Horticultural Uses for Flue Gas Desulfurization Gypsum

Abstract

Gypsum (calcium sulfate dihydrate (CaSO4 · 2H2O)) is a naturally occurring colorless and odorless solid mineral, with a very fine particle size. Gypsum is usually mined from natural deposits in the earth, but can also be produced synthetically when the exhaust gases from the burning of high sulfur-containing coal are released into calcium carbonate filters, a form known as flue gas desulfurization gypsum, or FGD gypsum. The natural form has been used for hundreds of years as a type of fertilizer, yet the synthetic form is not widely used within agriculture. Three studies evaluated the horticultural uses of FGD gypsum including: occurrence of blossom end rot on greenhouse tomotoes; the production of three greenhouse crops that included fern, geranium, and petunia; and stem strength of poinsettias. Most of the experiments performed used a rate of 3.26 kg/m3 (5.5 lbs/yd3) of FGD gypsum incorporated into the substrate mix, increasing by 3.26 kg/m3 (5.5 lbs/yd3) up to 19.58 kg/m3 (33 lbs/yd3). All treatments were compared to a control of no additional FGD gypsum. The FGD gypsum was incorporated into the soil mix and the plants were grown to a marketable size, or in the case of tomato, a harvestable and marketable size and color. Results varied across all experiments, with no detrimental effects observed even at the highest FGD gypsum levels. In tomato experiments, there was a reduction in the occurrence of blossom end rot, and increased in fruit weight and number were observed with increasing FGD gypsum level. There were varying results for the three greenhouse crops. One of the most promising results in the poinsettia experiments showing greater stem strength at higher rates of FGD gypsum were added. Throughout all experiments, plants were tolerant of the high FGD gypsum rates, and Abstract iii results indicate FGD gypsum could be a suitable replacement for lime as a calcium source when a substrate pH adjustment is not needed. However, plant growth and development in response to supplemental FGD gypsum in our greenhouse studies in soilless substrates were not as distinct as responses reported from studies using mineral soils. Therefore, we conducted a final study to evaluate the stability of supplemental FGD gypsum in two soilless substrate blends commonly used in greenhouse crop production. Our results show that FGD gypsum is rapidly leached from the soil column and does not remain as effective in soil solution as expected. Therefore, FGD gypsum does not appear to be to be suitable for long-term supply of calcium and sulfur, and is not likely to provide reduction of phosphorus in leachates from soilless substrates. Specifically, our data shows that FGD gypsum incorporated into a soilless substrate washes out of the container in less than 10 irrigation events when a standard irrigation regimen target of 10% leaching fraction is used.