81 The Food of our Agricultural Crops. 
the sugar ; in other countries additional crops are grown, such 
as rice, maize, sugar-cane, sugar-beet, &c. ; and it may be said 
that the amount of starch or sugar which we are able to produce 
upon a given area of laud, depends greatly upon the amount of 
nitric acid which the crop can obtain from the soil. Mineral 
substances, however, are quite as essential to the growth of a 
crop as nitrogen, and, amongst these, phosphoric acid and potash 
are of the greatest importance. Lime is largely taken up by 
some crops ; and where potash is deficient and soda is abundant, 
the latter substance, although it cannot take the place of potash 
in its most important functions, can at all events aid in promot- 
ing the growth of crops. 
It is extremely important to understand clearly the capacity 
of our crops to obtain a supply of mineral food from the soil. 
In our four-course rotation, which has been kept entirely with- 
out manure for forty years, ten crops of wheat and ten of 
barley, grown in a rotation with swedes and a leguminous crop, 
have given an average yield of 28 bushels of wheat and 30 
of barley. Each of these crops would carry off every year 15 
to 20 lb. of phosphoric acid ; that is, as much altogether as 
would be equal to 000 or 700 lb. of phosphate of lime. Between 
each crop of wheat and barley swedes are sown, but no bulbs will 
form, and the produce is only a few hundredweights per acre. 
It is evident that this crop cannot take up phosphate of lime 
from an unmanured soil, and that the growth of the swedes was 
arrested from the want of phosphates, and not from that of other 
substances, as the application of a mineral phosphate in the ad- 
joining experiment was sufficient to produce a fair crop. Liebig 
suggested that the extension of the growth of root crops in Great 
Britain was due to a want of sufficient phosphates in the soil 
to grow more corn crops, analysis having shown that root crops 
contain less phosphates than corn crops. At the time Liebig 
wrote, the capacity of one crop to take an important food/rom 
the soil, and the inability of another crop to take it, was alto- 
gether unknown. 
When we consider that cereal grain crops furnish by far 
the largest amount of food to the whole human race, we cannot 
but recognise how essential is the power which they possess — 
first, of obtaining so large an amount of their food from an ordi- 
nary unmanured soil, and, secondly, of growing continuously 
upon the same soil — neither of which properties is possessed by 
the other crops. The difficulty which root crops have of obtain- 
ing mineral food from the soil, has led to the idea that they are 
not dependent upon the soil for a supply of nitrogen, but can 
obtain it from the atmosphere. There is, however, every reason 
