Improving water quality impacts from agriculture
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Climate change may be dominating environmental headlines, but water quality continues to be a principal concern for communities affected by agriculture. Although animal feeding and other livestock operations have been implicated in many examples of agricultural waste compromising municipal water supplies, crop production bears responsibility too: sediment, crop nutrients, and crop protectants are still entering surface and groundwater from farm fields.
Sediment from fields is simply lost topsoil, or the organic-matter-rich A horizon. Once that soil is washed away by rain or irrigation, it cannot be returned or replaced. Thaler et al. (2021) found that the A horizon has been completely lost on approximately 35% of agricultural land in the U.S. Corn Belt. Agricultural soil erosion is a serious problem on many levels. For the farmer, losing topsoil means losing productivity. It is estimated that A-horizon soil losses account for an approximate 6% decrease in yield across the Corn Belt, which is associated with a whopping $800 million profit loss.
For communities, sediment in waterways can be dangerous and extremely costly to fix. Sediment pollutes drinking water and complicates municipal water treatment operations. Waterways used for shipping become too shallow for safe navigation and must be dredged when sediment accumulates, at great cost to federal and state governments. Keeping the Mississippi River navigable is extremely important for the cost-effective export of commodity crops from the Midwest, like corn and soybeans. In 2020, the Army Corps of Engineers launched a project to dredge 256 miles of the Mighty Mississippi, to allow larger ships to carry products out to the Gulf of Mexico, at a cost of nearly $1 million per mile of channel dredged (Baratta, 2020). This project will deepen the main shipping channel by 5 ft, from a present depth of 45 ft to a target depth of 50 ft (Office of the Governor, 2020).
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Soil washing away from farm fields carries crop inputs with it. Soil-applied crop protectants and nutrients show up in drinking water and rivers, streams, and lakes used for recreational activities. Nitrates (NO3–) in drinking water are known to cause serious health problems, including infant methemoglobinemia (colloquially known as blue baby syndrome) and certain cancers (Ward, et al., 2005). To protect residents from nitrates in the water, cities like Des Moines, IA have installed nitrate removal systems from their municipal water supply, and citizens are encouraged to have private wells regularly tested.
The environmental impacts resulting from nutrients in waterways are troubling as well. Phosphate from fertilizer stimulates algal growth in waters. Some of those algae produce toxins that are dangerous for people, pets, and aquatic organisms. Cyanobacteria, which commonly “bloom” in lakes and ponds affected by agricultural phosphates, produce cyanotoxins, which can make people and pets extremely ill or even kill them if enough of the toxin is ingested (USEPA, 2014). State and local governments regularly close surface waters for recreational activities when algal blooms take over bodies of water; these blooms can also disrupt the water supply of agricultural operations that depend on those bodies of water.
Identifying Water Quality Concerns in Your Region
The USEPA has an online tool to identify local water quality concerns called How’s My Waterway (https://mywaterway.epa.gov). The tool allows users to look at national and state water quality issues and offers a great deal of detail on specific local conditions and improvement efforts. Trusted advisers can use this tool to uncover any ag-related concerns in the watersheds nearest the farms they serve.
Advisers can enter the zip code of their client’s farm or find it on the map to learn in which watershed the farm is located. In Figure 1, the Lake Mendota-Yahara River watershed, near Madison, WI, was selected. This watershed faces several impairments, and 96% of the assessed waters are impaired with nitrogen and/or phosphorus. This suggests there is significant room for improved nutrient stewardship.
Farm-Level Assessment
Tools like How’s My Waterway are great for identifying water quality issues in local watersheds. Understanding an individual farm’s impact on the local watershed is considerably more complex. Direct measurement is the most accurate method of assessing the quality of water leaving the field, either through surface runoff, drainage tiles, or infiltration. This method involves taking a direct sample of the water and having it analyzed for pollutants. Due to myriad financial and logistical challenges accompanying this approach, we cannot expect this type of data collection to occur at scale any time soon. The most feasible approach to assessing water quality impacts from agricultural production continues to be the application of science-backed tools designed for this purpose. Here we describe two such tools available to users at NRCS offices or using other platforms.
