Garden Workshop : Green manuring
Green manuring is the practice of digging growing plants into theto increase its humus content. To ensure fertility, a soil must contain enough in a sufficiently decomposed state; by the activities of bacteria and other organisms in the soil, this organic matter becomes converted into humus, which in turn makes for a healthy, well-structured soil. Because of this process of breakdown by soil organisms and its uptake by plants, organic matter must be replenished periodically.
In nature, organic matter is constantly being replaced as plants die and their flowers, fruit, leaves, twigs and branches fall to the ground, and also in the form of animal droppings, dead animals and so on.
In the garden, however, the humus content of the soil is constantly being depleted as crops are grown and harvested, and the waste plant and animal material that falls to the ground is either burnt or composted; when the material is composted, a proportion of its humus content is returned to the soil, but it takes a great deal of gardento supply enough humus to keep your soil healthy. The balance can be made up with well-rotted animal manures, seaweed and other manures, but these may not be readily available. For instance, animal manures are usually difficult to obtain in urban areas. There may be a riding-stable offering horse manure, but the supply is likely to be limited, and difficulty of access or storage may make it impossible to use the manure.
The alternative in such cases is to practise green manuring. This is one of the cheapest methods of increasing the organic matter and humus content of the soil. It is not a new idea: for generations, farmers have dug cereal stubble into the soil after harvesting, and have grown green crops such as rape and clover and ploughed them in to enrich the soil. Sometimes market gardeners dig in lettuces that have gone to seed, for the same purpose.
Green manuring is clean and extremely cheap compared with farmyard manure, and labour-saving, compared with both manures and garden compost, which must be brought to the plot and spread over it before being dug in. In addition, there is a certain amount of effort involved in starting and maintaining a compost heap. Green manuring, on the other hand, involves only sowing the seed, watering it, and finally digging it in.
The main value of green manuring is not so much that it improves the content of plant food in the soil, since the amounts added are very small indeed, but that it adds humus, which improves the soil structure, and also absorbs valuable plant nutrients already in the soil which would otherwise be leached out or lost in some other way. (Plant nutrients are best supplied by adding fertilizers and manures.) Green manuring also increases the rate of bacterial activity in the soil.