GREEN MANURE

Ian
What is green manure and when is the best time to incorporate it into the garden? Y. N by e-mail


Reply
Adding fast growing plants into the soil to improve fertility between crops in vegetable gardens, or around fruit trees and bushes, is called green manuring. It is also a weed suppresser and soil-improving preparation for borders. After green manuring soils are easier to work, even though young soft growth contributes little organic matter. Green manuring supplements rather than replaces manures and fertilisers. The most common types of green manure are clover, beans, ryegrass, fenugreek and lupins. The seeds can be bought from most of the well-known seed catalogues and specialist shops.

When to sow
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 very hardy, growing all winter before being dug in during spring to release nutrients as they rot. Fast growing fodder radish or mustard sown before mid-September can be incorporated in October, or leave their frosted remains left as a mulch. Summer grown green manures, buckwheat and fenugreek for example, consume useful space and leave soil very dry. However, they can form dense foliage that is ideal for smothering weeds, much in the same way that spud plants do. Nitrogen fixing plants such as legumes (a pod, such as that of a pea or bean, that splits into two valves with the seeds attached to one edge) accumulate nitrogen into themselves with help from bacteria in their root nodules. These function best in summer, and although field beans and vetches can be sown in autumn for incorporation in spring, summer crops of lupins, clovers and peas fix more nitrogen. Other benefits of green manures include winter protection of the soil from compaction by rain and shelter for beneficial insects such as ground beetles, although slug control may be needed after green manuring.

Sowing and digging in

Green manure seeds are sown broadcast and raked into the soil. They are dug in when lush and leafy, before flowering. Flowering green manures such as clovers benefit helpful insects. But after flowering plants become woody and can temporarily use up soil nitrogen when dug in, so add extra organic nutrients before sowing or planting if you leave the plant too long before digging in. Decaying green manures can suppress plant growth; allow at least two weeks between incorporation and planting or sowing.

Horticultural.