Water Quality Index for Agricultural Runoff
The USDA-NRCS offers the Water Quality Index for Agricultural Runoff (WQIag), which rates the quality of water leaving farm fields on a scale from 1 (very poor) to 10 (excellent). To run the WQIag tool and obtain water quality scores, the tool uses inputs that include specific, inherent soil and site conditions as well as grower-controlled factors such as tillage, pest management, nutrient management, drainage, and irrigation management along with any implemented conservation practices. Growers and their advisers can run different scenarios using WQIag to test how installing or implementing various conservation practices can impact the score for an individual field. The lower the score, the more opportunities for improvement. The scores are an aggregate of several factors (field properties and erosion, nutrient applications, tillage, and pest management) and subcomponents (drainage, irrigation, and conservation practices), which allow farmers and users to identify the main opportunities for improvement.
Stewardship Tool for Environmental Performance
Another water quality tool offered by USDA-NRCS is the Stewardship Tool for Environmental Performance (STEP). The tool assesses a field’s risk for nitrogen and phosphorus loss through surface runoff and leaching while offering insights into how effectively the applied management practices are mitigating that risk. Although STEP is not designed to predict quantitative levels of contaminant loss, it is extremely useful for supporting conservation management decisions that are appropriate for the natural resource concerns of each site.
Calculations for STEP are computed in three main phases:
- Water quality sensitivity rating. Each field has characteristics that influence how likely it is to lose nutrients. These characteristics include soil leaching potential and soil runoff potential—which are largely impacted by slope, water table depth, and hydrologic soil group (Table 1)—and rainfall intensity and irrigation. For fields with diverse soils, STEP conducts an area-weighting of the major soils present to determine the soil leaching and runoff potential across the field as a whole unit.
- Risk mitigation. The second phase of STEP calculations evaluates field practices (crop rotation, tillage, irrigation, and nutrient management) to determine the level of risk mitigation (RM) achieved by the actual field management system. The tool evaluates a full rotation (typically over multiple years), including any fallow periods or winter cover crops, and requires management data for each crop in the rotation period. The STEP scores will be as accurate as the crop rotation entered into the model. Table 2 includes an example of a two-year crop rotation.
Table 1. Characteristics of soil hydrologic groups
| Soil hydrologic group | ||||
|---|---|---|---|---|
| A | B | C | D | |
| Sand % | >90% | 50% – 90% | <50% | <50% |
| Clay % | <10% | 10% – 20% | 20% – 40% | >40% |
| Runoff potential | low | moderately low | high when wet | high |
| Leaching potential | high | high | low | very low |
Table 2. Example of a winter wheat–peanut rotation
| Operation name | Year | Day | Crop |
|---|---|---|---|
| Fertilizer application surface broadcast | 1 | Oct. 10 | |
| Drill or air seeder, single disk openers 7-10 in spacing | 1 | Nov. 1 | Wheat, winter, grain |
| Sprayer, postemergence | 1 | Dec. 15 | |
| Sprayer, postemergence | 2 | Jan. 15 | |
| Sprayer, postemergence | 2 | Apr. 15 | |
| Harvest, killing crop 50% standing stubble | 2 | June 1 | |
| Planter, in-row subsoiler low disturb | 2 | June 10 | Peanut, runner |
| Sprayer, postemergence | 2 | June 20 | |
| Sprayer, postemergence | 2 | July 15 | |
| Sprayer, postemergence | 2 | Aug. 15 | |
| Harvest, shallow root crops, residue surface | 2 | Sept. 15 |
Field practices included in the STEP calculation are listed below:
- Tillage: timing and level of tillage is used to determine the amount of residue on the field.
- Nutrient management: rate, form, timing, and application method (4R nutrient stewardship), precision ag, N stabilizers, etc.
- Crop type is used to determine the N ratio (N removed in crop harvest)
- Conservation practices (a sample is described below)
3. Site vulnerability and risk mitigation score. The calculations in Phases 1 and 2 will then provide a site vulnerability and risk mitigation score for each of the nutrient loss pathways (Table 3). The STEP tool considers the relative points of risk mitigation achieved compared with the sensitivity rating for that field in the conservation-planning process with individual producers.
