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 difficulties
 in obtaining supplies cheaply, but what the
economic limits are has not yet appeared. The
necessity for supplies of potassium is now better understood
 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