How to restore soil fertility with green manure crops

Did you know that the key to creating healthy soil is to cover it over autumn and winter?

With abundant rainwater and a drastic reduction in pests and disease, winter crops are the easiest to grow. But, whether or not you like to grow a winter food garden, it is paramount to cover your soil to prevent weed growth and to restore the soil food source.

The easiest way to improve soil fertility is to sow green manure crops. Their long root systems gather nutrients from the depths of the soil and they absorb nitrogen from the air, fixing it into the nodules on their roots.

Green manure crops also provide a valuable carbon source and increase the humus in the soil. The best thing about these plants is that they are easy to sow and germinate, and very simple to care for. They only require a couple of weeding sessions and, in return, they will replenish your soil for the next three months.

Mustard is a fast-growing green manure crop, which can be directly sown after your summer crops. Once you’ve cleared out the weeds from your growing space, water the soil and scatter the seed liberally. Mix the seeds into the soil with a rake or by hand, press down and water until germination occurs. If you are in a dry area, water once a week until the rain comes.

Broad beans and lupins are green manure crops that have a longer growing cycle. You can sow seeds in rows and push into the soil, or scatter the seeds and cover with a thin layer of soil or compost, then press down and water.

The key with these crops is not to sow the seeds too deep, otherwise they will deplete all their energy before reaching the surface. Lupins need to be sown about 1cm deep and covered, and broad beans sown twice the diameter of the seed.

The most crucial thing to remember is to not let these crops go to flower. These are sown specifically for fixing nitrogen to the soil, so cut them down or pull them out before flowering occurs, otherwise the nutrient goodness will leave the soil and transfer to the flower.

Once ready, green manure crops can be cut just above the soil and dropped straight on top, so that the root will release nitrogen into the soil and become soil-available carbon, which will build your soil sponge and soil web.

Alternatively, green manure crops can be dug into the soil by using a spade and turning it over, or you can pull the whole plant, and then chop up stalks and use as a mulch elsewhere in your garden.

Whichever method you choose, these fantastic crops will deliver their magic! So, give your soil some love, and sow a green manure crop today.

This article was published in the New Zealand Herald’s food and wellbeing magazine, Be Well. Look out for my monthly column in the New Zealand Herald’s Monday food and wellbeing magazine Be Well and on

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