Fall provides the perfect opportunity to increase our soil’s organic matter. Organic matter, the stuff that darkens our soil to an earthy brown colour, enhances so many different soil functions that it almost seems made up. But seriously, the right amount of organic matter in the soil improves soil structure and soil drainage, holds more water (i.e. you don’t have to water so often), and feeds soil microbes (which then feed the plants). And that’s just the short list of organic matter benefits.
There’re two ways we can naturally increase soil’s organic matter in the fall: growing green manure cover crops and mulching with fall leaves. This blog will cover green manure crop. Next week, I’ll cover fall leaves.
Green manure cover crop
Green manure. Sounds like an environmentally sound manure or a manure that hasn’t matured yet. Enough with the jokes. Green manure is a crop grown especially for turning under to supply the soil with organic matter. It’s a technique that is as old as the hills and yet it’s still appealing and applicable. The earliest record of using green manure appeared in 500 BC in China and the latest farming trend is to use “cocktail” seed mixes.
Green manure feeds soil organisms. As in a good bartering system, the soil organisms go on to improve soil structure by providing the glue to form soil aggregates. As an extra bonus, the soil organisms enhance soil fertility by slowly releasing the excess nutrients they don’t use as they break down the green manure.
Usually a green manure crop is sown in the fall (mid- September to mid-October) and then tilled in or turned over with a spade in spring. Timing is everything. The general advice is to turn in the green manure a month before you plant to give the green manure enough time to rot. If we’ve left it too late to plant this fall, don’t worry. We can seed in the spring too! But it’s still definitely better to seed in the fall. The green manure plant root exudates feed the soil microbes until spring.
The most popular green manure crops are legumes because they supply a high amount of nitrogen when you turn them over in the spring. Think clover – red, crimson, white and sweet – as well as hairy vetch, cowpea, soybean and field pea. But grasses – rye, wheat, barley, oats, ryegrass and sorghum – as well as brassicas and mustards are also great. They score high in the organic matter scale and they mop up excess nitrogen from the soil so it doesn’t leach in the fall and winter, and release it the spring. Forage radish/daikon radish has the added bonus of breaking up compacted soil.
The cocktail cover mix idea originated in Brazil in the 1970s, and has taken off in North American farms after a 2006 presentation by Brazilian researcher Dr. Ademir Calegari. A single mix could contain from 7 to 20 varieties of seed. A mix of crops appears to feed a wider diversity of soil microbes. Soil microbe diversity is a good thing. Makes the soil more resilient.
“A good cover crop mix will have a grass, legume, & broadleaf component,” says Kevin R. Elmy, P.Ag. seed farmer at Friendly Acres Seed Farm, Saskatchewan. “A great one will have warm & cool season species.”
If I interpret that correctly, you could plant a 3 species mix to fill that requirement
Rye (grass, cool season),
Crimson clover (legume, cool season),
Buckwheat (broadleaf, warm season).
How about a 6 species mix to rapidly increase organic matter?
Rye, oat, Sudan grass (2 cool season grass, 1 warm season grass)
Mustard, buckwheat (broadleaf: 1 cool season and 1 warm season)
Vetch and soybean (legume: 1 cool season and 1 warm season).
You can play around with combination by using the USDA cover crop chart.
Green manure for small spaces
It’s all very well and good if you have the space, but what green manure species can small space urban gardeners use?
“Any of them would work, says Gary Ogle, Vesey Seeds, “but the best bang for your buck might be clover or vetch. If weeds are the issue though, buckwheat is better.”
The final step!
The critical last step: reaping the rewards of green manure. We need to incorporate the green manure back into the soil somehow for the green manure’s potential to be realized.
The traditional route is to grow the green manure cover crops in the fall. In the spring cut down the cover crop and turn over the whole plant (roots and all) into the soil with a spade.
“Best to incorporate [green manure crop] 3-4 weeks before planting so as not to tie up available nutrients,” says Gary Ogle, Vesey Seeds.
I scoured the Internet for the other ways to incorporate green manure into the soil. Renee’s Garden Seeds featured three different methods. Which one you use depends on your climate and the height of the plants.
Method 1: Instant green manure
Grow the cover crop to 5-6 inches tall. Cut down the cover crop with a weed-whacker (also known as a whipper-snipper) and turn the cover crop into the ground using a garden fork, shovel. The green manure will completely decompose in a about a month, enriching the soil for planting. This method works best when a cover crop will be growing for only a short time.
Method 2: Compost it first
Pull out the cover crop when it reaches as 1 foot in height. Shake off soil excess soil from the roots and drop the cover crops into the compost. Add compost to the garden once it’s done. Composting time? Well, it depends on your composting method and climate. This method works well for small areas.
Method 3: Cook before turning
Grow the cover crop to 1 to 1 ½ feet tall and then chop it down with a weed-whacker. Cover the chopped plant debris with black plastic to speed up decomposition. Two to three weeks before sowing/planting, turn the rotted residue into the soil.
A couple weeks ago I purchased Renee’s Garden Seed cover crop mix, which is a blend of legumes (Austrian winter peas and hairy vetch), grass (winter rye grass), roots (purple top turnips and daikon radish), and canola (used to be called rapeseed). Really looking forward to trying it on my vegetable beds in my backyard this year.
I am also trying Renee’s Garden Seeds’ soil building Kodiak mustard (Brassica juncea) on the low fertility community post-box community garden.
Got to practice what I preach!
Colorado Extension. Cover crops and green manure crops.
Extension. What is organic matter? Benefits.
NODPA. Cocktail Cover Cropping
SARE Nonlegume cover crops.
USDA Cover crop chart
Vesey Seeds #groundchat on green manure cover crop