Green Manures

Manure is usually something that we think of as smelly, albeit useful for our garden. However, it is sometimes difficult to obtain from a reliable source, especially if you live in the big city. Added to this, there’s the problem of using manure that may come from animals that have been inoculated with hormones and antibiotics. These are not going to be very good for your organic vegetable garden. Which is why it’s time to take a look at green manures.

According to Gardening Know How, ‘Green manure is a term used to describe specific plant or crop varieties that are grown and turned into the soil to improve its overall quality’. This is a wonderful dual benefit. The plant can be useful as a growing crop, and as a manure once it has been harvested and put back in the soil.

Once your green manures have been dug into the soil, you should ideally leave the beds for about two weeks or more before sowing your next crop. The green material needs time to decay and not hamper growth of new seeds or seedlings.

Green manures are also sometimes cover crops and smother crops. Cover crops protect your soil from erosion by wind and water, and smother crops keep weeds under control.

Which are suitable green manures?

Interestingly, there are many plants that are useful as green manures, but here are a few that are familiar to most of us.

Broad beans

Broad beans (Vicia faba) is a wonderful bean grown in winter (green beans are summer crops). It is part of the legume family and has pretty flowers and a prolific crop of large, flat, green pods. Obviously the pods and shells can be put in your compost for added nutrition. All legumes provide ‘free’ nitrogen to your soil for your next crop, by fixing gaseous nitrogen in the soil. The legume plants have a symbiotic relationship with rhizobium bacteria, which appear as white nodules on their roots. These convert the nitrogen in the atmosphere into plant-useable nitrogen. Nitrogen is essential for healthy leaves.

After harvesting the bean pods, simply cut down the remaining plant growth and lightly turn this into the soil bed. This organic matter improves the physical condition and the structure of the soil. This will also assist with the protection of the soil from compaction by adding extra plant matter into the soil to break down. One of our biggest problems on the planet right now is the lack of good topsoil for growing food.


Buckwheat (Fagopyrum esculentum) is a fast growing, half-hardy spring and summer plant. It grows well in soil that may be acidic, has low phosphorus and/or is considered nutrient poor. Once cut, buckwheat decomposes rapidly, so it is useful as a green manure. While it does contribute to the organic matter in the soil, it doesn’t contribute as much as a legume crop would. It can regrow if it is left to do so, and is also good as a smother crop for weedy fields.

Brassicas, especially the mustard family

Mustard (Sinapis alba) is a beautiful, large-leaved plant. Red mustard has a gorgeous, deep purple-red, large leaf that is delicious to eat. Remember your crop rotation rules and don’t follow your planting with another crop of brassicas. This could encourage a build-up in your soil of disease particular to the brassica family, such as clubroot. A lot of brassicas are cool-season crops, such as turnips, cabbage, kale, broccoli. There also summer brassicas, like rocket, and year-long brassicas such as radishes. All are useful because of their roots – in some cases where the taproot is long, they will assist with the break-up of compacted soil. Many brassicas are also good forage crops.

Red/crimson clover

Red/crimson clover (Trifolium pratense) is a short-lived perennial. It’s tolerant of acidic soil, which is usually caused by poor drainage, and is also quite able to grow with less water than most. Clover is grown in early spring and summer right up until autumn. However, it can easily be under-sown in midsummer if there is sufficient moisture for the existing crop and the clover.

All of these and many more can be incorporated into your planting planning, and you will be thrilled with the results of nutrient-enriched soil. Remember: healthy soil = healthy plants.

The Gardener