Nitrogen is the most expensive of all the substances required by growing plants. A sufficient supply of nitrogen can most cheaply be added to soils, which are deficient in this respect, by the practice of green manuring. This method consists in growing some plant which is capable of taking up nitrogen from the air. The nitrogen is added to the soil by cutting down the plants at the proper stage of growth and either burying them or using them as a mulch. It is important therefore that this process should only be undertaken when there is plenty of labour available for dealing with the green crop at the proper stage. The plants used for green manuring belong to the natural order leguminosae. They may conveniently be divided into (1) creepers and climbers, (2) low growing bushy plants and (3) trees.
Creepers and climbers are not generally to be recommended. They are difficult to deal with, are liable to smother the young rubber trees if neglected, and form a convenient cover for snakes and other vermin.
Among low growing bushy plants some of the best, according to experiments carried out at Peradeniya, are Crotalaria striata, Indigo/era arrecta, Tephrosia Candida and Leucaetia glauca. The last named plant grows into a good sized shrub if permitted to do so, but it can be kept coppiced and then provides a heavy mulch of green shoots. Any of the above named plants may be sown broadcast on level land or in transverse lines on sloping ground. A maximum of green material is generally available just before flowering, after four or five months growth, when the plants have reached a height of from four to six feet The plants are then cut down close to the ground and mulched round the trees. On slopes they may be spread in the form of a crescent some feet below each tree, in order to lead to the formation of small natural terraces. Such a mulch is specially valuable in times of drought, but the method can only be applied with safety in countries where the drought begins at stated periods. The cutting and mulching must be carried out at the beginning of such a period, otherwise the green crop will dry up the soil by evaporating water. The dry stems left after cutting lead to no appreciable loss of water, whilst the green mulch is a valuable protection. After cutting, the plants throw up fresh shoots, and in a moist climate they can be cut repeatedly at intervals of four or five months. Ultimately their growth is checked by the shade of the rubber and they should then be uprooted.
In situations where it will grow freely, one of the best types of tree for green manuring is the dadap, Erythrina lithosperma. This may be planted alternately in the rows of rubber, and can be conveniently raised from cuttings after a stock has been obtained from seed. The best cuttings are stout branches five or six feet long and three or four inches in diameter. These are planted like so many posts, and develop into good sized trees in little more than a year. Care must therefore be taken that their growth is not allowed to interfere with that of the rubber. The green branches are pollarded at suitable intervals and mulched as already described in the case of smaller plants. The shelter afforded by the dadaps is often valuable while the rubber is young, and their presence tends to draw up the latter into tall saplings and to check branching close to the ground.