GARDENING Use green manures to improve soil fertility

Use green manures to improve your soil’s fertility


Growing
Hans Wieland

Today, I will start with my own quote of the day: “In a nutshell, or dung heap for that matter, modern society is built on cow manure!” (Hans Wieland).
Once people settled and began growing food in the same spot again and again, soil fertility became a problem and keeping soil fertile or even improving soil fertility became the big challenge. And who stepped in and helped solving the problem: The cow.
Hopefully everyone – at least every gardener and grower – now knows about the benefits of farmyard manure and compost for that matter. So here, in the first of a mini series of fortnightly articles on soil fertility, I will focus first on the newish kid on the block: Green manures.
Good fertile soil will produce healthy and nutritious vegetables. Soil fertility can be improved through the addition of farmyard manure and compost, and also by sowing ‘green manures’.

Why go green?
Green manures are grown primarily to uproot and dig into the soil, where the dying plants enrich it and improve soil fertility and structure. If even a small piece of ground looks like being vacant for a few weeks it is worth sowing a green manure. They act as a supplementary source of fertility, but should not be relied upon as the sole means of fertilising the land and supplying plant nutrients. Green manures can increase organic matter in the soil and also increase nitrogen levels. They protect the surface over winter, help suppress weeds and increase drought resistance. In some cases the flowers attract beneficial insects.  

Three are three different categories of green manure:

• Fast-growing, leafy crops, such as mustard, rape seed, fodder radish and phacelia, quickly provide a leafy canopy to suppress weeds and give an almost-immediate supply of bulky organic material when dug in.
• Leguminous crops, including various types of clover, alfalfa, bitter blue lupin, winter field beans, field peas, winter tares, and common vetch have nodules on their roots which fix atmospheric nitrogen in the soil releasing it slowly for the crops that follow them. Winter-hardy legumes such as field beans, tares and crimson clover can be used for over-wintering.
• Fibrous-rooted crops develop dense fibrous root systems which improve soil structure.  When dug in they increase the organic matter in the soil. Grazing rye and phacelia fall into this category.
  
  

How to choose a green manure?
Green manures can be used either as short-term summer catch crops for over-wintering or as long-term green manures.
It is essential to choose manures that suit your soil. Mustard for example prefers a fertile soil, buckwheat thrives also on poor soils. When planning your rotation you need to consider when they will leave the ground available for the next crop.
Green manures (which can be ordered online from the Organic Centre Shop) can be broadcast, sown in narrow or wide drills or in the case of field beans sown individually.  Generally they are dug in while still young and succulent, before the stems harden, become woody or start to flower.  If they are very bulky they can be tricky to dig in.  It is easier to cut off the top growth, leave it to wilt and then dig in or put on compost or ground for mulching.
Ideally, allow two to three weeks before sowing the next crop, as it takes green manures time to break down in the soil and they may suppress seed germination. In cold weather you may need to wait four weeks.
At the Organic Centre we are trialling a variety of green manures in our kitchen garden and the jury is still out which are the ones to recommend. We found that phacelia and winter vetch can be dug in easily. Rye is much harder to dig in but also good to improve heavy clay soils.

Phacelia facts
Phacelia is a fast-growing hardy-annual green manure. It germinates at low temperatures and is ideal for sowing from March until September. It will grow up to a meter, and can overwinter if it doesn’t get too cold. While it especially loves dry soil, it likes most soil types.
Phacelia produces attractive, scented purple/blue flowers with dense, fern-like foliage. It smothers weeds, and its large root system improves the soil structure. However, because self-seeds easily it should be dug in before flowering. It flowers six to eight weeks after sowing for a period of six to eight weeks weeks.
Phacelia is listed as one of the top 20 honey-producing flowers for honeybees and is very attractive to bumblebees and hoverflies. (Hoverflies eat a lot of aphids, so they’re good to have around.) A small patch could be left to flower, especially near to vegetables to attract pollinating insects to the area – but don’t have too many as the insects will feed on the Phacelia rather than the veg. It also makes an excellent cut flower and has a long vase-life with strong stems.
Phacelia is part of the Hydrophyllaceae family so fits into any bed in a crop rotation plan.Source: www.greenmanure.co.uk

Hans Wieland is training manager at The Organic Centre, Rossinver, Co Leitrim, which offers courses, training and information on organic growing and cooking, and runs an Eco Shop and an online gardening store. For more information, visit www.theorganiccentre.ie.
Gardening questions or comments? Contact Hans at living@mayonews.ie.