For a new market farmer, understanding the difference between cover crops and green manures may be a little tricky.
There is a difference between the two, albeit a slight one at that.
Cover crops are used to protect bare soil from strong winds and heavy rains that will inevitably cause erosion and degrade soil quality.
Green manures are crops grown not for the purpose of selling, but rather to add nutrients and organic matter back to the soil.
Since I don’t have a ton of experience with cover crops or green manure, yet, as our field is still covered with hay, which I guess is technically a cover crop, just not one we chose, but that’s a story for another day!
I am going to outline how J.M. Fortier uses cover crops and green manures on his market farm so you can get an overall picture of how to use them in your own market garden.
What Is A Green Manure Cover Crop?
Green manure cover crops are mainly grasses and legumes, which after being mowed down are then plowed under and incorporated into the soil to boost its fertility.
The grass and legume crops (beans, peas, soy, alfalfa, clover, etc) have the ability to capture nitrogen from the air and feed it to the soil; referred to as ‘N fixation.’
When the crops are turned under, the degradation of plant material allows the nutrient held within the green manure to be released and made available to the succeeding crop.
When cereals (oats, rye, wheat, etc.) are mixed in with the legumes, the resulting crop residues provide not just nitrogen, but carbonaceous organic matter as well.
Green manures are considered amendments just like compost or animal manure.
Green Manure Advantages
The primary material used to make the amendment is produced onsite and requires no work other than to seed the crop, shred it and mix it into the ground.
Generally speaking, green manures are highly recommended in organic vegetable farming.
They are an economical and effective way to add nitrogen to the soil. They also fertilize crops with nitrogen without adding phosphorus to the soil.
However, there are some disadvantages to using green manures for market farming.
Green Manure Disadvantages
They take up growing space that would otherwise be devoted to vegetable crops.
Green manure takes time to grow, from six weeks to a whole season, depending on the chosen plant.
An additional two weeks may also be required for the microorganisms to completely break down the green manure and make the nitrogen available to the plants.
Multiple successions don’t allow for much time to grow green manures and keeping plots idle does not jive with the principle of making optimal use of cultivated land.
Also, without the use of a flail mower, it is difficult to turn them into the soil with a rototiller.
Despite these drawbacks of green manure on a market farm, J.M. still grows legumes and grasses but refers to them more as a cover crop than green manures.
Here’s how he integrates the beneficial uses of green manure in his market garden.
J.M. finds ‘holes’ in his crop plan that will allow him to precede a crop with green manure. This is called ‘catch cropping.’
They generally use field peas and common vetch. In both cases, they mix the legume with oats to give it something to climb on as it grows.
There are two things to keep in mind in order to get the most fertilizing action out of a leguminous green pasture.
1. The best time to mix in green manure is just before it flowers. At this stage, the plant has stored its maximum nitrogen and will make the greatest contribution to soil fertility.
With the crop still young and tender, it will decompose quickly and easily, in time for the following crop.
2. Legumes do not fix nitrogen from the air by themselves. The fixing is done by bacteria of the genus Rhizobium, which form nodules on the roots of the leguminous plants and permit the exchange to take place.
However, these bacteria may not be present in the soil if legumes have not been grown there for some time.
It is a good idea to inoculate your legume seeds with proper rhizobia. To ensure you are getting maximum nitrogen fixation, make sure you inoculate the seeds of each legume species you use.
You can buy this inoculum from seed suppliers as a powder that you mix with your seeds along with some water.
It should be noted that because micro-organisms need to consume a great deal of nitrogen in order to break down the fibrous green manures, they may actually draw nitrogen from the soil. Essentially, stealing the available nitrogen from the soil, leaving the following crop in deficit.
For this reason, J.M. primarily relies on lots and lots of compost for soil building and instead just grows cover crops late in the season to cover the soil for winter.
This also adds significant amounts of organic matter to the soil.
What Are The Benefits Of Cover Crops?
There are many benefits to employing the use of cover crops in your market garden, including:
Protecting the Soil
Leaving the soil bare for months is a bad idea. Strong winds and heavy rains can lead to soil degradation and erosion.
In some areas, snow provides adequate protection during the winter, but come spring the melting snow saturates the soil with water. The runoff can cause serious erosion problems that decrease soil fertility.
Come spring, the soil will be easier to work on instead of being hard and compacted on the surface.
It is best practice to allow enough time to seed any cereal crop, at least 6 weeks before the first heavy frost is expected so that the root system of the plant has sufficient time to develop.
