72 
The Food of our Ayr (cultural Crops. 
just when it was wanted, and was followed by less than the 
average rainfall during the next three months. This unusually 
large crop of wheat, grown under adverse summer conditions, 
can therefore only be accounted for by the fact that the large 
amount of nitric acid which was formed in the soil, and had ac- 
cumulated there owing to the extremely dry weather prevailing 
from seed-time to harvest, was available for the use of the crop 
in the most important period of its growth. A great increase 
in the available nitric acid in the soil was evidently the cause, 
in the same year, of the exceedingly large yield of wheat 
(amounting to 21 bushels per acre) on our permanently un- 
manured land ; for we know that at any time, by the applica- 
tion of nitrate of soda, the crops on this land can be increased 
by several bushels per acre. 
The use of phosphates for root crops has sometimes led to 
the idea that these crops and the cereal crops do not obtain 
their nitrogen from the same source, and that the former are 
less dependent upon a supply of nitric acid in the soil. It is 
quite true that fall agricultural crops of roots are frequently 
grown by the use of a manure which contains a soluble phosphate, 
and no nitrogen ; but in these cases the soil must be equal to 
furnishing the amount of nitric acid required by the crop. It 
is known that the formation of nitric acid in the soil is much 
more rapid in the summer and autumn than at other times, and 
that constant stirring the soil so as to expose fresh surfaces to 
the air increases this formation. Root crops, beginning their 
active growth at the time when cereals are ripening, can obtain 
from the soil large quantities of nitric acid which were not 
available for the corn crops ; the great exhaustion of the soil 
which follows the removal of a large root crop also proving that 
it obtains its nitrogen from the same source as the cereals. It 
is somewhat remarkable that maize, which is so largely grown 
in the United States, is said to be mostly benefited by the appli- 
cation of mineral manures, and to derive its nitrogen from the 
atmosphere, and not from the soil. Owing to the growth of 
maize taking place during the summer and autumn, it would, 
like our root crops, be less dependent upon a direct supply of 
nitrogen in manures ; but, at the same time, it is reasonable to 
suppose that the source of the nitrogen in maize is the same as 
that of other cereal crops. So far as our crops are concerned, 
the evidence at our command points to the conclusion that our 
cereal crops, pasture-grasses, and various root crops, depend for 
their supply of nitrogen almost entirely upon the nitric acid in 
the soil. 
There is a very important class of plants, included under tho 
