One way of keeping your soil healthy and in balance is to use cover crops to protect and fertilize it. Cover crops aren’t just for gardeners: many farmers are now using cover crops to improve their yields. For instance, some farmers find their soil ‘slumps’ over winter, becoming heavy and difficult to work; using a cover crop keeps the soil open and loose, ready for the first planting in spring.
Growing a cover crop is simple. You need to find a plant that can be sown as soon as the main crop is harvested and that can be mown or will naturally die off before the next cash crop is sown. It needs to be a fast-growing annual plant and one that will create a lot of biomass. The crop covers the soil while it’s growing, preventing erosion or evaporation of moisture, and when the crop dies, it is left to decompose where it grew, increasing the amount of organic matter in the soil.
Growing a cover crop between two cash crops (or if you’re gardening for your own family’s needs, two edible crops) should boost yields, as it keeps the soil in better condition and replenishes nutrients that have been used by the previous crop. Farmers in dry climates are sometimes reluctant to use cover crops, as they’ll need to water them, but leaving soil bare causes water to evaporate out of the soil, so the net impact of a cover crop is positive even if there is a cost to cover irrigation.
Cover crops also help to address two difficulties with organic farming. Without using pesticides, weed control can become a burden; a cover crop helps to suppress weeds during the early growing season for the main crop, acting as an organic mulch. Maintaining soil fertility can also be difficult without using chemical fertilizers; a cover crop helps put nutrients back in the soil so that there is less depletion of essential elements.
Some governments now offer subsidies and other encouragements for farmers to use cover crops; for instance, in the UK, there is a payment per acre for farmers using these techniques.
Other benefits that cover crops provide include:
- Slowing down the velocity of running water when there is rainfall
- Allowing the water more time to sink into the soil
- Binding the soil together with their roots
- Providing a beneficial environment for soil microbes
- Acting as insulation, keeping the soil warmer in winter and cooler in summer than if it was bare.
- Roots growing all year round help to loosen the soil and give the next crop the ability to sink roots deeper.
- The crops attract beneficial insects—early in the year, they may be the only source of flowering plants for some pollen-hungry species—and provide cover for other wildlife. And by keeping the soil open, cover crops encourage earthworms to stay active.
Since cover crops can be broadcast (that is just throwing the seed to scatter it and then raking a little soil over) without tilling the soil, they reduce compaction of the soil. This is particularly the case when farmers use the roller crimper in front of their tractors to break the stems of cover crops and crush them into mulch while sowing the cash crop behind the tractor at the same time. This single visit to the field replaces, in some cases, up to five different operations including deep tillage.
Types of cover crop
Different types of cover crops are available both in the cool season and in the warm season. Winter cover crops include annual cereals and grasses such as rye, oats, and barley, and legumes such as clover, vetches, and brassicas. Warm-season cover crops include numerous legumes as well as millet.
Several factors influence the choice of cover crop. For instance, you’ll need to consider:
- The type of soil (as always!)
- The planting date for the cover crop
- The intended planting date for the next crop
- Your first and last frost dates
- Any particular benefits required (such as nitrogen-fixing)
- Rotation of crops
- The likely temperatures over the cover crop’s growing period
A cover crop generally needs to be in the ground as long as possible to produce as much biomass as it can before it is killed off. However, you don’t want a cover crop to set seed, so you would usually aim to get it just to the flowering point before chopping it and sowing the cash crop.
It’s often better to plant a mixed cover crop. For instance, a mixture of vetch and rye, sown in the later summer, will help to fix nitrogen, and the mixture of one shallow-rooted and one deep-rooted plant will help to keep the soil open and fertile all the way down. Farmers who are going to plant sugar beet or corn can use this cover crop mix; it’s a good mix in the garden, too, for instance, if you want to fix nitrogen for a tomato crop next year.
Whichever cover crop is planted, needs to be planted densely. With the main crop, you tend to space it out a bit more to get the benefit of higher yields; with a cover crop, weed suppression and biomass production are your big targets. You really want lots of foliage covering the soil.
It may be a good idea to experiment with different cover crops in different beds, or even to leave a ‘control’ patch where you don’t sow a cover crop. You can then test the soil at different stages to see if there is a difference and also compare your yields on the different beds.
Choosing your cover crops
Consider the kind of soil you have and its needs. If you have heavy, claggy soil, choose cover crops that are deep-rooting and will help to break up compact soil, like brassicas. Fall-sown radish or white mustard have deep tap roots and will grow fast; they will help with any drainage problems you might have.
