387 
instance $25 per ton exclusive of transport and labour. This particular 
fertilizer supplied only one of the essential constituents necessary to 
plant growth and this may or may not have been that most needed in 
the soil in question. The increased yield of latex or rubber was un- 
fortunately not stated for any period whatever — this might have 
been interesting ! Some figures of increase of girth were given, but 
these were too small to be of value, being probably within the limits 
of experimental error, and no statement made as to how many trees 
were used on the manured plot or the control. 
There are one or two points, which it may be advisable to 
emphasize, as they may not be generally known, in connection with 
the use of fertilizers. 
(1) . The first application of any fertilizer usually produces the 
greatest increase in yield, subsequent treatments producing a gradu- 
ally diminishing increase till a level is reached. 
(2) . Double the increase is not necessarily obtained by doubling 
the amount of manure applied. 
These points explain why excessive expenditure on fertilizers in 
connection with crops of low price are not justified. The more valu- 
able the crop, the greater the amount— to a certain limit— which can 
be profitably expended on fertilizers, and this principle particularly 
applies in the case of Para rubber cultivation at the present time. 
The problem of manuring as applied to Para rubber trees is 
somewhat different to that of an annual crop such as roots, cereals, 
vegetables, etc., and resembles the cultivation of fruit trees. 
Para rubber trees, like fruit trees, require a prolonged prelim- 
inary period of growth before any crop is obtained and growth still 
continues while the tree is fruiting and being tapped, whereas with an 
annual crop, growth ceases when the plant ripens. 
We'are dealing also with a continuous cropping of one kind, thus 
removing the same proportion of plant food every year. Nothing 
except the constituents in the fallen leaves is returned to the soil. 
General principles still apply, however, in this cultivation, since it 
is essential to replace constituents removed from the soil and to main- 
tain the general health and vigour of the plant ; any treatment which 
increases the girth of the tree also enables it to be tapped at an earlier 
age. 
Manurial Requirements. 
Of the principal contituents which need replenishing in the soil 
v i z - Nitrogen, Phosphoric acid and Potash, each to a certain extent 
exercises a definite function, and if one is deficient, the application of 
the others has no beneficial effect. 
