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.
