The Food of our Agricultural Crops. 
71 
bear upon phosphates to increase their solubility. By degrees 
it became known that the three most important foods of plants — 
phosphoric acid, potash, and nitrogen — exist in the soil in a very 
insoluble form, and that by the action of the roots of plants the 
two former substances are dissolved, and taken up as food ; 
while organic nitrogen, called humus by the older chemists, is 
the source of the nitric acid found in the water of the soil ; and 
that, until the nitrogenous compound of the humus has become 
nitrified, it possesses comparatively small manuring properties. 
When it was found that the drainage-water taken from the soil 
when the crop is in the full vigour of its growth contains no 
nitric acid, while at all other times it does contain it in more or 
less quantities, there could no longer be any doubt that nitric 
acid is the chief som*ce of the nitrogen in some of our crops, 
and that the necessity for the application of nitrogen in a 
manure depends upon the capability of the soil to liberate a 
sufficient amount of nitric acid to supply the wants of the crop. 
It is evident that nitric acid and its salts, such as the nitrates 
of soda, potash, and lime, being very soluble in water, and 
forming no fixed compounds with the soil, like phosphoric acid 
or potash, cannot accumulate ; what is not taken up by the 
crops is more or less completely washed out during the winter. 
Each year, therefore, fresh quantities of nitric acid are formed, 
partly from the organic matter existing in the soil and partly 
from that supplied in the ordinary manures of the farm. The 
quantity available for the crop varies every year. For instance, 
after a very wet winter and spring a wheat crop may find 
very little left for its use when its active growth takes place 
in the spring, and must depend upon the amount liberated 
in the soil during its growth, or upon a direct application of 
nitrates. 
The immense importance of dry weather in preventing the 
loss of nitric acid is well illustrated by the season of 1 853-54. 
The wheat crop of 1851 was one of the typical great crops 
of the present century, the yield being very large upon all 
sorts of land, from the lightest to the heaviest; and yet, if 
we were to judge by the climate of the summer months, which 
are supposed to decide the yield of wheat, we should have ex- 
pected that the crop would be below the average. May was 
cold and wet, June cold and sunless, July had not one day of 
summer heat, and the harvest was ten days or a fortnight later 
than usual. But now let us turn to the winter months of the 
same season. From seed-time to the end of April we find a 
great deficiency of rain, only half the average amount having 
fallen. In May there was a great excess of rain, which came 