Radish also pulls nutrients up with its root network (use oilseed radish as a clay buster, not salad radish with its tiny little red roots). Mustard is not frost-hardy, so if you plant it in late summer, it will really open up the soil but will be killed off by the first hard frost. You won’t need to kill it off yourself and it will start to decompose quite naturally as it lies on top of the soil.
On the other hand, if you have soil that’s already well conditioned, you might prefer to plant phacelia with its shallower roots and little blue flowers. Phacelia isn’t related to any major food crops, so it’s less likely to transmit diseases.
That possibility of disease transmission is a reason you might decide not to use some types of the cover crop; for instance, mustard, being a brassica, might pick up clubroot, which it could then transmit to your cabbages, cauliflower, and broccoli.
Legume species are good for nitrogen-fixing. If you’ve just grown nitrogen-hungry crops like tomatoes, bok choy, or sweetcorn, a nitrogen-fixing cover crop like vetch, berseem or red clover, or field peas is just what you need.
Some have relatively shallow roots, so won’t help soil structure and might need to be paired with a deeper-rooting cover crop. But they will help rebuild soil fertility.
Don’t use nitrogen fixers just after a pea or bean crop. Radishes are the best cover crop then, as they will help to pull in all the right nutrients; the peas and beans should already have fixed enough nitrogen in the soil.
Field peas grow quickly in spring, or in early fall, and will die naturally, in late spring or in winter. Fall-planted peas will decompose by the time you get around to springtime planting.
Red clover has a taproot and will build nitrogen; it’s also a good choice for suppressing weeds and for attracting pollinators. It needs to be mown, cut, or crimped when it’s budding as if it’s allowed to flower fully, it self-seeds too easily. In zone five and colder zones, fall-planted clover will die out over winter in any case.
Cereals such as oats and rye have vigorous fast growth and give superb winter ground cover. They tend to have shallow roots, spreading out just under the surface, which will help improve the conditioning of sandy or light soils by making the topsoil crumblier, as well as introducing more organic matter once they decompose.
Forage rye can be used for grazing, as well as being a good cover crop, and it will work in all soil types, including the sandiest. It suppresses weeds particularly well since its roots are allelopathic (that is, they inhibit other plants’ growth) and can prevent weed germination. Rye is also a good choice for late sowing, as it will germinate even at low temperatures. But you will need to ensure that you kill it before it goes to seed.
Pair rye with radishes if you want a good root network to break the soil up as well as the topsoil conditioning that rye will give you.
Ryegrass is cheaper than rye and is a good cool-season cover crop. Purely annual ryegrass will die over the winter and can just be mowed and smothered. However, many commercially sold green manures combine annual and perennial ryegrass; the perennial ryegrass needs to have the roots killed off and then the tops mowed too.
Buckwheat is frost intolerant, so as a fall-planted cover crop, it will naturally be killed off by frost. It’s particularly good for supplementing phosphate in the soil.
Oats make a good planting choice for early spring, and can also be sown in the fall. Oats grow vigorously and have fibrous roots that help to improve the soil structure. These plants are also good at finding phosphorus and capturing other nutrients. They need to be mowed after two to two and a half months while the seed heads are still green. Oats won’t survive over winter in zone six and colder.
Farmers who can leave stubble standing (so they leave the cut stalks of the crop they have harvested, instead of plowing them in) can allow annual weeds to grow between the stalks or can sow a cover crop between the stubble rows.
