Green manure crops: feed your soil while it rests

Green manure is a crop you grow not to eat but to feed the soil. You sow it into a bed that would otherwise sit empty, let it grow for a couple of months, then cut it down and return it to the soil. It's one of the cheapest ways to add organic matter, and a resting bed covered in green growth is far happier than bare dirt.

Why grow green manure

• Adds organic matter. The leaves, stems and roots all break down into the soil.
• Adds nitrogen. Legumes such as peas, beans and vetch work with bacteria in nodules on their roots to take nitrogen from the air. Much of that stays in the soil when the crop is turned in.
• Improves structure. Roots open up the soil, and cereal crops like oats have fine, fibrous roots that help bind soil into crumbs.
• Protects bare soil. It shades the ground, slows erosion and reduces crusting.
• Smothers weeds. A thick crop leaves little room for weeds.
• Feeds soil life. Worms and microbes have something to work on.

Winter mixes for Adelaide

Autumn, from March to May, is the main time to sow green manure in Adelaide. The cool, damp months suit these crops, and they'll be ready to cut in late winter or early spring, just in time for summer veggies.

A mix of a cereal and a legume works well. The cereal gives bulk and fibre, the legume adds nitrogen, and the cereal stems give the legume something to climb.

Many garden centres and seed suppliers sell ready-made winter green manure mixes. You can also mix your own from bulk seed, which is usually cheaper. If you grow broad beans to eat, you're already doing a version of this. Cut the plants at the base after harvest and leave the roots in the ground.

Summer options

If a bed is resting through the warmer months, a summer crop keeps it covered.

• Buckwheat grows very quickly in warm weather and flowers within a few weeks. Bees love the flowers.
• Cowpeas are a heat-loving legume that adds nitrogen and copes with dry conditions.

Summer green manure still needs regular water in an Adelaide summer, so it's best in beds you're watering anyway.

How to sow

1. Prepare the bed. Clear old crops and weeds, and rake the surface level.
2. Water first if the soil is dry.
3. Scatter the seed thickly and evenly. Green manure is sown much more densely than food crops, so follow the rate on the pack.
4. Rake it in lightly to cover the seed, or sprinkle a thin layer of compost over the top.
5. Keep it moist until it comes up. Autumn rain often does the job.

Birds love pea and bean seed. A light cover of netting or a little extra soil over larger seeds helps.

When to cut

Cut at flowering. This is the key timing. At this stage the plants have built up plenty of leaf and stem but are still soft and green, so they break down quickly. Legumes also hold most of their nitrogen in the plant and nodules around this time.

If you wait until plants set seed:

• Stems get woody and break down slowly.
• Legumes use much of their stored nitrogen to make seed.
• The crop may drop seed and come back as a weed.

Cut a bit earlier if you need the bed sooner. Smaller plants simply give you less bulk.

Dig in or chop and drop

There are two ways to return the crop to the soil.

Dig it in. Slash the crop down, chop it up with a spade or shears, and dig it lightly into the top 10 to 15 cm. This is the traditional method and breaks down fastest.

Chop and drop. Cut the plants at the base and leave everything on the surface as a mulch. The roots stay in the ground and rot in place. This suits no-dig gardens and keeps the soil covered. Add a thin layer of compost on top if you like, and plant through it.

Either way, chop the material into small pieces so it breaks down evenly. Water well afterwards to get the soil life working.

Wait before planting

Give the bed 3 to 4 weeks after cutting before you plant. While fresh green material breaks down, soil microbes are busy, and it can briefly hold back seedlings and seeds. After a few weeks the bed settles and the nutrients become available.

That timing lines up nicely in Adelaide. Cut a winter crop in late winter, wait a few weeks, and the bed is ready for spring plantings of tomatoes, pumpkins, sweet corn and other summer crops.

Tips

• Rotate your mixes. Legume-rich mixes are great before hungry crops like tomatoes and corn.
• Don't let it go to seed unless you want it back.
• Small beds count too. Even one resting bed covered in oats and peas is worth doing.

In Patchmates

Add a note to the resting bed when you sow, and a task for when the crop starts to flower so you cut it on time.