PRACTICAL MANAGEMENT OF SOILS 29 
has been cultivated and manured for so many 
years that it has become a blackened mass over- 
charged with humus, and probably also teems with 
ground insects and fungoid diseases. No soil, as 
we have already said, can without humus grow 
plants satisfactorily, but when humus is continually 
added to the soil year after year the inevitable 
result will be that the soil becomes sour. Here 
we find perhaps the greatest of all the useful functions 
of lime, for a good dressing of quicklime will sweeten 
a sour soil, bum up the excess of vegetable matter, 
and by combining with acids which in themselves 
are harmful to plant life will render them harmless 
and even convert a portion of the acids into useful 
plant food. If the soil of an old garden was 
originally heavy the excess of humus is likely to 
make it sluggish and stagnant, whereas a light soil 
overcharged with humus will always be spongy 
and lack the degree of firmness necessary to promote 
sturdy and vigorous growth. The incorporation 
of sand, burnt earth, wood ashes, etc., in the former 
case will be more beneficial than the further appli- 
cation of manure, at any rate for a year or two. 
In the case of the light spongy soil it is weU if 
possible to work in some clay, which, however, 
should be spread over the surface for two or three 
months to become * weathered ' before digging in. 
There are one or two further points that must be 
touched upon in connexion with soil treatment, 
one of the most important being that no digging 
