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  Section: Principles of Horticulture » Soil organic matter
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Green manures

Soil organic matter
  Organic matter in soil
  Living organisms in the soil
  Nutrient cycles
  Dead organic matter in the soil
  Organic matter levels
  Organic soils
  Benefits of organic matter
  Addition of organic matter
  Green manures

Unlike leys, green manuring is the practice of growing a cover crop primarily to incorporate in the soil. It is undertaken to:
  • provide organic matter which can improve soil structure, aeration, water-holding capacity and, on decomposition, increase microorganism activity in the soil;
  • add some nutrients, especially nitrogen (depending on the plants involved), for the following crop;
  • take up and store nitrogen that would otherwise be leached from bare soil over the winter period;
  • deep rooted plants can bring up nutrients which have become unavailable to shallower plants;
  • suppress weeds;
  • provide cover to protect the soil from wind or water erosion;
  • provide flowers for pollinating insects.
The seeds for green manuring are typically broadcast sown in the autumn when there are no other overwintering plants, but it can be undertaken at other times when the ground is to be left bare for several weeks instead of planting bedding or taking a catch crop. The plants are then dug or ploughed in when the land is needed again.

Plants used are typically agricultural crops that cover the ground quickly and yield a large amount of leaf to incorporate. The choice of plants needs to take into account the time of sowing, growth rate, soil type, winter hardiness, as well as particular characteristics of the species involved, e.g. legumes which fix nitrogen. Most commonly used are:
  • legumes including bitter lupins, clovers, fenugreek, tares and trefoils;
  • non-legumes including buckwheat, mustard, phacelia and rye.
Green manuring has many benefits, but there are some points to note in their management. If the plants are left to the stage when they become fibrous or woody, e.g. when allowing flowering to help pollinators, they will not provide extra nitrogen but are likely to ‘rob’ the soil of it (see C:N ratio p325). There can be difficulties when the following planting requires a fine seedbed, especially if this is to be early in the season; alternative approaches might be to cut and compost the foliage, cut or hoe off and use as a mulch or grow a plant killed by cold and remove the residue. Whilst it is highly valued in organic gardening, the value of the result when the cost of seeds, time and energy is taken into account is less clear cut in other systems.

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