The practice of growing a green manure comes from agriculture. Historically, it can be traced back to the fallow cycle of crop rotation, which was used to allow soils to recover. They are used as a type of fast growing ‘cover’ crop primarily grown to add nutrients and organic matter to the soil. Leaving your land bare of plants can lead to the break down of soil particles, leaching of nutrients, and poorer drainage.
Green manures, especially autumn sown ones, are effective in mopping up nutrients remaining after crops, preventing them being washed away by rain. Italian ryegrass, and rye sown in September are both very hardy, growing all winter before being dug in during the spring in order to release their nutrients as they rot. Fast growing fodder radish or mustard sown before mid-September can be incorporated in October, or their frosted remains left as a mulch.
Using plants from the legume family as a green manure will help to accumulate nitrogen from bacteria in their root nodules. Although they will function best in summer, field beans and vetches can be sown in autumn for cutting in the spring.
Green manures will not only add nutrients to the soil but they will also protect and improve the soil structure – especially important over the winter period – as well as restrict weed growth, and encourage the proliferation of beneficial soil borne organisms. Green manure can also attract native wildlife providing cover and food for predators like frogs, and hoverflies, and - if left to produce flowers - for beneficial insect pollinators.
Typically, a green manure crop is grown for a specific period – usually immediately after one crop finishes but before the next crop is sown - and is then ploughed back into the ground to rot down to release its nutrients. The crop should be cut back before the stems become woody and before flowering to make the most of the available nutrient. At this stage the available nitrogen content is relatively high. Depending on the species of green manure used, you can cut it and just leave it on the soil surface to decompose naturally.
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