Table 3. Summary of the risk and mitigation factors for the four nutrient loss pathways assessed by STEP
| Loss pathway | Description | Risk factors | Mitigation factors |
|---|---|---|---|
| Surface N | Mobile nitrate (NO3–) dissolves in soil water, readily running off fields, into tile drainage and leaching through soil profile to groundwater |
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| Sub-surface N | Same as above |
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| Surface P | Phosphate attached to sediment leaves the field via wind and water erosion. It should be noted that it takes much less phosphorus to cause environmental harm than nitrogen. |
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| Sub-surface P | Dissolved phosphate leaches through the soil profile into groundwater |
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Conservation Practices to Improve the Quality of Water Leaving Farms
The following are several edge-of-field practices that can be implemented to reduce the amount of sediment, nutrients, and crop protectants entering waterways from agricultural fields (STEP recognizes more than 35 unique conservation practices):
- Sediment basins—water-holding structures that intercept runoff at the field edge to trap soil and attached crop inputs before entering surface water
- Riparian forest buffer—trees and shrubs planted along waterways to intercept sediment and crop inputs
- Filter strip—herbaceous cover planted along waterways to intercept sediment and crop inputs
- Grassed waterway—channel planted with grasses to slow water movement and reduce gully erosion
- Tailwater recovery system—captures and reuses irrigation water
- Strip cropping—alternating strips of erosion-prone crops with strips of equal size of erosion-resistant crops to reduce erosion
- Drainage water management system—adjusts the volume of drainage water and the elevation of the water table
- Riparian herbaceous cover—forbs, grasses, ferns, and other herbaceous plants that tolerate saturated soils planted along waterways to protect banks against erosion, trap sediment and crop inputs running off of fields, and provide forage and habitat for wildlife
- Bioreactors—tile drainage is directed to woodchips or other organic substrate buried in a trench where microorganisms living on the substrate denitrify nitrate in the drainage water before it is released into waterways.
Management Techniques
Along with conservation practices, management techniques are based on avoiding the need for adding nutrients, controlling nutrient losses, and trapping nutrients before they leach out of the rootzone or leave the field boundaries.
Management techniques for nitrogen include the use of nitrification and urease inhibitors and slow-release fertilizers and adjusting rates based on tests such as the corn stalk nitrate test, fall soil nitrate test, or pre-sidedress nitrogen test, among others.
Management techniques for phosphorus include drawdown strategies such as reducing fertilizer P inputs combined with harvest removal and removal of cover crops. Both nitrogen and phosphorus fertilizer applications receive credit when applied with precision agriculture equipment.
Compounding Effects of Conservation Practices
Farms are often considered miniature ecosystems unto themselves with their complex interactions between living organisms and the various components of the physical environment, like soil, air, and water. Interventions of any sort, such as field prep, pest management, or harvest, will likely have cascading or compounding effects on those interactions and the resulting environmental impacts. Practices that support good water quality outcomes from agriculture can also have significant positive effects on biodiversity, energy use, greenhouse gas emissions, irrigation water use, soil carbon, and soil conservation.
Dig deeper
Barrata, B. (2020). Deepening Lower Mississippi River to enhance key link in Iowa soybean farmers’ supply chain. Iowa Soybean Association. https://bit.ly/2SSKv2n
Office of the Governor. (2020). Mississippi River deepening from 45 to 50 feet authorized. https://bit.ly/35MouoG
Thaler, E.A., Larsen, J., & Yiu, Q. (2021). The extent of soil loss across the US Corn Belt. PNAS, 118(8), e1922375118. https://doi.org/10.1073/pnas.1922375118
Ward, M.H., deKok, T.M., Levallois, P., Brender, J., Gulis, G., Nolan, B.T., & VanDerslice, J. (2005). Drinking-water nitrate and health—recent findings and research needs. Environmental Health Perspectives, 113(11), 1607–1614.
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