Annual cool weather cover crops
|Crop||N Fixer||Soil Preference||Tolerates Poor Soil||Height||Insectary||Comments|
|Austrian winter pea||★||Heavy||2ft||★||Hardy to 0°|
|Barley||Loam||2-4ft||Mild winters only|
|Bell bean||★||Loam||★||3-6ft||★||Opens heavy soil|
|Blando bromegrass||Many||★||2-4ft||Drought tolerant|
|Clover, alsike||★||Heavy||2ft||★||Can take acid soils|
|Clover, berseem||★||Many||2ft||★||Hardy to 18°|
|Clover, crimson||★||Loam||★||18in||★||Hardy to 10°|
|Clover, red Kentland||★||Loam||2ft||★||Short-lived perennial|
|Clover, sweet white||★||Heavy||3-6ft||★|
|Clover, sweet yellow||★||Loam||3-6ft||★||Drought tolerant|
|Clover, nitro Persian||★||Many||★||2ft||★||Hardy to 15°|
|Daikon radish||1-2ft||Nutrient accumulator|
|Fava bean||★||Many||4-8ft||★||Hardy to 15°|
|Fescue, zorro||Many||2ft||Mix with legumes|
|Fenugreek||★||Many||2ft||★||Opens heavy soil|
|Garbanzo bean||★||Many||3-5ft||★||Slow in cold soils|
|Mustard||Heavy||★||2-4ft||★||Opens heavy soil|
|Oats||Many||2-4ft||Mild winters only|
|Oil seed radish||Many||2-4ft||★||Hardy to 20°|
|Phacelia||Many||★||2-3ft||★||Hardy to 20°|
|Rapeseed||Loam||★||2-3ft||Opens heavy soil|
|Ryegrass, Annual||Many||2-4ft||Mix with legumes|
|Vetch, common||★||Many||3-6ft||★||Hardy to 0°|
|Vetch, hairy||Many||★||3-6ft||★||Hardy to -10°|
|Vetch, Purple||★||Many||3-6ft||★||Hardy to 10°|
Annual warm weather cover crops
|Crop||N fixer||Soil preference||Tolerates poor soil||Height||Insectary||Comments|
|Black-eyed peas||★||Many||3-4ft||★||Chokes weeds|
|Cowpeas, red||★||Loam||★||1-2ft||★||Drought resistant|
|Pinto beans||★||Loam||2-4ft||★||Drought resistant|
|Soybeans||★||Many||2-4ft||★||Mix with nonlegumes|
|Sudan grass||Many||★||6-8ft||Mix with legumes|
|Sunn hemp||★||Loam||★||3-6ft||★||Tolerates acid soil|
|Sunflower||Many||★||Up to 15ft||★||Need sun|
|Sweet potato||Many||★||1ft||Ground cover, Nutrient accumulator|
Perennial cover crops
|Crop||N fixer||Soil preference||Tolerates poor soil||Height||Insectary||Comments|
|Alfalfa||★||Loam||2-3ft||★||Well limed soil|
|Birdsfoot trefoil||★||Many||★||3-5ft||★||Drought resistant|
|Chicory||Heavy||★||2-3ft||★||Opens heavy soil|
|Clover, strawberry||★||Many||1ft||★||Needs moisture|
|Clover, white dutch||★||Many||6-10in||★||Needs moisture|
|Clover, white ladino||★||Many||1ft||★||Needs moisture|
|Clover, white New Zealand||★||Many||1ft||★||Needs moisture|
|Fescue, creeping red||Many||2-3ft|
|Timothy grass||Heavy||2-3ft||Needs moisture|
I already mentioned nitrogen-fixing legumes as cover crops. Let’s look at how they benefit the soil and help your next crop.
Nitrogen is the element that boosts leafy growth, helps flowers to bud and bloom, and helps fruit to set. It’s very easily depleted, particularly by crops like sweetcorn or tomatoes. And most plants need to access nitrogen in the soil; they can’t breathe it in from the air.
However, a few plants can access atmospheric nitrogen and they then store it in their roots. To do this, though, they rely on a symbiotic relationship with a little micro-organism, a bacterium called Rhizobium. Rhizobium, technically, I suppose, is a plant infection, but it’s a beneficial one. The plant can take in atmospheric nitrogen (N2) and convert it to ammonium (NH4+) and then the bacteria convert it to NO2 and NO3 nitrates which plants can access.
If you pull up a pea or bean plant, you’ll probably be able to see the little nitrogen nodules on its roots. When these decompose, they will add nitrogen to the soil, don’t pull up your old bean plants by the roots—leave the roots in the soil to do it well! However, remember that the rhizobium needs to be present for this to work, so if you use a nitrogen-fixing cover crop, you will see better results if it has been pre-treated with a rhizobium inoculant.
Good examples of nitrogen fixers among possible cover crops are lupins, clovers, alfalfa, sweet pea (Lathyrus latifolius), soybeans, and (perhaps you didn’t expect this one) peanuts.
Balancing carbon and nitrogen in cover crops
We already looked at the nitrogen and carbon mix when we talked about the nutrients that plants need. Whenever you’re thinking about soil, you need to remember that there isn’t one single answer to everything, like Miracle-Gro; instead, you’re always looking to get the right balance. The interventions that you make are always addressed at moving the balance a little way to one side or another, rather than at dramatic, overnight change.
