264 CAUSES OF FERTILITY AND STERILITY [chap. 



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 

 action of lime as a liberator of potash is illustrated 

 by the effect of a dressing of chalk applied in 1881 

 to part of the permanent grass plots at Rothamsted; 

 by 1884 differences began to be manifest, the chalk 

 caused a change in the herbage of those plots which 

 had been receiving potash each year for twenty-five 

 years previously, increasing the production as a whole, 

 and particularly increasing the proportion of leguminous 

 plants in the herbage. On the plots, however, which 

 had been receiving no potash, and therefore contained 

 no recently accumulated reserves of this material, the 

 chalk had practically no effect, either in the weight or 

 character of the crop. 



To some extent lime seems able to act as a liberator 

 of phosphoric acid in the soil. As pointed out by 

 Thdnard, lime is able to act upon the very insoluble 



