We next come to the vexed question of weeding. It is the universal opinion of practical planters that clean weeding from the burn off is cheapest in the long run, and leads to better growth of the trees than any other system. Cover plants should not be regarded as a substitute for weeding. Leguminous plants if grown as a source of nitrogen are to be regarded as a part of the general cultivation and manuring of the estate. These processes are additional to weeding and should not be looked upon as an alternative Weeding is almost universally done by hand, but on suitable land the use of agricultural machinery is strongly to be recommended during the earlier stages of growth. The disc harrow drawn by oxen is a most effective implement for keeping down weeds and preserving a surface mulch, whilst on large estates steam power might very well be introduced. However, the presence of drains or unevenness of the land ^renders the use of such implements impossible on a large majority of rubber estates.
In connection with weeds and the use of cover crops, one remarkable fallacy, which has been again and again repeated, requires to be very clearly pointed out. It has been asserted that a close cover of leafy plants is a preservative against the effects of drought; and the moist surface of the soil beneath such a crop has been pointed out as evidence of the conservation of moisture. It is perhaps difficult at first sight to realise that a dry powdery surface is losing less moisture than a moist surface, but a little thought will show that this is certainly the case. The best preservative against the effects of drought is a thick covering of dead leaves, such as is actually present under old rubber when the leaves have fallen, an event which occurs towards the beginning of the dry season in Ceylon. Failing such a mulch, the next best form of protection is afforded by keeping the surface of the soil loose and powdery, in the form of what is known as a dust mulch, since a caked surface evaporates much more water than a loose surface. A leafy crop however is capable of evaporating three or four times as much water as.a bare surface of soil. In times of drought therefore a thick covering of living weeds is a special danger, since the weeds draw off the supplies of water available for the rubber roots, and the crop is liable to suffer severely in consequence.
On steep slopes a cover crop may be of distinct utility in checking soil wash, especially if it is grown in definite lines across the slope, and periodically cut down and laid along the contours in order to form a series of miniature terraces. For this purpose various species of Crotalaria, Indigofera and Tephrosia may be used with advantage, since these are leguminous plants which also collect nitrogen from the air.