This section is from the book "Rubber And Rubber Planting", by R. H. Lock. Also available from Amazon: Rubber And Rubber Planting.
Few data exist with regard to the effect of cultivation on growth. Frequent deep forking has been tried on a small scale in Ceylon, and has apparently no harmful effect upon the trees. It is hardly to be expected that this expensive operation can lead to much financial profit when carried out over wide areas, although on sloping ground an occasional forking has a marked effect in checking wash. Mr Tisdall informs me that cases of wonderful development in growth have occurred as the result of annual deep forking in fields which were thought unsuitable for Hevea, owing to the slowness of growth during the early stages. Where the conditions are favourable for the employment of agricultural machinery, deep cultivation may profitably be employed between the rows of rubber, in connection with the eradication of weeds during the early stages of growth. More often than not the use of such machinery will be prevented by the frequency of drains, by the presence of rocks or other obstructions, or by excessive gradients. Later on it is probably better to conserve the mulch of fallen leaves and to imitate forest conditions as closely as possible.
With regard to the use of manures, no really reliable experimental work has yet been published, and opinions can only be based on general principles and on .plantation experience. The latter shows considerable unanimity in favour of the use of artificial manures on the poorer classes of soils. Such manures, especially those containing nitrogen and potash, are said to exert a markedly beneficial effect on growth, yield and renewal during the earlier stages of the life of the tree. The application of potash is said to be specially beneficial in connection with renewal of bark. Phosphorus is found in practice to have less influence on the functions named; and this is what would be expected on general agricultural principles, since the use of phosphates is generally closely associated with the formation of seed. The application of phosphates may therefore perhaps be suggested in connection with seed-bearers. Manures containing potash have the advantage of being considerably cheaper than nitrogenous and phosphatic manures. An excess of nitrogen is to be avoided, as it is said to make the trees top-heavy and brittle. Such an excess is well known to lead to an extravagant production of foliage in the majority of plants. Apart from this, it does not seem likely that the application of artificials in moderate quantities can have any deleterious effect upon a rubber plantation. The experience of the best authorities in Ceylon shows that the annual application of well chosen artificial manures leads to increased growth and yield and to a better renewal of the bark as well as to some immunity from disease.