27(6 Food of oar Agriculturjil Crops. 
85 
to suppose that they derive their nitrogen from nitric acid in 
the soil, and require even more than the cereal crops. If they 
are less dependent upon an artificial supply of nitrogen than 
corn crops, it is because they grow during the summer and 
autumn, when nitrification of the soil is most active ; this, and 
the constant stirring of the soil, enables them to obtain the 
necessary supply, independent of any aid from manures. 
Leguminous crops possess qualities quite distinct from the 
other crops grown in a rotation : they are more dependent upon 
soil and climate, and less upon a direct supply of manure. 
When they are not grown, the food they require accumulates in 
the soil, and, where one kind of leguminous plant has exhausted 
the soil of its own special food, another may find in abundance 
food suitable for its own growth. How far these exceptional 
properties in leguminous plants are due to special compounds 
in the soil, or to special micro-organisms which prepare their 
food, is at present quite unknown ; and while acknowledging 
their great value to agriculture as restorative crops, we must 
admit that science is not yet in a position to give a full explana- 
tion of these exceptional properties. 
We see, therefore, that the economy of a rotation of crops is 
due to the special functions performed by the different crops. 
We have, first, the corn crops, which possess the remarkable 
power of obtaining a supply of food from an ordinary unmanured 
soil, and also of growing upon the same soil for very long 
periods. They derive their nitrogen from the soil, chiefly in 
the form of nitrates ; and although the crops do not carry off 
large quantities, they are indirectly responsible for a great loss 
of nitrogen, owing to the ear\j period of the year at which they 
cease to take up nitrates from the soil. The root crops, on the 
contrary, can obtain very little food from an unmanured soil, 
and are largely dependent upon a supply in manure, especially 
of phosphates. But they also take large quantities of nitrogen 
from the soil, chiefly as nitrates ; and owing to their growth 
taking place in the summer and autumn, more nitrates are 
formed in the soil, and taken up by the crops, and there is less 
loss than with the corn crops. As cleaning crops, also, the 
value of roots is very great, especially to the corn crops. The 
leguminous crops, and especially those which are sown with 
the corn crops, such as the red and white clovers, are great 
restorers of fertility, although there is still wanting a clear 
proof that, under the ordinary conditions of agriculture, these 
crops obtain their nitrogen from the atmosphere. By sending 
their roots deep into the subsoil, and covering the land with 
green vegetation for so long a period, they are able to collect 