So, you need to look at the CNR ratio and remember that microbes need it to be around 24:1 (the ratio looks at the mass of carbon and nitrogen). Put too much nitrogen in the soil and you risk killing off the beneficial microbiota. This can affect crop residue decomposition, as well as crop nutrient cycling.
Different cover crops give you residues with different C: N ratios. Alfalfa already has an almost perfect C: N ratio at 25:1, so it won’t alter anything in the soil and it will be decomposed very quickly. But wheat straw goes to an extreme with eighty times more carbon than nitrogen. Eating carbon, remember, consumes nitrogen, so if you’re going to be growing a crop that needs plenty of nitrogen, don’t use wheat as a cover crop!
Hairy vetch is at the other extreme, with only an 11:1 C: N ratio. It will deliver loads of nitrogen to the soil and consumes very little of it in eating up the carbon.
Now there is a bit of finesse that comes in here because there is a timing effect. Microbes need to find additional nitrogen to balance the excess carbon (for instance if they’re eating straw), so they have to take it out of the soil, causing nitrogen depletion. But when the carbon has been fully eaten and the microbes die, their nitrogen-rich bodies will return that nitrogen to the soil. That takes a bit of time, and that’s why you need to give organic materials time to rot down properly.
You should also be aware that when organic materials decompose, their original C: N ratio tends to narrow. For instance, rotting manure starts off with a ratio of 20:1 but will end up as humus with a 10:1 ratio. That means that if you choose a cover crop with a C: N ratio of between, say, 20 and 33:1, it won’t change the carbon and nitrogen balance of the soil.
Extremely nitrogen-rich residues are decomposed quickly by the microbes, but little residue is left on the surface of the soil. Carbon-rich residues tend to leave more residue on the surface, so if you want to cover the soil well, you might shoot for a cover crop with a higher C: N ratio.
C: N ratios
Cover crops with CNR below about 20:1 will add nitrogen
Cover crops with CNR above 33:1 will deplete nitrogen and add carbon
If you’re in an area of high rainfall, you will need a nitrogen boost. That’s because nitrogen tends to get washed out of the soil very easily when it rains. So, you would want to pick cover crops with a lower C: N ratio.
|Low C: N||High C: N|
|Pea – 10:1||Corn stalks – 57:1|
|Lentil – 16:1||Wheat straw – 80:1|
|Clover – 23:1||Alfalfa hay – 25:1|
|Phacelia – 18:1||Sawdust – 400:1|
|Turnip – 23:1||Rye straw – 82:1|
|Radish (root) – 22:1||Oats – 70:1|
|Rapeseed – 28:1||Flowering rye cover crop – 37:1|
|Mustard – 33:1|
|Young alfalfa – 13:1|
|Hairy vetch cover – 11:1|
|Rotted manure – 20: 1|
You also need to think about the nitrogen needs of the different crops in the rotation. You can follow a cover crop with a low C: N ratio (legumes—pea, lentil, cowpea, clovers, or brassicas—turnip, radish, canola) with a high C:N ratio of crops like corn or wheat. This can help residues decompose while enhancing nutrient availability for the next crop.
Similarly, a high C: N ratio cover crop might include sunflowers or millet that can provide soil cover after a low-residue, low C: N ratio cash crop like peas or soybeans. Still, they can decompose during the next growing season to make nutrients available to the following crop.
If you apply organic material with a high C: N ratio, either a cover crop like wheat or a mulch or amendment like sawdust or cardboard, then you might want to use an adjunct (a supplement) to boost nitrogen levels. Blood meal, bone meal, chicken manure (composted), fish meal, cottonseed, or soybean meal are all organic ways to add nitrogen. Or, of course, you can mix your high C: N cover crop with a lower C: N one to balance it out.
Sowing your cover crop
The practicalities of using a cover crop in the garden are very simple. You don’t need a roller-crimper as an organic farmer does. Once you’ve harvested your previous crop and composted any vines or stems (leaving the roots in the ground to decompose), just rake the top half inch or so of soil, scatter the seeds over the surface, and rake again to cover them. Then—and this is very important because it’s the main influence on germination—water the soil really well and make sure it stays moist till the seedlings come through.
No space is too small to use a cover crop. You can even just fill in one row in a bed rather than leave bare dirt. Ideally, if you’re harvesting at different times, you’ll be starting off cover crops as you go to fill in the gaps. But over winter, if you clear the entire vegetable bed, make sure you’ve sown a cover crop to protect and improve your soil.