APPENDIX A XXXIII 
million bushels of grain. In the remaining 44 years, from 1767 to 1811, 
during which the population rose from 8 millions to over 12 millions, 
an increase more rapid than British agriculture could quite keep up with, 
the total excess of imports over exports of grain was about 132 million 
bushels in all, equal to about 2,800,000 bushels of grain annually. 
During the last five years of the period referred to, from 1811 to 1815, 
notwithstanding further increase in the population, and the waste and 
expense of wars, Great Britain, favoured with good harvests, was able 
to raise a sufficient food supply to sustain her own people. 
It is interesting to think of that fertile little island feeding all her 
own population and having a balance for export up to about 150 years 
ago. Notwithstanding the rapid increase in the number of food con- 
sumers during the next 49 years (up to 1815) she was able to occupy 
about the same independent position. How different things are now 
with a population of about 44 millions and an annual demand over and 
above all home production of over 200 million bushels of wheat. 
The growing of regular crops of cereals and other food products is 
rendered possible by the large stores of plant food laid up in the arable 
soils which cover a large part of the earth’s surface. Of the constituents 
which enter into the composition of these soils, quite a number are taken 
up by living plants, in varying proportions; but, of many of these, the 
quantities used are small and the store of such contained in the soil is 
usually ample. There are, however, three ingredients, nitrogen, potash 
and phosphoric acid, which plants take from the land on which they 
grow, in considerable proportion. The presence of these important con- 
stituents in sufficient quantity, and in available form, determines in large 
measure under reasonably favourable climatic conditions, the character 
and weight of the crop. 
It is estimated that an acre of soil, a foot deep weighs on an average 
about 3,500,000 Ib. and the results of many analyses of good ordinary 
loam in Europe, where the soil has been long under cultivation, show 
that it contains in most instances not less than 3,500 lb. of nitrogen 
per acre, and sometimes more. The quantity of potash varies from 
5,000 to 8,000 Ib. and the phosphoric acid from 3,000 to 6,000 Ib. In 
all fertile soils we find these elements, in considerable proportions, 
associated with smaller quantities of others, such as lime, magnesia, 
silica, etc., together with large quantities of humus, the latter the result 
chiefly of the decomposition of vegetable matter. These are all necessary 
to the production of healthy plant growth. 
An average crop of wheat will take from the soil for the grain and 
straw about 41 Ib. of nitrogen, 20 Ib. of potash and 18 Ib. of phos- 
Proc., 1907. 3. 
