Cane Soils and Artificial Manures. 289 
the actual amount in any given extent of the average 
soil is sufficient to supply the wants of even a large 
number of successive crops. The weight of any consti- 
tuent in a specified area of soil can easily be ascertained 
from the analytical figures, by remembering that a 
tenth of one per cent, represents about two tons (4480 
lbs.) to the acre, of soil 1.2 inches deep. As we already 
know the nature and quantity of the mineral matters 
removed by an acre crop of 30 tons, we can also easily 
calculate the number of such crops that a soil would 
sustain before complete exhaustion ensued. Thus taking 
the "mean" figures given in the above table to represent 
an average cane soil we should find that — 
An acre 12 inches deep 
contained 
Or 
a sufficient quantity 
of:— 
for : — 
Potassa 
... 9408 ] 
pounds 
162 crops 
Soda 
... 8960 
„ 
1389 „ 
Lime 
... 8960 
„ 
277 „ 
Magnesia 
... 22100 
„ 
1068 „ 
Sulphuric acid 
... 3584 
>> 
138 „ 
Chlorine 
... 1344 
92 „ 
Phosphoric acid 
... 4032 
„ 
208 „ 
Of course it is not to be understood that any soil would 
continue to support crops until it became quite exhausted ; 
although, on the other hand, it would be impossible to say 
at what point it would become sterile. To insure that 
the soil is kept permanently in proper condition, no 
exhaustion whatever of its constituents should be per- 
mitted. Any loss that it sustains on account of the 
crop should at once be made good by means of the 
properly selected manures. If sufficient manure can 
be added to cause a gradual increase in its fertilising 
constituents, so much the better. The figures given 
above have therefore onlv a relative value, yet serve to 
