A well-documented threat to human and environmental health comes from agricultural runoff, water containing phosphorus and nitrogen fertilizers leaving farm fields and entering lakes and rivers. While there are a number of concerns around contaminated agricultural runoff, a primary issue is that algae growth dramatically increases when it is exposed to these excess nutrients. The algal bloom that results chokes out natural vegetation by consuming all of the oxygen in the water and creating “dead zones” in which most animal life cannot exist. Sometimes the algae also generates toxic substances that are harmful to people, resulting in closed beaches and the elimination of other recreational opportunities.
The Eagle Creek reservoir is one of the primary drinking water sources and recreational areas in the Indianapolis metropolitan area. The watershed that feeds it is also heavily farmed, and the integrity of the reservoir’s water quality is a primary management concern.
Greenleaf Communities is currently overseeing field-scale research into farm management practices that could help to reduce the amount of phosphorus and nitrogen lost from the farm fields into the watershed. Primarily, we are studying the use of gypsum as a soil amendment for watershed environmental benefits. Additional benefits of the research include improving soil health and reducing erosion while increasing crop yields and providing an economic incentive for farmers to participate.
Demand for increased crop yields to meet global population growth has led to depleted soils. Nearly all U.S. agricultural soil is somewhat degraded and much of the soil in the Midwest is very degraded (“Land Degradation”, 2010). Poor soil conditions inhibit plant uptake of important nutrients, like phosphorus and nitrogen, reducing yields and producing less healthy plants. To compensate, farmers often over-apply fertilizers and pesticides, which further stresses soils and loads nearby waterways with excess nutrients. This causes algal blooms, eutrophication, aquatic life loss, water quality degradation and public health concerns. Additionally, degraded soils retain less water, stressing plants during drought conditions.
Research indicates that certain soil amendments can aid in remediating soils, resulting in higher yields and reduced sediment runoff (Chen and Dick, 2011; Stout et. al, 1998). One promising amendment is gypsum, a sulfate mineral made up of calcium sulfate dihydrate (CaSO4•2H2O). Gypsum has been used as a fertilizer for centuries (Chen and Dick, 2011). Gypsum is found naturally in sedimentary rocks around the world, including the United States which has several active gypsum mines. However, mined gypsum is not economically feasible in many locations in part due to transportation costs (Rhoton, 2011). Gypsum is also produced when sulfur dioxide is scrubbed from coal-fired power plant exhaust stacks. This kind of gypsum, flue gas desulfurization (FGD) gypsum, contains fewer impurities than mined gypsum, with a 90 to 99% purity concentration compared to 66 to 98% concentration for mined gypsum (Chen and Dick, 2011). FGD gypsum has been recognized as a beneficial additive for agricultural application, equivalent to or better than mined gypsum. Compared to mined gypsum, FGD gypsum has good spreading characteristics, which allow it to be applied easily (Dontsova et. al, 2004). With more coal fired power plants installing pollution scrubbers, FGD gypsum production has increased and will likely double in the next decade and remain economically priced (Wolkowski et. al 2010).
Improved water quality and quantity:
Gypsum also improves chemical properties of soil such as remedying aluminum toxicity caused by subsoil acidity (Chen and Dick, 2011; Dontsova et. al, 2004). Results include better rooting and uptake of water and nutrients, especially during periods of water scarcity (Chen and Dick, 2011). One study in the lower Mississippi River valley showed an increase in total water infiltration by 71% (Rhoton, 2011). While gypsum addresses subsoil acidity and aluminum toxicity, it is important to note that gypsum is not a liming agent, and does not alter pH levels in the soil (Fisher, 2011). Gypsum can also amend problems associated with excess phosphorus. High phosphorus levels pose a threat to waterways. The calcium in gypsum lowers the amount of phosphorus released in surface runoff. Additionally, gypsum aggregation of soils also reduces the amount of surface runoff. Water-soluble phosphorus decreased by 50% in one study (Stout et. al 1998). In a lower Mississippi River valley study the total runoff decreased by 30% and soil loss by 77% (Rhoton, 2011). A study on gypsum application on soils where poultry litter was used as fertilizer also significantly reduced soluble phosphorus (Sheng et. al, 2012). In another study in Indiana (Norton, 2008) soluble phosphorus was reduced by more than 50% with the application of gypsum in comparison to a no-till control plot (Norton, 2008). In a south Florida study (Andersen et. al. 1995) dairy manure was the source of phosphorus loading in surface waters, and the application of gypsum resulted in higher retention rates of phosphorus in the soil, thereby reducing soluble phosphorus in waterways. Grass buffers can also reduce soluble phosphorus runoff and are even more effective when gypsum is applied to the buffers. One study (Watts et. al, 2009), showed a reduction in soluble phosphorus by up to 40% when gypsum was applied to the grass buffers in Alabama.
Overall, gypsum is an effective soil amendment which improves soil conditions and their effective nutrient processing capabilities in certain soil types. It helps to restore degraded soils and produce more nutritious plants. Gypsum enhances water infiltration and improves soil composition, which contributes to lower erosion rates and less nutrient loading to area waterways. Gypsum is most effective as one component of a whole systems approach to managing agricultural lands that takes into consideration soil type, crop, and hydrology. Healthy functioning soils contribute to healthy crops and healthy waterways, and gypsum can be an important component of this result.
Learn More: Gypsum Literature Review (with citations included)
Our research is being led by Dr. Pierre Jacinthe, Associate Professor of Earth Science at Indiana University-Purdue University Indianapolis, and is being conducted on working farm fields Brownsburg, IN in the threatened Eagle Creek watershed. Fields with historic use of gypsum applications are being compared to fields with similar soil types, crops, and other management practices, but no history of gypsum use. Soil and water samples are collected regularly to compare the movement of nutrients (fertilizers) from the field and into the drainage ditches that eventually lead into Eagle Creek Reservoir.
Over the two-year initial phase of this study, the relationship between gypsum soil amendment, crop yield and water pollution. It is helping to define best management practices that will be used by farmers and soil conservationists across the country to improve soil and water conditions and environmental integrity.