Cover Crops: Feed your soil; feed your plants.
One of the easiest and most economical ways to improve your soil is to plant green manures, commonly called cover crops. Most garden soils can be maintained at their highest level of productivity by sound soil management practices that involve a combination of soil tillage, crop rotation, and most importantly, the addition of organic matter through green manures.
Organic matter is the food component of soil. Soil-dwelling fungi and bacteria work to break down organic matter. When these soil microorganisms eat organic matter, nutrients are released back into the soil in a form that is usable by plants. This process is called nutrient cycling. The addition of organic matter builds soil structure, which increases water absorption and nutrient-holding capacity, buffers the soil pH, and improves aeration.
Cover crops choke out weeds by restricting sunlight to the soil, stabilize the soil surface, and through their deep-reaching roots, help to break up hardpan and bring minerals to the surface for other plants to utilize. As part of a long-term rotation plan, cover crops can provide a stable habitat within your garden for beneficial insects and microorganisms.
Here is a breakdown of some of the main types of cover crops:
Legume Cover Crops
Growing legumes and incorporating them back into the soil increases the organic carbon content, and improves the soil fertility and water holding capacity of the soil. Legumes obtain their nitrogen from the air, and can provide as much nitrogen for your next crop as fertilizer, capturing 100-150 pounds of nitrogen per acre. For maximum nitrogen yields, legumes should be incorporated into the soil at peak bloom.
Cereal Grains and Grass Cover Crops
Cereal grains and grasses grow very quickly and provide quick ground cover. They can provide a tremendous amount of biomass that not only smothers weeds and prevents soil erosion, but also puts huge amounts of green matter, or green manure, back into the soil which improves the soil’s tilth. These cover crops are nature’s great nutrient recyclers. The plant’s extensive root system pulls nitrogen and other minerals from deep within the soil, and stores these elements within its roots and leaf structure. Upon turning under the crop, nitrogen and other elements are released or recycled back just underneath the soil surface so the next crop can utilize the nitrogen that once was beyond reach and leaching away.
Brassica Cover Crops
The rapid growth of brassicas supplies a thick ground cover that protects the soil from erosion and helps suppress weeds with a dense amount of biomass. Some brassicas have a large taproot that can break through, plow or rototill pans, thus aerating the soil. The roots also scavenge nutrients from deep in the soil and bring them back to the surface where they can be utilized by your next food crop plantings. Other brassica species release chemical compounds that may be toxic to soil borne pathogens and pests such as nematodes, symphylans, and even some weeds. And if left to flower, brassicas are especially popular with beneficial insects.
Winter Wheat Cover Crops
We suggest planting winter wheat late in September to early October. It is a great all-purpose crop that works three ways. When fall sown, it grows quickly and overtakes fall weeds, then as the cold days of winter begin, it goes dormant. Spring's arrival brings re-growth that can be turned under as a green manure. If left to grow, winter wheat makes good forage for livestock; or if allowed to mature it can be harvested mid-summer. Winter wheat is an exceptional, inexpensive and fast growing cover crop that suppresses weeds and disease with its allelopathic (the suppression of growth of one plant species by another) effect.
So, what do we use at our farm?
We recently added 44 acres of operational fields onto our existing 44 acres; fields varying in sizes from 5-10 acres each. Before we plant vegetables in the new fields (whether it be for seed crop or trials) there are several things we do to prepare the soil.
When we break ground on a brand new field (like a hay field) we do three things: chisel plow the field, disc the field and till the field. Generally speaking we will start new fields in the fall of the year. So, in the late summer we will break ground and before the fall rains comes we will begin our cover crop rotation. We start by planting an overwintering mix—TSC Fall Mix (Austrian Field Peas, Crimson Clover, Hairy Vetch, Annual and Winter Rye)—which offers a variety of ingredients that are beneficial to the soil. The peas and clover provide the soil with nitrogen and the grasses help to build organic matter and act as a smother crop for weeds. We water once or twice to get the cover established so it’s of good size going into the winter. Throughout the winter months the crop will stay relatively short, growing about 6-8 inches tall.
