3o6 CAUSES OF FERTILITY AND STERILITY [chap. 



plant, but only sets in action the dormant residues 

 already present in the soil, the forcing of crops by the 

 aid of lime alone soon results in the exhaustion of the 

 land. Hence the old saw : 



" Lime, and lime without manure, 

 Will make both land and farmer poor." 



The exact effect of lime in promoting fertility 

 depends upon the plant food in question. We have 

 already seen that all the decay processes which result 

 in the oxidation of the humus are promoted by the 

 presence of a base to combine with the organic acids 

 produced by the decay, and, in particular, that the 

 presence of an easily attacked base is necessary for 

 nitrification. As a nett result, the oxidation of the 

 humus and the formation of nitrates is much increased 

 by a dressing of lime, which, indeed, is the first 

 indispensable step towards rendering available the rich 

 organic residues accumulated in a sour soil. As regards 

 the mineral constituents, lime has a very marked power 

 of bringing potash into a soluble state ; the double 

 hydrated silicates of potash and alumina, etc., which 

 result from the partial breaking down of felspars and 

 are the sources of the potash of our soils, are de- 

 composed, lime being substituted for the potash going 

 into solution. It is a case of mass action, where the 

 addition of one soluble constituent to the soil will 

 increase the amount that goes into solution of all 

 the other constituents which are capable of being 

 replaced by the base added ; the extent of the action 

 is therefore dependent upon the amount of lime used. 

 The fact that more potash has been rendered avail- 

 able in limed soils is clearly seen in the character of 

 the vegetation, e.g., in an increased proportion of 

 clovers in the herbage of pasture or hay land. The 



