Green manure crops play an important part in field-crop production and can be used to break up pest and disease life cycles, increase soil fertility and nutrient levels, suppress weed growth and improve soil structure.
Green manures can be easily incorporated into a growing system, either field-wide or inter-row. They are particularly useful in tree production, where plants are in place for a number of years and demand access to good levels of soil nutrients. Lifting and selling trees affects soil structure and green manures can help to stabilise these areas.
Some green manures are best sown in autumn and can be effectively fitted in between some crops. They can provide good control over any potential nitrate leaching, which can occur on bare soils, particularly over winter. For crops such as Sudan grass, tagetes or brassicas, planting should occur when the risk of frost has gone - late spring and summer.
For land available prior to planting, consider a green manure crop six months in advance of that planting.
For the prevention of nitrate leaching over the winter months, grazing rye is one of the best green manures to sow. It establishes quickly and will grow year-round even in cooler months. When the rye is subsequently incorporated, it is important that the following crop is planted as soon as possible afterwards to maximise the available mineral nitrogen and prevent leaching.
Legumes, such as lupins and vetch, will produce nitrogen during their growing stages, which results in significant levels of soil mineral N available to crops that follow. It is important, therefore, to time the sowing - and subsequent incorporation - of a legume crop to maximise on the amount of nitrogen produced.
Phacelia has vigorous and highly branched roots that penetrate the soil and help to break it down. It is a valuable green manure for soils with poor structure. Its roots quickly reach their maximum development when the plant is in flower, usually around two months after sowing.
Its dense foliage can reach about 45cm high, can protect soil from the impact of rain and smother weeds. Damage by frost (at -5 degsC or below) results in foliage collapse, which provides good soil cover - particularly useful on capped soils.
As phacelia decomposes, it releases high levels of accessible forms of minerals, particularly phosphorus, calcium and magnesium, which are then available for the following crop. When incorporated into the soil, the foliage and stems quickly release high levels of nitrogen one or two months after decomposition begins.
Planted as an autumn crop, it can help to reduce nitrate leaching in mild winters, retaining up to as much as 50 per cent nitrogen produced by cultivation.
The bright-blue flowers attract hoverflies, which will predate on aphids on nearby crops. Bees are also attracted. It can be sown on all soil types (March to September) and grows in most UK climate ranges.
Sudan grass Sorghum spp.
Heat-tolerant, drought-resistant and able to grow in a wide range of soils, Sudan grass produces a deep, penetrating network of roots. This can be increased further by cutting the top growth part-way through the growing season.
Sudan grass does not produce flowers but forms a large, dense canopy layer that suppresses weed growth and prevents erosion. Soil nematode populations and Verticillium wilt propagules may be reduced in an infested soil after growing this crop. The plant produces a harmless glucoside called dhurrin, which when the plant is damaged by cutting, frost or drought is converted into hydrogen cyanide.
During growth, roots will also exude dhurrin, providing some initial control. Young tissue produces the greater amount of hydrogen cyanide but it must be incorporated and sealed immediately after flail cutting for maximum effect.
Marigolds eg: Tagetes 'Ground Control'
Tagetes species have the added benefit of being able to produce flowers and can be sown in between crops such as field-grown trees. The roots' naturally occurring broad-spectrum biocides act as nematicides, fungicides and bactericides.
There are several varieties on the market but the Caliente brand (Tozer Seeds) has shown significant promise abroad and in the UK. A wide range of soil diseases can be reduced including Verticillium wilt, silver scurf, Sclerotinia and nematode damage, onion pink root, Sclerotinia minor in lettuce, Pythium in carrots, Fusarium in tomatoes and Sclerotinia, Pythium and Fusarium in beans.
A significant amount of organic matter is also produced that can be incorporated into the soil to release a similar product to Sudan grass.
Flowering time and incorporation will depend on the variety and sowing date. A period of 21 days after incorporation is recommended prior to planting a new crop to prevent the risk of poor seed germination.
Nemat Eruca sativa
A white-flowered rocket that has shown similar biofumigant properties to the mustards. It can also be used as a trap crop for various nematodes including some root knot and cyst species. The roots contain the highest glucosinolate levels.
The dense foliage produced will provide a good level of biomass for incorporation. Mowing the crop before it flowers can extend its growing (and trapping) period. It is tolerant of a range of temperatures, can take a light frost and is relatively drought-tolerant once established.
Fully updated by Dove Associates
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