150 THE CHEMICAL ANALYSIS OF SOILS [chap. 



acid is also somewhat of an accidental figure, as it 

 depends very much on how far the previous treatment 

 of the soil has forwarded the weathering process, for 

 there remains in all soils rich in potash much material 

 that will not tyield potash to strong hydrochloric acid 

 even after forty-eight hours' digestion. For example, 

 the soil from one of the plots in the Broadbalk wheat- 

 field at Rothamsted only yielded 0-5 per cent, of potash 

 to hydrochloric acid, but when completely broken up 

 by ammonium fluoride it was found to contain 2-26 per 

 cent, of potash. 



Of all the soil constituents calcium carbonate shows 

 the widest fluctuations ; it may constitute 40 or 50 per 

 cent, of some of the thin soils resting on the chalk, 

 or it may sink on some of the sands and clays to such 

 small proportions as only to be detected by the most 

 refined analysis. 



The importance of the calcium carbonate lies not 

 in the calcium that it supplies for the nutrition of 

 plants, but in that it acts as the chief base, maintain- 

 ing the neutrality of the soil. Many plant diseases, 

 like the slime fungus which causes " finger-and-toe " 

 in turnips, etc., are only prevalent when the soil is 

 losing its neutral condition, and are not found when a 

 sufficiency of calcium carbonate is present. The normal 

 changes in a soil are brought about by bacteria, which 

 only flourish when the medium is neutral or very faintly 

 alkaline ; as soon as the soil becomes acid the bacterial 

 actions are largely suspended, and in their place moulds 

 and other micro-fungi become predominant. It is for 

 this reason always desirable to test the reaction of a 

 soil by putting a little on litmus paper, moistening 

 it, and after a few minutes washing away the soil. 

 What proportion of calcium carbonate is required for 

 fertility and health is difficult to say, probably an 