In the spring when the days become a little longer and start to warm up, the TSC Fall Mix really starts putting on growth. The optimal time to work under the cover crop is when the peas and the clover first begin to bloom. Growth is about 4 feet tall at this point depending upon how fertile the ground was to begin with. Working in the cover crop happens in stages. First we flail mow with a 10 ft. flail mower that’s attached to an 85 hp tractor. The flail chops the cover crop up into very fine pieces. We have discovered over the years that fine pieces are ideal for the ground to digest the cover crop. If it’s cut too big or too long, in bigger plantings like we do, we can’t work it into the soil as well. We let the freshly mowed cover crops sit there for a short time, about 4-5 days, to dry. We will then come in with a disc and turn the mowed cover crop into the earth. We wait a week then disc it again because generally we don’t get it all turned-in the first time. At this point we are approaching the end April or the first part of May, which is when we begin tilling the field.
After May 15th, when the threat of frost has passed, we start the second rotation which is buckwheat. Buckwheat provides good cover and organic matter. While it doesn’t fix nitrogen it helps to keep the nitrogen in the soil, towards the surface. Buckwheat also makes a great bee attractant because it flowers right away. And just when it starts to flower (like we did for the TSC Fall Mix), we till it all in. If we were to let it go too far into flowering the buckwheat will start using all of its energy to produce seed, and we want that energy captured and put into the ground. Because buckwheat grows so fast we can generally get two buckwheat crops planted in one summer.
After the second planting of buckwheat has come and gone, fall has arrived again. Depending on what we are going to do with that particular piece of ground (in the spring) we may plant another round of the overwintering cover crop (TSC Fall Mix), but what we do a lot of the time—once we have put our three covers in—is plant an early season crop, peas for example. Just before fall comes we will spread our [on-site created] compost all throughout the field. (So, basically all we have been doing up to this point, for the past year, is feeding the ground.) Once the compost is disked in, we let it rest over the winter. The field stays relatively weed free. Then, come February—it’s a quick till. We normally have two weeks of dry weather in February so we can till the field and plant our early season crops such as peas or lettuce.
Besides preparing new fields, what else do we use cover crops for?
Say we know we want to plant broccoli seed crop in the spring; we’ll plan ahead!
We will put the TSC Fall Mix in that field the previous fall, and after our first crop of buckwheat, instead of a second, we will plant a crop of clover. So in that particular field we have now fixed a lot of nitrogen, and can plant a nitrogen-loving crop like broccoli.
Groundhog Daikon Radish Cover Crop
Over the past few years we have started using a tillage radish. The Groundhog Daikon Radish sets huge roots into the soil, making them great miners of nutrients. Once you have fed your field and it starts to rain a lot (as it does here in Oregon) your nutrients are going to seep further and further down into the soil. A root cover crop, like Daikon radish, is going to mine your nutrients and bring them closer to the surface. We used the Groundhog Daikon Radish quite a bit last year and found that it did a great job of subduing overwinter weeds as well.
MightyMustard® Pacific Gold Cover Crop
In a previous post we talked about the issues we have with flea beetles at our farm—enter Mighty Mustard®! We use this cover crop most commonly when we are planting our winter trials (a brassica-heavy planting). We ring the field with the mustard about 2-3 weeks before setting out our winter crop. The idea is that the flea beetles will like the mustard better than the brassicas, and we found that in doing this we’ve had a lot less bug problems.
Something new in the last few years.
More recently we have started planting cover crops between rows of crops. The less often we till the ground, the better, so we have started using a low-growing cover crop like a vetch or clover in between our rows of vegetables. As long as we keep it away from our row crop then we are able to go through and mow instead of cultivate. Then, at the end of the season, we can just switch! For example, say we plant lettuce in one row and plant a row of clover right next to it. Then, after the lettuce has been harvested, we would till in the clover and plant a winter crop such as kale where the clover used to be. We continually grow cover crops within the same field as our vegetables by just switching rows. By utilizing our space in such a way we are able to go right from a spring crop directly into a winter crop.
Authors: Tom Johns & Mel Reynolds