( 15 ) 
and hilly land, the cultivation of suitable cover crops, 
particularly leguminous plants, such as Centrosema 
Plumieri , is undoubtedly preferable to grass and 
weeds for the conservation of the soil, and is more 
•easily eradicated if necessary. 
Green Green manuring, i.e. the cultivation 
Manuring. and digging in of green cover crops, 
is generally unnecessary, but may be 
beneficial before planting up lalang-covered slopes, 
on which the soil is frequently friable and a poor 
water retainer. 
Manuring. Manuring has hitherto scarcely been 
practised; and, except on worked-out 
tapioca, gambier, or pineapple lands, Hevea has done 
remarkably well without the application of manures 
or artificial fertilisers. No experiments are avail- 
able showing the effect of manuring on yield; 
and, taking into consideration the almost universal 
response of yield to thinning-out met with in Malaya, 
it seems improbable that good results are likely to 
accrue generally. The collection of latex involves 
the removal of negligible quantities of essential 
plant foods from the soil (e.g/a crop of 500 lbs. 
of dry rubber removes less than 2 lbs. of phosphoric 
acid P 3 o.,), and there seems no strong likelihood 
that manuring will have to be resorted to generally 
for a long period, except in cases where, owing to 
the previous history of the area, the soil has become 
exhausted, or on areas on which rubber should not 
have been planted on account of original poverty or 
poor character of the soil. 
In some cases it has been possible to correlate poor 
yields with the condition of the sub-soil; in such 
cases, any application of fertilisers would be entirely 
useless and uneconomical. 
