42 THE SHADOW OF THE WORLD’S FUTURE 
"This quantity of 300 million tons annually is, however, 
for the entire earth’s surface; for its land-surface it 
may be taken to be about 80 million tons. ‘This 
quantity, however, is not a simple function of the 
amount of rainfall. It may be added further that the 
necessary statistical surveys to enable one to state 
exactly what is available on given areas do not exist. 
It is seen at once that, in general, the scale of Nature’s 
operations are, compared with Man’s, colossal. Eighty 
million tons annually is, however, only about 2% times 
the total weight of chemical fertilisers produced 
annually. Only a part of the nitrogen arriving on the 
earth in this way is effective. Considerable loss occurs 
through drainage, and some return is made to the 
atmospheric nitrogen through the agency of anzrobic 
organisms, as already stated. 
Although a shortage of nitrogen will not occur, the 
same cannot be said of phosphorus or possibly of 
potassium. The supplies, economically exploitable, of 
natural phosphates, of guanos and of bones, are very 
limited. A shortage must occur in the near future, as 
those concerned in the phosphate business well realise. 
There may also be economic difficulties in obtaining 
phosphorus even where it is known to exist. 
The sources of potassium are the ashes of land and 
marine plants, sea-water, alkaline springs, saline deposits, 
felspars, micas, wool, etc. Already there are diffi- 
culties in obtaining supplies cheaply, but what the 
economic limits are has not yet appeared. The 
necessity for supplies of potassium is now better under- 
stood than it was even a short period ago, and it is 
certain that the near future will witness greater effort 
to solve the problem of securing sufficiently cheap 
supplies. At the same time it may be noted that the 
experience of the Chinese appears to show that very 
high yields can be obtained without artificial fertilisers; 
and it may be added that should the world’s population