This is the second in a series of posts that explore the world of cover crops. Much gratitude, again, to Mallory Daily for her time and excellent feedback in editing this post!
“Realize that everything connects to everything else.”
-Leonardo Da Vinci
These words ring true across experiences, but a particularly tangible expression can be found in the cycling of water throughout the Earth. The rain, the ice caps to the north and south, the rivers and lakes, the oceans, and the sweat on our skin are united in their aqueous nature. Astoundingly strong and transient, water is an absolute necessity to life. Many of our childhood memories contain an element of water: that rainstorm you danced in after your last day of high school, the swim lessons you splashed your way through, perhaps an unaddressed fear of being dunked by a pesky sibling.
The presence of water colors my childhood memories with laughter, wonder, and serenity. I grew up traveling to Bull Shoals Lake with my family every other weekend in the summer. Bull Shoals lies on the southern border of Missouri, with the majority of the lake resting in Arkansas. To the northwest, Bull Shoals is fed by Lake Taneycomo and Table Rock Lake, stretching up towards Branson, Missouri. To the southeast, the dam of Bull Shoals releases the lake into the White River, which winds its way across Arkansas, eventually spilling into the Mississippi River.
As a child, I didn’t realize that Bull Shoals was connected to so many geographies, not even while watching the rain, a demonstration of the cycling of Earth’s water, splash against its surface. I respected it, I was awed by it, but I never thought of the lake as more than a vacation destination. Then, one day it started to make sense. As a teenager, my family drove for one hour across the lake to the White River dam in Arkansas. I remember walking down to the river with my father and dipping my fingers into the water. It was as cold as ice, and moving so fast. I looked up at him, wondering how water from Bull Shoals could’ve ended up here.
I started thinking about how the water of the lake might be impacted by the land surrounding it and the water flowing into it. And about how the water of Bull Shoals Lake might impact the places downstream of its dam. I had previously thought about fish and insects near my swimming domain, but it never occurred to me to think about what else might be in that water.
Several years later, I traveled to Costa Rica to live and work at Finca Luna Nueva – an eco-lodge and biodynamic farm in the rainforest of San Carlos. There were bountiful opportunities to be an adventurer in this place, a land ripe with exciting landscapes and natural treasures. One day, several of my friends wanted to go swimming in a river about 15 minutes from our home base. When discussing this in front of the owner of Finca Luna, I couldn’t help but be surprised by his objection. He assertively stated that we should think twice about swimming there. The river was lined by farms where pesticides and manure were inappropriately managed. Even by just swimming in this water, he said we could be exposed to chemical or bacterial pollutants on contact.
I heeded his advice, and forwent the adventure of swimming in that river. But when I returned to Missouri at the end of that summer and lake season was in full swing, I started to think about Bull Shoals Lake in the way that the owner of Finca Luna had thought about that river in Costa Rica. What was in the water I’d been playing in all my life? How was land managed around and upstream from the lake? What sort of impact did this have on me, a biotic being with porous skin so vulnerable to pollution?
Water quality is of huge concern to global populations, whether its a Central American river or a Missouri lake. In 1972, the U.S. government instituted the Clean Water Act to protect the waters of the country from pollution. Although regulatory measures like this one are taken to ensure water remains of livable quality, pollution is still a constant issue for local and federal governments to address.
I introduced the problem of water quality issues by briefly discussing erosion in my first post about cover crops. In this post, I will continue exploring erosion and other environmental themes to better understand the ways that cover crops can help steward the land. Cover crops can provide us with a host of “ecosystem services,” without which we would struggle to survive. As we walk through these ideas, think about your own experiences with the environment. Maybe you didn’t grow up visiting a lake, but instead enjoyed spotting butterflies in your yard or watching vegetables grow in your parents’ garden. Think of these experiences as we learn about the way the environment is integrally connected to human management decisions. We should not take those decisions lightly, and implementing cover crops is definitely a step in the right direction. In the words that follow, we’ll explore the ways that cover crops impact water quality, biodiversity, soil health, and greenhouse gases.
