by Sally Colby
Using cover crops isn’t a new concept for most farmers – many have adopted the practice for a variety of reasons. For organic farmers, cover crops can mean the difference between so-so yields and a tidy profit.
Julie Fine, who earned a master’s degree in plant biology at UMass-Amherst, pointed out the value of cover crops and offered suggestions about selecting the most effective cover crop combinations.
“The heart of it is organic matter,” said Fine. “Without sufficient organic matter, there isn’t enough food for the thousands of pounds of organisms that work the soil, cycling nutrients, building soil aggregates and reducing populations of negative bacteria.” Fine added that an increase in organic matter of just 1% to 3% can reduce erosion by 33%.
Each percentage increase in soil organic matter (SOM) can also increase the number of gallons of plant-available water, which translates to higher crop yields. Building SOM is essential to healthy soil, and cover crops can help boost SOM levels.
“One of the major benefits of cover crops is living roots,” said Fine. “Root residues have the largest effect on SOM. Fifty percent of SOM is provided by root carbon, and only 13% from shoot biomass.”
Fine referenced NRCS’s best practices for soil health: cover the soil, feed the soil, reduce tillage and cultivate diversity. Fine explained that cultivating diversity means crop rotation for cash crops as well as cover crops.
“There are different relationships between peas and oats in terms of which soil bacteria are attracted to plant roots in the soil,” said Fine. Varying cover crops provides more diversity in what’s available to the soil biome. “Root architecture has a big influence – deep roots move carbon much deeper into the soil. Branching roots and surface roots do a lot to feed the more active population of soil bacteria at the soil surface.”
Fine’s graduate research focused on cover crop mixes to achieve maximum benefits. One finding was that erosion control improved with a mix of rye and vetch compared to vetch alone. “By using different species, you attract different soil biological populations,” she said. “There’s different capacity for nutrient scavenging in soil based on root architecture or plant requirement.” Cover crops have varying levels of weed suppression ability, and in many cases, cover crop combinations improve weed suppression.
Quantity and quality are the main factors to consider when designing a cover crop mix for soil health. “If you want to build SOM, you have to put down a fair amount of above-ground soil biomass every year – about 5,000 pounds per acre per year to build organic matter above current levels,” said Fine. “It can be a challenge to manage that amount of biomass.”
The quality of the biomass influences soil health. Above-ground biomass can contain high levels lignin. Fine said rye persists with a tough, almost straw-like residue by the time it’s tilled under in spring. Conversely, cover crop residue can be light in structure with high levels of cellulose. The goal is to determine which species can provide a balance of lignin and cellulose as well as the appropriate carbon to nitrogen (C:N) balance.
The C:N balance is important because soil microorganisms are decomposing the material and making it available to plants. The goal is to achieve a ratio in which microorganisms will thrive, which Fine said is a ratio of around 24:1.
The C:N ratio in a hairy vetch cover crop is 11:1, and will decompose fairly quickly – perhaps more quickly than crops can take it up. A rye cover crop has a lot of carbon and will take a long time to break down. “Meanwhile, the nitrogen is immobilized in the cover crop tissue and ties it up,” said Fine. “That’s a good reason to use a combination.”
Cover crop mixes should be selected based on commonly used cover crops in a particular growing region along with goals such as soil cover, weed suppression, biomass production and nitrogen scavenging.
Consider one commonly used combination – vetch and rye. Vetch has highly efficient nitrogen fixation, prostrate growth habit, low biomass, rapid decomposition in spring and low C:N. Rye doesn’t fix nitrogen but is a good nitrogen scavenger, grows vigorously, produces high biomass volume and grows rapidly in autumn. Vetch uses the rye as a scaffold support and rye uses nitrogen fixed by vetch, and both are terminated at the same time in spring.
Fine suggested designing a cover crop mix by first identifying the top three services desired from a cover crop. “For most of us that would be ground cover to protect the soil, nitrogen scavenging and biomass production,” she said. “Then figure out when you’re going to plant it, how you’re going to terminate it and when.” Select a group of cover crops that will perform the desired services in the area’s growing window, then narrow down the options.
Make sure there’s complementarity among the species in the cover crop mix, including above-ground growth habit and a variety of root types. Once species are selected, determine any potential drawbacks from a species and drop that from the mix.
Fine said there’s a significant difference between broadcasting and drilling seeding rates. For cover crops that are broadcast, increase the drilling rate by 50%. One challenge with seeding cover crop mixes is setting the planter for a variety of seed sizes. In this case, alternate row seeding is an option because the drill can be reset for precise, accurate seeding of each species.
“Winter-hardy grasses should be reduced to one-quarter to one-half of the monoculture rate,” said Fine, adding that rye should be reduced to 20% of monoculture rate. “Reduce the rate of any species that are functionally redundant – if you have annual ryegrass and triticale, divide the recommended seeding rate for each species by two. That reduction is done after you halve the rate for winter-hardy grasses.”
Some final suggestions for cover crops: Carefully determine the goals for cover crops and manage/adjust soil pH. If the goal is to cycle nutrients, soil tests should indicate those nutrients (nitrogen, boron, sulfur) are present to begin with. “Reduce the amount or intensity of tillage,” said Fine. “And don’t plant too much rye.”