Agriculture Reference
In-Depth Information
selves and by taking up a lot of the water that would otherwise be heading back to the wa-
ter 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 of 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 con-
tent 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 in-
flux 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 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 mi-
crobial biomass. Without plants, many microbes will go to sleep, but if we keep plant cov-
er on the soil they'll keep working, even through winter, albeit more slowly. This is espe-
cially 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-sudan-
grass does the same in the summer. The chemicals can inhibit germination of your veget-
able 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 di-
versity 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, orch-
ardgrass and crimson clover may attract armyworms. This is one of 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
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