Let’s begin in a familiar landscape. Although many environmental factors impact water quality, the most prominent to me is erosion. If you recall from my first post about cover crops, sediment displaced by erosion is the most common pollutant in waters across the United States.
This is bad news. Sediment is a broad term that can imply many things, like debris from urban areas (carrying urban chemical pollutants, like oil and fluids from cars) and soil from farm fields. We can see why the presence of soil in rivers and oceans is severely lamentable, because that’s the soil we need to produce food. But why might the presence of farm soil in water be a threat to its overall quality?
For one, if we have a high volume of topsoil entering waterways that are used for commerce, that soil will eventually settle to the floor of those waterways. That makes it harder for barges with goods to pass through. For many government entities, like the Army Corps of Engineers, this presents a problem that can only be solved by dredging. Dredging is the mechanical process of removing sediment from stream beds and banks to clear a path for barges, and requires money and energy to complete.
According to figures from 2012, the average rate of erosion from cropland in Missouri was 5.64 tons per acre per year (National Resources Inventory, 2012). Given that there were about 10.7 million acres of cropland in Missouri in 2012, let’s do the math.
5.64 tons/acre/year * 10.7 million acres * 1 year = 60.3 million tons
About 60.3 million tons of sediment from cropland alone was displaced by erosion in 2012 in Missouri. If we assume that a dump truck can hold up to 25 tons of material, this amount of sediment would require about 2 million dump trucks to move. Now, sure, not all of this sediment was transported to streams and rivers, and certain cropland is better managed than others, but on scales such as this, even knowing that erosion rates can occur at 5.64 tons/acre/year (that’s about 5 dump trucks per 5 acres of cropland), the pollution from erosion becomes impossible to ignore.
What’s more, when soil erodes it drags along nutrients, like Nitrogen (N) and Phosphorus (P), and chemicals from pesticides. When these substances enter water, they are introduced to the ecosystems of aquatic organisms. Like humans, aquatic organisms require specific conditions to survive. Unfortunately, too much N, P, or pesticides offset the ecosystems in which these organisms live and cause untimely deaths. A real-world example of this can be seen at the outlet of the Mississippi River, in the Gulf of Mexico. Nutrients that have run off from the agricultural fields surrounding the Mississippi River have traveled thousands of miles to the Gulf. These nutrients are then consumed by algae, and as algal populations bloom because of the sudden increase in their dinner options, they also consume the oxygen in the water, essentially suffocating other organisms that depend on that oxygen to survive, like fish and shellfish. This is a phenomenon known as hypoxia, and it is so dire that the EPA formed a Hypoxia Task Force in 1997 that still focuses on re-oxygenating the Gulf of Mexico today.
Another program, targeted at reducing pollution in Madison, Wisconsin, will pay farmers and municipal water system operators $140 million over the course of 20 years to reduce phosphorus pollution of the surrounding waters . This pollution, if allowed to continue, would cost taxpayers about $270 million in the same time frame. Steven Verburg wrote an article about this program, stating that leaders of the local community feel that:
“… more and more farmers are seeing themselves as stewards both of the land and the water.”
Fortunately, cover crops have a lot to contribute in the national effort to halt erosion. The reason is quite simple: by covering the ground for the 6-8 months of the year it would otherwise lie fallow, cover crops keep the soil in place. Their root systems develop and stabilize the soil, while their stems and leaves act as a shelter, protecting soil particles from wind and rain. Some species of cover crops, like cereal rye and oilseed radish, are especially good at scavenging nutrients and improving nutrient cycling within the soil. This prevents nutrients like N from escaping farm fields during erosive events. Studies have shown that:
- Cover crops, such as ryegrass, reduce rates of erosion by up to 78-99% (Sharpley et al. 1991)
- Oats, rye, and wheat, when used as cover crops, reduce nitrate losses (a result of N fertilizer application) from farms by 13-94% (Kladivko et al. 2014)
If cover crops were human, they’d certainly be considered superheroes for accomplishing feats such as these.
