Saturday 10 October 2009

Green manure to upgrade soil

It is becoming increasingly difficult to obtain good quality horse or cattle manure, so a different tack can be employed.
In agriculture and horticulture, a green manure is a type of cover crop grown primarily to add nutrients and organic matter to the soil. Typically, a green manure crop is grown for a specific period, and then ploughed under and incorporated into the soil. Green manures usually perform multiple functions, that include soil improvement and soil protection:

  • Leguminous green manures such as clover and vetch contain nitrogen-fixing symbiotic bacteria in root nodules that fix atmospheric nitrogen in a form that plants can use.
  • Green manures increase the percentage of organic matter (biomass) in the soil, thereby improving water retention, aeration, and other soil characteristics.
  • The root systems of some varieties of green manure grow deep in the soil and bring up nutrient resources unavailable to shallower-rooted crops.
  • Common cover crop functions of weed suppression and prevention of soil erosion and compaction are often also taken into account when selecting and using green manures.
  • Some green manure crops, when allowed to flower, provide forage for pollinating insects.

Historically, the practice of green manuring can be traced back to the fallow cycle of crop rotation, which was used to allow soils to recover.
Our own green manure crops have been planted and are growing well, we are growing mustard on two beds and forage peas on another two.
Mustard is a very quick growing green manure, that can be sown from March – September and has a growing period of 1-2 months and can reach 60-90cm. The half hardy annual grows on most soil types and produces large volumes of green matter and residual fibre which is particularly good for those soils that lack organic matter it helps to improve soil texture and moisture retention.
After 4-8 weeks of growth chop down the plants and cultivate into the top few inches of your soil. On sandy ground let mustard reach 40cm in height or wait for the first flowers to appear before digging in. This will produce more fibrous plant matter which can help very free draining soils retain more moisture and nutrients. It isn’t a nitrogen fixer though but is still a very worthwhile leafy green manure.
As  part of the brassica family, repeated sowings and digging in of the mustard at 4-8 week intervals may help to clear infected land of club-root. Mustard is part of the brassica family and it will activate the dormant spores of this fungal disease, and will allow the grower to break its life-cycle as the plants are prematurely destroyed. An excellent plus, as club-root disease can remain dormant in the ground for up to 20 years but this is not a quick fix and it will take a number of repeated sowings.
As it is part of the brassica family don’t use mustard in crop rotation plan before sowing a brassica crop as this increases the likelihood of diseases.
Another control is that if allowed to fully mature it can clear a site of wireworms- these insects can devastate a crop of potatoes. Wireworms feed on the dug in and decomposing mustard crop, they mature quickly and fly off as beetles to lay in pasture, so the cultivated area is now clear.

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