10 The Effect of Climate and Weather on the Soil. 
certain amount of soluble material, and a good deal more 
soluble material is added in the form of manure Some o 
this, notably the phosphates and potassium salts gets absorbed 
by the soil in such a way that it becomes foe the time b g 
locked up, and is only slowly given up to plants and still 
slowly to the drainage water. Two very striking except 
occur, however. The calcium carbonate dissolved n the 
drainage water does not become re-absorbed to any notab 
extent, but is- quickly removed as the drainage watei Ao 
away. The nitrates, also, which are among the most potent 
all nitrogenous plant foods, are not absorbed but are spee i y 
washed out. Both these substances-calcmm carbonate and 
nitrate— are exceedingly important to the fertility of the s , 
and their loss is a serious matter which has to be made good 
In the case of calcium carbonate this is readily done bj addin* 
lime or chalk, but the process of increasing the nitrate is often 
m ° Often, ^of course, a nitrate is added to the soil and then the 
process is as simple as when chalk is added. But it ^ common 
to add some other nitrogen compound, such as sulphate 
ammonia, or a complex organic material such as f&rmjar 
manure, the residue of a clover ley, or some kind of guano. 
In this case a manufacturing process has to go on m ie 
soil through the agency of the soil bacteria, and not till this 
is complete does the nitrate appear. The complication arises 
through the fact that the soil bacteria are themselves affected 
by the weather, so that the whole manufacturing process may 
be brought abruptly to an end by an adverse change m this 
direction. But even this relationship is not entirely direct. 
There is now evidence that the bacteria producing ammonia 
and nitrates are not the only organisms living in the soil, but 
that others are also present, destructive to the useful ones. 
The amount of action at any time depends on the difference in 
activity of these opposing groups. Fortunately the detrimental 
forms are more readily put out of action than the use u 
bacteria, and a period of adverse conditions is really an 
advantage to the useful forms and leads to a greater production 
of plant food. Thus exposure to prolonged frost or drought or 
to the baking of the sun does not permanently injure the 
useful bacterial activity in the soil, but on the contrary leads 
to an increase as soon as the conditions become normal again, 
because the detrimental organisms suffer the greater check, so 
that the balance shifts in favour of the useful ones. 
Further, the physical condition of the soil is affected very 
much by the weather. Frost helps to make a tilth, rain tends 
to destroy it. Neither action is quite understood, but the tact 
is incontrovertible. 
