15 
agriculture it is usual to practice a rotation of crops 
whereby plants with different requirements as regards 
muneral salts succeed each other, and with the periodic 
introduction of a leguminous crop prevent the exhaustion 
of the soil. Though this is not so necessary in horticul- 
tural practice where the soil is generally richer, yet it is 
equally useful here to vary the crops in different portions 
of the plot, firstly because as shown above peas and beans 
enrich the soil in nitrogen, and secondly because such 
alternation often puts a stop to certain plant diseases 
which can only propagate themselves when the same plant 
is grown year after year on the samé plot. This is the 
case with such diseases as “club root,’ of turnips and 
cabbages, and the “wart disease” of potatoes. 
Another feature of some importance must be touched 
upon. Probably every gardener has noticed that the 
deeper soil of his garden differs very considerably from 
the surface soil. On heavy land the subsoil, as it is 
called, consists partly of solid clay, and is not so porous 
and crumbly as the surface soil. Itis also of less nutritrve 
value for it contains little or no decaying organic matter. 
Both physically and chemically, therefore, it is less suited 
to the nutrition of plants than the surface soil. It is 
important, therefore, when cultivating deep-rooted plants 
to improve the subsoil. This is done by ¢renching, a 
process which may be regarded as the cultivation of the 
subsoil. The surface soil is lifted off by removing one 
spit of soil, and then the subsoil may be improved both 
chemically and physically by digging in farmyard 
manure, or other forms of decaying organic matter. In 
the case of clayey subsoil, the addition of lime should 
not be forgotten. If the clay of the subsoil is too solid 
it may be necessary to replace it by loam, or it may be 
improved by burning it, whereby it 1s rendered more 
friable. By thus improving the subsoil, a much better 
nutrition of deep-rooted crops will be secured. In the case 
of lighter soils, where there is less difference, at any rate 
physically, between the surface soil and the subsoil, it is 
a common and beneficial practice.when trenching to put 
the surface soil at the bottom of the trench and cover it 
with the subsoil, to which manure has been added. 
Another useful process for the amelioration of soil is 
that of ridging, this 1s particularly beneficial in the case 
of heavy and tenacious soils. By digging up the soil and 
