Cultivation and Chem Istry Rubber Botany

Cultivation.

Hevea brasiliensis is planted on many types of soil and thrives remarkably well as long as reasonable precau tions are taken to avoid swampy, undrained or exposed lands. It is sometimes grown at an altitude of over 2,000ft., but the trees do not flourish so well at this altitude as at a lower level.

In opening up a new plantation the land is cleared of all growth as soon as possible not only to make room for the rubber trees but also to avoid the possibility of disease from rotting timber.

At one time it was the practice on rubber plantations to remove all weeds and leave bare the ground between the trees, but this is no longer regarded as an attribute of a well-kept estate. It is still the practice to remove the weeds, but heavy tropical rains have caused such loss of top-soil, particularly on sloping land, that many estates find it necessary to plant cover crops. Shrubby types of plants such as species of Crotalaria and Tephrosia which are periodically lopped and mulched into the soil, or herbaceous types such as Centrosema pubescens and Vigna oligosperma are among those used.

On sloping land cover crops are not sufficient to prevent the loss of valuable top-soil. In Ceylon for example stone walls are sometimes built across the hills. As a general rule however it is considered better to level the land in a series of contours about 15 to 2oft. apart, cutting into the side of the hill at a slight gradient to a depth of about six feet.

The loss of top-soil can also be reduced by drains which pre vent the accumulation of a continuous stream of water during heavy rain. In Ceylon a series of lateral drains empty into main drains (herring-bone drains) which carry the water away, but in Malaya and Sumatra the water is trapped in blind drains (silt pits) from which it ultimately percolates into the soil.

Where considerable erosion has taken place or the soil has been impoverished by previous cultivation it is sometimes possible to effect great improvement in the health of the trees by the addition of manures, particularly those containing nitrogen and phosphorus. This increased health is reflected in a more vigorous canopy of leaves, better replacement of bark removed for collection of latex, and a greater yield of latex.

The number of trees planted per acre on estates is largely dic tated by local conditions, such as the quality of the soil and the contour of the land. Most estates plant out more trees than will

eventually be required and thin them out, removing weak ones or those which prove low yielders, leaving about 90 trees per acre.

The rubber tree is by no means free from disease but a careful watch is kept by scientific officers, and nowhere in the East have the diseases assumed serious proportions. The most troublesome are an abnormal leaf-fall (not to be confused with that which occurs while the trees are wintering) and a pathological condition of the bark often associated with heavy tapping. Measures have been devised to counteract both, but they still occur.

Selection of Planting Material.

Most trees on estates yield 4 to 51b. of rubber per annum, but there are a few which yield as much as 3olb. growing by the side of others which yield only 2 1'b. As yield capacity is partly hereditary, it seems probable that the yield per acre may be greatly increased by propagating from high-yielding trees only. For this purpose two methods of propa gation have received considerable attention. In one the plants are raised from seeds from carefully selected high yielding mother trees. In the other a bud from a high yielding mother-tree is grafted on to a vigorous young plant grown from seed. Unless spe cial care is taken a high yielding mother tree is liable to be ferti lised by pollen from neighbouring low yielders, so that the daugh ter trees grown from seed may not be particularly good yielders. On the other hand it is to be expected that the budded material will have the vegetative characteristics of the mother tree.

Tapping.—The trees are ready for tapping for latex when about five years old, but the yield of latex and the quality of rub ber obtained are not so good as when the trees are a few years older. Tapping is a very delicate and important operation, consist ing in the removal of a shaving of bark with a sharp knife. The cut passes through the latex tubes and there is a flow of latex in consequence. If the cut is too deep, it penetrates into the cam bium and bark renewal is hindered, but if it is not deep enough only a portion of the latex tubes are pierced and the yield of latex is reduced. For perfect tapping it is necessary to cut within Ain. of the wood, an operation requiring practice and skill.