Peering into Murky Waters: A Look at Nutrient Pollution and Soil Erosion

Figure 1: A Gulley forming in a plowed cornfield.

Figure 1: A Gulley forming in a plowed cornfield. Photo courtesy of Corn+Soybean Digest

When we hear about water pollution, we think of factories spilling toxic waste into streams. Maybe you picture green sludge poured out of a barrel with a skull and crossbones on it. However, this is not the biggest issue we face. A water pollution source that is often unconsidered is our agricultural fields. I’m talking about nutrient pollution and soil erosion, which occur when fertilizers and our precious top soil wash out of our fields and enter our streams, lakes and rivers.

Let’s start with erosion. Row crops and constant tilling leave soil exposed to wind and water. Fields with gullies running through them and areas where the topsoil is gone and the reddish subsoil is exposed are sure signs that erosion is happening (Figure 1).

Erosion leads to sedimentation of our waterways. Besides losing the limited top soil that we depend on, an increase of sediment in the water means decreased amounts of sunlight in the water. This reduces growth of our native aquatic plants; plants that many species depend on for food, shelter and reproduction. It also clogs up fish gills and the feeding tubes and gills of clams and mussels (Figure 2).

Figure 2 Freshwater mussel in stream.

Figure 2 Freshwater mussel in stream. Photo courtesy of Prepare and Protect

Tile systems also cause problems with erosion. When it rains, tiles drain that water fast and shoot out into drainage systems and streams. This fast flowing water leads to water levels that rise and fall quickly instead of slowing filtering through the soil into the water table. The fast flow of water out of tiles leads to the undercutting of stream banks and increased erosion (Figure 3). All this eroded soil has to go somewhere, and some of it ends up being deposited in the floodplain during a flood. Some sediment deposit is good, but getting 2-4 inches of mud can inhibit native plants. A lot of this sediment ends up deposited and causing problems in the fragile estuaries of the Gulf of Mexico.

Figure 2 Example of different types of bank erosion.

Figure 2: Example of different types of bank erosion. Photo courtesy of Stream Tender

We do our best to harvest as much as possible out of our fields, but with our current practices, this requires a lot of phosphorous and nitrogen (ammonium) fertilizer. Unfortunately, the ammonium quickly converts to nitrate which dissolves into water. It then gets washed away with rain into tiles and into our water. Phosphorous binds to soil and is lost through erosion. When these nutrients get into our water at high levels, they cause problems like algae blooms. These blooms can then lead to decreased oxygen in the water. This can be so severe in places, like the Gulf of Mexico, that it causes a dead zone. It has so little oxygen that nothing can survive. The Gulf of Mexico Dead Zone is expected to be approximately 5,483 square miles this year (Figure 4)1. That is about the size of Connecticut.

Figure 3 The Gulf of Mexico Dead Zone.

Figure 3: The Gulf of Mexico Dead Zone. Image courtesy of NOAA

You may be wondering how algae blooms lead to lower oxygen. Although plants like algae produce oxygen, it’s what happens afterwards that’s important. During the spring rains, high levels of nutrients enter the water. This is when the algae blooms happen, but when nutrient levels drop, the algae can’t survive. They die and are decomposed by bacteria which use up the oxygen.

These are some big problems, but scientists and farmers are working together to find solutions. One way farmers are helping prevent erosion is using buffer strips and grassy waterways in their fields. They leave areas that are prone to gullies untilled and plant grasses to slow water. They also keep wide strips of land along the edge of the field unplowed. Both of these help keep soil and nutrients on the fields where they belong. However, tiles carry water and nitrates under these buffers and into streams. One way to help prevent this is tile water management.

Figure 5 Drainage Water Management System.

Figure 5: Drainage Water Management System. Image courtesy of Purdue University

This is the idea of installing devices that hold tile water back in reservoirs that will can help remove the nitrogen and also keep water for dry periods (Figure 5). Constructed wetlands also help with erosion and nutrient loss (Figure 6). They can create wildlife habitat, remove nitrogen from the water, and slow water flow into streams which allows sediment to settle out. Groups like Illinois Natural History Survey, Army Corps of Engineers, Nature Conservancy, and several University researchers all work with constructed wetlands and experiment with how they can benefit our farming system.

Figure 6 Constructed wetland within an agriculture system.

Figure 6: Constructed wetland within an agriculture system. Photo courtesy of Colorado State

There is also a lot of promising results coming out of using no till and cover crop practices. These two practices help prevent a loss of topsoil, and scientists are finding that good soil conservation practices decreases the need for heavy fertilizer use.

If you are interested in this subject or want to get involved, there are a few ways you can do that. First, there is the CRP and CREP programs that partner with farmers to increase conservation. To read more about these and other programs head to the USDA’s Conversation Programs website.

You can also check out the Delta Institute’s site for even more information. This is a fascinating organization that I have just recently learned about. They are doing great things when it comes to working with farmers in a way that benefits everyone. You can also attend the ISRS Biennial Symposium, which has public events on Sunday, August 23 in La Crosse, Wisconsin.

If you are interested in helping scientists monitor water quality across the state, you can join the RiverWatch team. For more information contact Matthew Young of The National Great Rivers Research and Education Center (NGRREC℠) at (618) 468-2784.

There is a lot of work and progress happening in this subject, and I think it will continue to expand as we learn more about how we can balance our agriculture and our environment.

Check in again on August 21 to learn about urbanization and our water in our next blog post.

  1. http://water.epa.gov/type/watersheds/named/msbasin/zone.cfm
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