Biodiversity & Soil Health
As cover crops shelter the soil, they also provide a habitat for beneficial insects, like bees and butterflies. Bees are well known for their critical role in pollinating our crops, but some insects may have functions besides pollination – such as the Lady Beetle, who can control many aphid pests that might attack crops (Clark, A. 2012, p. 28). Cover crops provide a habitat for these beneficial critters, and this can increase the diversity of our farming systems.
Much like the social climate of today’s world, we need to be talking about diversity in farming systems and soils, too. And that’s because a diverse soil is a healthy soil. In fact, just one teaspoon of a healthy soil is home to billions of microorganisms.
What are all of these little organisms doing in there? They are living – making shelters by borrowing tunnels into the soil, eating food and leaving their digested food behind, reproducing, transporting nutrients and water from place to place. All of these tasks are super important to help humans and larger critters have healthy, productive soils that will grow our food. For instance, when earthworms tunnel through the soil, they’re creating tiny channels and openings for water to enter. And when they poop, they’re excreting some pretty nutrient-rich fertilizer for other little microbes and plant cells to use.
Life is fascinating, and none more so than the diverse ecosystems we can’t physically see, though we can certainly perceive their impact.
So, when we plant cover crops, we are enabling these ecosystems to live all year long, and not just when we are planting cash crops. Some soil health experts think of it this way… We have about one ton of bacteria in just one acre of soil, and that’s the equivalent weight of two cows. So if we start to think about how we’re going to keep those cows alive all year long, we know that we’ve got to give them something to eat, or else they’ll starve! In the same way, we need to be feeding our soil populations all year long.
Earlier I mentioned that earthworms plow through the soil. This is an example of one small way to reduce soil compaction, which is an important issue on many farms. The soil becomes compacted over time as heavy farm machinery (like tractors and combines), repeatedly pass over fields throughout the season. As soil compacts, the pores within the soil where air and water reside disappear. This prevents root systems from growing large and strong, and it also prevents water from entering into the soil layer.
To combat compaction many farmers choose to till their fields, which will loosen soil to about 30 cm below ground. Because tilling reduces compaction in the surface layer, planting crops becomes much easier. But this method is a short-term solution. Over time, the tilled soil becomes heavily eroded. It also breaks up all of the incredible networks formed by microorganisms.
It’s a good thing, then, that cover crops are so great at preventing soil compaction. Particular species, like the oilseed radish, have long roots that will grow deep into the soil (at least two feet deep in some situations), and act like earthworms to create channels for air and water to enter.
In fact, the oilseed radish excels so much at relieving compaction that it was planted underneath the St. Louis arch in the fall of 2015. Renovations occurring at the park around the arch prompted the project team to look for cost effective ways to revive the soil, so that it could support the trees that will be planted there soon. A reporter from the St. Louis Post Dispatch wrote about the project:
Sotillo called the effort an “unbelievable success.” He said he had convinced New York’s Central Park to do something similar, thanks to the work here. “They were completely sold,” he said.
It turns out the benefits of cover crops aren’t limited just to the farm setting. Our superheroes can efficiently and economically revive soil health for many different sectors of society!
By relieving the compaction of the soil, cover crops also allow for more rainfall to penetrate the soil layer. This is significant because with each rain event, a farmer is given natural, free access to water resources. In order to maximize these resources, it’s best if the water is given paths to enter the soil, rather than running off the surface (which contributes to the problem of erosion). Studies have shown that cover crops can increase rainfall infiltration to the soil layer by 37 – 147% (Folorunso et al. 1992).
And we’re not done yet! Cover crops also work to add organic matter to the soil. Organic matter is living material (plant leaves, stems, roots, and organisms) that is introduced to the soil system and will decompose over time. Organic matter adds nutrients to our soils, but that’s not all. It also acts like a sponge in our soils, helping to store more water at any given time. In fact, just a one percent increase in organic matter in one acre of soil can allow the storage of an additional 27,000 gallons of water. On a national level, if we increased the soil organic matter on all of our cropland by one percent, we would have the capacity to store an additional amount of water equivalent to the flow over Niagara Falls for 150 days!
