by Phil Nauta
The following is an excerpt from the book, Building Soils Naturally: Innovative Methods for Organic Gardeners, and is reprinted with permission from the publisher.
Cover crops are traditionally thought of as plants used when the garden or field is empty, such as over winter and sometimes over summer, but I’d like to broaden this definition to also include plants used during the growing season, inter-uplanted with food crops or even in ornamental beds where they’re sometimes called groundcovers or living mulch. They’re also referred to as green manures, generally when they’re going to be incorporated into the soil after a certain period of growth.
A good goal is to make sure your soil is always covered with plants, and cover crops help achieve this goal. Some soil experts consider this more important than composting. It’s certainly more natural, and more feasible in large gardens and on farms where it’s difficult to make enough compost to cover everything, let alone distribute it.
The main reasons home gardeners use cover crops are to improve soil fertility, increase organic matter and control weeds and plant predators. There are also many other uses, such as to prevent erosion, send roots deep to break up compaction, conserve moisture and increase water infiltration, attract insects and other animals, and even provide food for humans and animals. Let’s look at the three main reasons.
Cover crops improve soil fertility in a couple of ways. The main benefit most people think of is the increase in nitrogen we can get from planting legumes, such as clover. Many leguminous plants and a few others form symbiotic relationships with nitrogen-fixing bacteria such as Rhizobia and Bradyrhizobium. The plant provides the bacteria with a home on its roots, as well as food. The bacteria give some nitrogen to the plant in return. Some of that nitrogen ends up in the soil when the plant is alive, adn more when it eventually decays. I’ve read the 22 lbs. of fixed nitrogen is as good as applying 220 lbs. of ammonium sulfate or 1,100 lbs. of sulfate of ammonia because the fixed nitrogen is efficiently used as it’s produced, whereas most of the chemical ammonia that is applied is leached and volatized.
Fertility is also improved by many cover crop plants that send deep roots into the soil and harvest minerals that aren’t reached by other plants. Even if they don’t send deep roots, just being there is hugely beneficial because many of the nutrients would otherwise leach. Cover crops can stop a lot of this leaching both by taking up some of the nutrients themselves and by taking up a lot of the water that would otherwise be heading back to the water table, bringing nutrients with it.
Nitrate nitrogen is a prime example of a leachable nutrient because it’s water soluble, so it doesn’t stay around for long in the soil when there’s a lot fo water moving through. While legumes fix the nitrogen, grasses and brassicas are much better than legumes at stopping it from leaching. Many other nutrients such as calcium and potassium can also leach without plants and organic matter to hang on to them.
The next big benefit is organic matter. Any plant will increase the organic matter content of the soil simply by the dropping of leaves (including evergreens) and the constant growth and death of roots, but cover crops are often turned into the soil to give a huge influx of biomass to it. Less talked about, but potentially more important, plants send a huge amount of carbon –– as well as protein, amino acids and thousands of other substances –– into the the soil as exudates. Grasses are especially adept at quickly getting big and building soil. For example, cereal rye adds large amounts of organic matter, sorghum-sudangrass sends deep roots to break up compaction, and annual ryegrass is great for stabilizing and drying out wet soil.
Along with the addition of plant biomass is the increase, or at least maintenance, of microbial biomass. Without plants, many microbes will go to sleep, but if we keep plant cover on the soil they’ll keep working, even through winter, albeit more slowly. This is especially important for mycorrhizal fungi, who need a host to stay active.
And then there is weed and pest control. Plants accomplish weed control by several mechanisms –– competing for water and nutrients, shading out the soil, crowding out the soil below ground with their roots, and sending out chemicals to inhibit other plants from growing. Cereal rye is an overwintering crop that controls weeds both physically and chemically by producing compounds that are toxic to many other plants. Sorghum-sudangrass does the same in the summer. The chemicals can inhibit germination fo your vegetable seeds –– a process called allelopathy –– so be sure to wait two to three weeks after incorporating the cover crop before seeding.
Plant predator control is achieved mainly by the cover crops inviting and hosting a diversity of microorganisms, nematodes and insects that keep the system more in balance, as well as producing their own antibacterial compounds. Some people worry about the cover crop crowding out their other plants. It’s possible, but I like to have them touching so the beneficials climbing on the cover crop will move over to my other plants, too.
It’s also true that cover crops can attract some predators. For example, cereal rye, orchardgrass and crimson clover may attract armyworms. This is one fo the reasons I like to plant a combination of two or three cover crops, to create more diversity, decreasing the chance of one insect causing problems.
Some cover crop benefits, such as organic matter or fertility improvement, are only substantially noticeable after a couple of years of cover cropping, while others such as weed control adn insect attraction are noticeable right away. I have my dad cover cropping his entire tree nursery with a mixture of grasses and legumes. It’s work to keep it mowed, but it’s slowly increasing organic matter, improving soil structure and fertility, controlling erosion and attracting insects and other small animals.
Cover crop selection is an involved decision for farmers and gardeners, who have to decide which of the above benefits are most important and select appropriate crops based on that, as well as choosing crops that are going to work in their climate conditions. We gardeners can spend a lot of time making this decision, but for most of us, I believe it’s more important to just get something in the ground that will satisfy the main goals of increasing organic matter and soil fertility. We can also enjoy the by-products of this choice, including some weed control, and we can always experiment with some different crops to see what we like best.