Replenishing soil with green manures

Mustard seeds are among the green manure crops used within a crop rotation cycle to re-nourish the soil between panting out your vegie crops. Picture: Shutterstock.
Mustard seeds are among the green manure crops used within a crop rotation cycle to re-nourish the soil between panting out your vegie crops. Picture: Shutterstock.

Green manure crops are used within a crop rotation cycle to replenish the soil of nutrients in preparation for further annual crops.

They offer the perfect way to feed, rest and activate your soil simultaneously.

Common green manure crops

Green manures suitable for cold seasons

  • Broad beans (also known as faba or tik beans) (Vicia faba)
  • Mustard (Brassica nigra)
  • Peas (Pisum sativum)
  • Lupins (Lupinus)
  • Oat grass (Avena sativa)
  • Rye grass (Lolium rigidum)
  • Vetch (Vicia)
  • Annual crimson clover (Trifolium incarnatum)

Green manures suitable for warm seasons

  • Buckwheat (Fagopyrum esculentum)
  • Lablab (Lablab purpureus)
  • Soybeans (Glycine max)
  • Cowpea (Vigna unguiculata)
  • Millet (Panicum miliaceum)
  • Marigolds (Tagetes)

The benefits

Different green manure options will have different benefits for the soil, as Sustainable Gardening Australia puts it:

Biofumigants, like marigolds (Tagetes patula) planted in spring, brassicas (Brassica napus and Brassica campestris) and mustard, planted in autumn help to control root knot nematodes and root rot fungal pathogens. These crops must be dug in to release beneficial gases as they decompose.

Legumes, like lucerne, clover, beans and peas, which fix nitrogen and will make it available to whatever follows the green manure crop.

Weed smotherers include lablab, cowpea, lucerne and buckwheat.

 Green manures help reset the soil. Picture: Shutterstock.

Green manures help reset the soil. Picture: Shutterstock.

How to plant them

We mix up a range of the seeds listed above in a bowl (usually a blend of mustard, peas and broad beans or lupins) and simply broadcast them across the soil. We then rake them in, not overly worried if some are still exposed. If needed, we water them in. But if rain is coming, we let nature water them instead.

Then what?

We make sure they're never allowed to flower, once they do the plant starts to put all their energy into flowering or fruiting instead of into the soil. After all, we're feeding the soil, not ourselves. So we will slash them down a couple of times over the season to ensure they're putting all their good stuff into the soil.

Once it's time for the next crop to be planted, you can either dig the plants into the soil (remove any excess green matter from the top of the plant first), slash, mow or sythe them to ground level and leave the roots in the ground. Or you can plant your crops amongst them (knowing you might have to manage any re-growth).

Mustard seeds are one of many ideal green manures ideal for 're-nourishing' soil. Picture: Shutterstock.

Mustard seeds are one of many ideal green manures ideal for 're-nourishing' soil. Picture: Shutterstock.

Another option is slashing them down, watering in and then smothering the garden bed with a non-toxic tarpaulin or silage tarp around four to six weeks before you want to plant the next crop.

This process encourages all the biology to the top soil level where they eat the whole plant - leaving no trace of it. This last method is our preferred one as it means you don't have to dig the soil at all (meaning you don't disturb the soil food web) and all green manures have perfectly "disappeared" into the soil, with all the biology having eaten and cycled them back into the soil profiles.

The idea of replenishing the soil to grow better crops, it also explores how we can grow a better world. Because as we still have months or years ahead of us in recovering from covid-19, it's the perfect time to ask ourselves "what type of world do we want to re-emerge into?"

Don't be afraid to sew both green manure and social seeds to nourish our soils and societies towards a better world for all.

  • Hannah Moloney and Anton Vikstrom are the founders of Good Life Permaculture.