Growing green manure is an age-old gardening technique that draws on a large range of fast growing crops to build, regenerate and maintain soil health.
I like to think of it as growing soil instead of food – which is all the same thing really.
The benefits of green manure
- Helps prevent and treat soil disease.
- Increases organic matter, earthworms and beneficial microorganisms.
- Increases the soil’s available nitrogen and moisture retention.
- Stabilises the soil to prevent erosion (we’ve used them extensively on our steep banks directly after fresh earthworks, which has worked brilliantly).
- Brings deep minerals to the surface and breaks up hardpans (when you choose plants with deep taproots, such as lupins).
- Provides habitat for beneficial insects and reduces populations of pests.
- Improves water, root and air penetration in the soil.
- Can smother persistent weeds (good choices for weed suppression include lablab, cowpea, lucerne and buckwheat).
What should I grow to create green manure?
Gardeners will often grow a mixed selection of green manures, including both legumes (which fix nitrogen into the soil) and grasses (which provide/build organic matter).
By ‘grasses’, I mean cereal grasses such as rye or oats, and definitely not weedy types like kikuyu or twitch.
Cool season/climate green manures include broad beans, fenugreek, lupins (have deep taproots), oats, rye, mustard, peas, sub clover (not recommended for your vegie patch as it’ll get weedy) and vetch.
Warm season/climate green manures include buckwheat, cowpea, French white millet, Japanese millet, lablab, mung bean and soybean.
Green manures are usually integrated into a crop rotation cycle so there’s
a seasonal rhythm to planting them.
In Tasmania, we usually grow them over winter to rejuvenate the soil for spring plantings; however, if you’re leaving your garden for an extended period of time, there’s always a type of green manure you can grow any time of the year – it’s best to not leave your soil naked.
How to grow green manure
Make sure your soil has good nutrients available, such as compost or aged manure, and that it’s not compacted, or use a garden fork to puncture and massage (not turn) the soil before planting.
If you have sandy soils, add lots of organic matter (compost, aged manure and mulch in warmer seasons).
To sow the seeds we simply broadcast them, which involves throwing handfuls of seed over the bed, and then using a rake and/or garden fork to shuffle the soil around until there’s a layer of soil covering all the seeds. Add water, and then you can pretty much forget about them.
Unlike growing food, don’t let green manures flower or set their crop. Otherwise the plants will divert most of their energy and goodness into the flower/crop instead of into the soil, which is where you want it.
Once you see the plants are about to flower you can either: chop them up and dig them into the soil; or “prune” them. We’ve been pruning for the past two seasons, which means we get an extended life out of the plants and, in the case of broad beans, have lots of salad greens for us or our chickens to eat, as the young leaves are super tasty.
Overall, green manures provide an affordable, easy and effective solution to restoring or maintaining soil health. And when we have healthy soil, we have healthy food, which of course means we have healthy people.