in addition to bermudagrass, halved the radon efflux again. The six inch overburden cap reduced the fluoride uptake by bermudagrass, but the overburden caused an increase in radium uptake. Vegetation cover on phosphogypsum, with or without a six inch overburden cap, resulted in a 30-fold decrease in the electrical conductivity and a 5-fold decrease in the fluoride concentration of surface runoff water.
Vegetation was successfully established on 2:1 and 3:1 slopes. It was possible to use conventional methods (rubber tired tractor with dual rear wheels, disk, broadcast spreader, and hay mulch blower) for tilling and planting on the 3:1 slope, but the 2:1 slope was much too steep to operate such equipment. Manually raking, broadcast seeding and fertilizing, and stapling a straw blanket mulch was a very effective method for establishing bermudagrass on a 2:1 slope of a phosphogypsum stack, but the practice would be expensive. A less costly method, hydroseeding/hydromulching, was also tested. Grass establishment was initially more sparse with hydroseeding/hydromulching than with the straw blanket mulch, but due to the spreading nature of bermudagrass, ninety percent or greater plant cover was achieved in the third growing season on plots that had been raked or furrowed to break up the surface crust on the phosphogypsum. The major difficulty in establishing and maintaining a vegetation cover on a 2:1 slope, in addition to the erosion potential, is the problem of access to the steep slopes for planting, mulching, fertilizing, mowing or herbicide application.
Common bermudagrass was the best adapted grass of those tested for establishing plant cover directly on phosphogypsum from seed. Observations of demonstration plantings indicated that Alamo switchgrass (Panicum virgatum) was also vigorous and persistent on phosphogypsum. Ryegrass (Lolium perenne) performed satisfactorily as a temporary winter cover. Three grasses, broomsedge (Andropogon virginicus), bushy beardgrass (Andropogon glomeratus) and smutgrass (Sporobolus poireti), were commonly observed natural invaders on weathered or leached portions of phosphogypsum stacks (pH 4 or greater). Saltbush (Baccharis halimifolia) and dog fennel (Eupatorium capillifolium) were also common invaders. Other plant species were seeded or transplanted in demonstration plots on the Estech and USAC phosphogypsum stacks. Those plants that appeared to be doing well included: wax myrtle (Myrica cerifera), red cedar (Juniperus virginiana), yucca (Yucca filamentosa), kleingrass (Panicum coloratum), and buffelgrass (Cenchrus ciliarus). Addition of amendments such as sand tailings, phosphatic clay, composted garbage (after weathering through the winter), and overburden, improved the ability of other species (e.g. weeping lovegrass, bahiagrass, and browntop millet (Brachiaria ramosa) to grow on phosphogypsum.
Steven G. Richardson, Florida Institute of Phosphate Research. December 1995.