When planting a cover crop for soil protection, be sure to seed it very densely so it quickly protects the surface.
Reducing the Spread of Weeds
Planting cover crops between two successive seedings or in certain beds that will be free for a long enough period is an effective way to interrupt the weed cycle in your garden.
J.M. uses buckwheat which forms a plant cover dense enough to suffocate weeds in less than a month.
Buckwheat is good to use because it grows flowers for the bees and increases biological activity in the soil when turned under. This increase in activity positively affects the following crops being planted in that bed.
Make sure you densely seed the bed (5-10X more than recommended). It’s better to spend the extra cash on seeds than spend the extra time on weeding.
Letting a Plot Rest
There may come a time where you don’t plant any vegetables in a bed for the season. In this case, you should seed a cover crop that would keep growing over a long period of time without being killed when mowed.
White clover is a good choice as it is hardy, feeds the soil with nitrogen, is low growing and low maintenance.
White clover is slow to establish, so it is best to mix it with a cereal. The first mowing would kill the grain and by that time, the clover would have enough time to cover the soil.
How Do You Manage Cover Crops?
To seed cover crops, use a generous amount of seeds.
To help ensure good germination, mix the seed into the soil with a quick and shallow pass with a rotary tiller or wheel hoe.
When you are ready to get rid of the cover crop, J.M. recommends shredding it using a flail mower.
Then you have two choices:
1. Leave the crop on the surface and cover it with a tarp to let it compost with the help of microbes and soil organisms.
2. Till it into the top 8 inches of the soil with the rotary tiller.
If you are using a low-till technique on your market farm, be sure to be careful with using the tiller, setting its slowest speed so the tines mix in the amendments without breaking up the soil’s structure too much.
To further minimize soil disturbance, the ideal solution is to use a spade or spading machine. However, this may not be very efficient or practical for 100 ft. beds.
J.M.’s current method to deal with cover crop residues is to shallowly mix them into the soil with a quick pass of the rotary power harrow or rototiller (depth of 3 inches) before covering them with a black tarp.
Sometimes he doesn’t even work the soil, he just sprinkles compost over the mulched residue, waters it, and leaves it to decay under a tarp.
Note that this biological method of working the soil requires a lot of time with bed preparation which sometimes can be problematic. When this is the case, the tillage option is what he uses.
Common Cover Crops
These are the cover crops J.M. uses on his market garden.
White clover is slow to establish but very hardy. It is difficult to get rid of once established.
He mostly plants it at the edges of his gardens where it adds nitrogen to the soil, survives the winters, and does not require regular mowing.
Oats and Legumes
J.M.’s main catch crop is an oat-pea mix. It can be seeded very early in the spring, as soon as the snow melts.
They add a lot of nitrogen and biomass to the soil and is a great weed smotherer.
Fall rye is useful in providing plant cover for beds where late harvests have taken place. It requires 4-6 weeks to fully develop and grows in cold conditions.
However, it is hard to eliminate even after shredding. A pass with the rotary tiller is not enough to kill it.
Aim to seed the first week of October to get the rye growing before winter. The plants come up again in the spring and produce lots of biomass by the end of May.
Useful for rapidly covering the soil and crowding out weeds. Buckwheat goes to seed 8-10 weeks after being seeded, so be sure to take note and mow it before then.
Buckwheat is sensitive to frost, so late August is the latest seeding date.
There is no doubt that cover crops and green manure are beneficial components to any market farm.
However, including them in a fertilization program is not easy.
Because of the time and space required to grow them, a systematic approach must be used so you can adequately use the cover crops without too much of a headache.
On J.M.’s market garden, he has incorporated cover crops in his annual crop planning process. This allows him to grow cover crops on larger areas with multiple beds, ideally entire plots or half plots at a time. Allowing for easier crop management.
In the end, incorporating cover crops and green manures in your market farm will be a trial and error process. What works for J.M. might not work for you and vice versa.
Taking the time to get educated on soil biology is important so that over time, you can establish best practices when it comes to fertilizing your garden plots.
Be sure to check out the market gardener, where most of the information in this article was gathered from.
Additionally, check out our booklist here for recommended reading before starting your market garden.
You can bet that we will be documenting our progress on utilizing cover crops and their result for our market farm when we start to use them.
Do you use cover crops in your market garden? If so, what have your results been so far? Let us know in the comments below.
Kathy & Jon
your friendly neighbourhood growers