6 
The Effect of Climate and Weather on the Soil. 
moister conditions a more leafy type of vegetation arises. 
These two types of vegetation break down m very Ameren 
manner in the soil : the large leafy plants yield a considerable 
supply of useful humus material, while the shrubbier and more 
leathery plants of the dry situation do not. There may be 
plenty of organic matter in these dry soils ; the light dry 
sands of the Sussex heaths sometimes contain -as much as It 
per cent., but it exists in the form of undecomposed bracken 
fronds and similar residues, and is of no agricultural value 
because it is not properly decomposed. Hilgard m California 
long ago drew attention to the great difference between the 
humus material in soils of dry and humid regions, and this 
difference arises from the fact that in humid regions the 
conditions are favourable for the growth of the best kind of 
plant to make humus material and also for the carrying on of 
the best type of decomposition process. 
Soil Losses. 
So far we have been considering only the building up of 
the soil ; we have now to turn to the other side of the account 
and study the losses that are going on. The processes that 
called the soil into being are still operative to-day, and the 
transport of material did not come to an end when the soil 
was brought into its present position but continues, and tends 
to remove the soil now that it is formed. The losses have gone 
on simultaneously with the formation of the soil and they still 
continue. The most important source is the rain. As rain 
falls on to the land and soaks in it dissolves out some substances 
and carries them away. Hence the drainage waters are always 
hard and often unfit for drinking. The constituent that is 
removed in largest quantity is calcium carbonate, and no less 
than 8 to 10 cwt. per acre of this are washed away each year 
at Rothamsted. The importance of this becomes evident when 
it is realised that calcium carbonate is a most potent agent in 
enabling a good tilth to be got and in preventing the soil from 
becoming sour. Other soluble constituents are also removed 
in proportions which are certainly less but which become 
considerable when the action is continued year after year. 
Thus in course of time a soil exposed to a heavy rainfall tends 
to become reduced to hard insoluble residues of unchanged 
mineral fragments ; finally it may become barren through loss 
of plant food, and “ sour ” through absence of calcium carbonate. 
On the other hand, a soil in a dry region of low rainfall keeps 
all its soluble constituents intact, indeed it may become so 
heavily charged with them as to become barren through this 
very excess. Again, heavy rainfall may wash the soil bodily 
away and leave only the bare rock or a wholly impossible 
