20 
Rotations. 
worked on a sort of three-course shift, viz., two corn crops 
and a bare fallow in constant succession. Continuous corn- 
growing fouled the land, while the yield became more and 
more stunted, and thus the introduction of the bare fallow as 
a means of cleaning and incidentally renovating the fertility 
was introduced. 
2. Are economical of food in soil as well as of manures. 
Sir Humphry Davy was one of the first in this country 
to inquire into the chemical composition of farm crops, and 
in later years the two veteran experimenters of Rothamsted— 
Sir John Lawes and Sir Joseph Henry Gilbert — carried 
similar investigations to a fuller issue, and we nowadays 
know that every crop takes from the soil so many pounds per 
acre of nitrogen, phosphoric acid, potash, lime, and a lot of 
other substances ; and further, that different plants require 
these in very varying quantities. We therefore economise 
manure by varying the crops grown, because, if one kind is 
repeated on the same land year after year, one or more of the 
special chemical bodies may have to be supplied in larger 
quantities than is necessary under a more general system of 
farming. If this is not done the crop grown on the same 
soil year after year may become stunted, having exhausted 
its own special form of available food, or at any rate greatly 
reduced the supply, by its incessant demands on that par- 
ticular area. 
A dressing of manure of any kind put on a crop is generally 
compounded to specially suit that crop so far as we know how 
to do so. For instance, we put superphosphate or some other 
phosphate manure on turnips, lime in some form or another 
on clover and beans, and so on, but there is in the case of 
many fertilisers an “ unexhausted residue ” left for the next 
crop, while in the case of dung or mixed artificials some 
ingredient would not be used up so fully as the others. A 
change to another crop will equalise matters, and thus the 
whole of the fertility becomes utilised more evenly ; the 
manures as well as the “ inherent fertility ” of the soil. 
3. Alternate dee p and shalloiv -rooted crops. 
Our different crops vary very much as to the depth to 
which their roots penetrate, and the layer of soil in which 
each feeds most largely. Thus wheat and barley are good 
examples of the difference between two corn crops ; wheat is 
deep-rooted, and barley very shallow-rooted. This explains 
why they go well together in a rotation, and why in some 
rotations barley will grow well immediately after wheat. 
They draw their nutriment from different layers, and thus 