Climate Change & Greenhouse Gases
Another benefit of increasing the amount of organic matter present in our soil is that organic matter is comprised of carbon. As plants grow, they take carbon from the atmosphere and convert it to sugars in their cells. This carbon is then stored within the plant, and as the plant decomposes, the carbon can enter the soil layer. This is a form of “biotic carbon sequestration,” and has the potential to sequester significant amounts of carbon from our atmosphere, thus mitigating some of the impacts of climate change. According to Poeplau et al. (2015), the cultivation of cover crops globally has the potential to sequester about 0.12 Pg of carbon each year, which would offset about 8 percent of the annual greenhouse gas (GHG) emissions from agriculture.
But carbon isn’t the only way that cover crops can offset greenhouse gas emissions. Other sources of GHG in agriculture come from the practice of using N fertilizer. Every step of the way, fertilizer releases GHGs into the atmosphere. It takes energy to create that fertilizer in N-fixing factories, to transport it to farms, and to fuel the tractors that apply it to the earth. This process doesn’t just happen with fertilizer, but with the production and application of all pesticides. Not to mention, when N fertilizer is applied to fields, any excess fertilizer is likely to transform to nitrous oxide, a greenhouse gas. Fortunately, certain species of cover crops (legumes like crimson clover), are natural N-fixers, so they can pull N from the atmosphere and replenish stores of N within the soil, thus offsetting the need of the farmer to purchase and apply N fertilizer. In fact, according to the Sustainable Agriculture Research and Education (SARE) Program in Managing Cover Crops Profitably:
“Legumes can contribute from 15 to 200 lb. N per acre to the subsequent crop, with typical values of 50 to 100 lb. N per acre.”
The average amount of N fertilizer applied to corn crops in Missouri in 2010 was 126 lb. per acre (“Fertilizer Use and Price”, USDA-ERS) . Legume cover crops have the potential to partially or complete offset the N fertilizer required to grow corn!
By sequestering carbon into the soil and offsetting the need to use chemically produced N fertilizers and pesticides, cover crops can contribute to global goals of reducing greenhouse gas emissions and mitigating the impacts of climate change. All of the amazing ecosystem services provided by cover crops, such as erosion prevention, enhanced, soil health and increased water infiltration, will help farmers adapt to a changing climate, making their farms resilient in the face of extreme weather events.
Cover crops are the guardians of our soil. They are our ecosystem superheroes destined to help us through tough times. Enlisting their services in the years to come will help provide clean water, and I can’t complain about that.
Next week, continue reading as I explore the economic implications of using cover crops.
Thanks for reading, until next time!
Clark, A. (2012). Managing Cover Crops Profitably, 3rd edition. Sustainable Agriculture Research and Education (SARE).
Folorunso, O. A., Rolston, D. E., Prichard, T. P., & Louie, D. T. (1992). Cover crops lower soil surface strength, may improve soil permeability. California Agriculture 46(6):26-27. Web. Retrieved February 18, 2016 from: http://californiaagriculture.ucanr.org/landingpage.cfm?article=ca.v046n06p26&fulltext=yes
Kladivko, E. J., Kaspar, T. C., Jaynes, D. B., Malone, R. W., Singer, J., Morin, X. K., & Searchinger, T. (2014). Cover crops in the upper Midwestern United States: potential adoption and reduction of nitrate leaching in the Mississippi River Basin. Journal of Soil & Water Conservation 69(4): 279-291. Ankeny, IA: Soil and Water Conservation Society.
Poeplau, C. & Don, A. (2015). Carbon sequestration in agricultural soils via cultivation of cover crops – a meta-analysis. Agriculture, Ecosystems & Environment 200: 33-41.
Sharpley, A. N., & Smith, S. J. (1991) Effects of cover crops on surface water quality. Cover Crops for Clean Water: 41-49. Ankeny, IA: Soil and Water Conservation Society.