THE COMPOSITION OF THE SOIL 109 



are cultivated only where large quantities of dung are avail- 

 able, or where the water-supply is exceptionally good. As 

 the amount of coarse sand increases, the soils become less 

 and less suited to cultivation, till finally the sand dune con- 

 dition may be reached, though in adequately moist conditions 

 cultivation may still continue even when 90 per cent, of coarse 

 sand and no clay are present (e.g. part of Anglesea). 



Fine gravel is not usually present to any great extent, and 

 is of importance only when the coarse sand is already danger- 

 ously high. Stones cannot be determined quantitatively by 

 any method of sampling in use, and their effect must be 

 judged by a visit to the field. If they are uniformly scattered 

 through a stiff soil, as in the Clay-with-Flints, they are on the 

 whole beneficial, because they facilitate tillage. Where they 

 form a bed underlying the soil they may do harm by causing 

 over-drainage. Some typical examples are discussed in 

 Chapter VII. 



Reactive Inorganic Constituents. 



It was formerly supposed that zeolites occurred in the 

 soil and accounted for many of the soil reactions and absorp- 

 tions. The direct evidence is so slight \that this view is now 

 generally given up. It seems necessary, however, to assume 

 that some of the inorganic constituents of the soil are very 

 reactive, since certain chemical changes are brought about 

 which can hardly be attributed to micro-organisms. Thus, 

 calcium cyanamide is decomposed by soil with formation of 

 urea, and the change proceeds whether the soil be fresh or 

 ignited ; this change can also be effected by certain minerals, 

 e.g. prehnite. So phenol reacts instantly with soil to form 

 some insoluble compound not decomposable by steam. These 

 obscure reactions are being studied at Rothamsted (p. 215). 



Soil Acidity. 



It has long been known that many soils are acid to litmus 

 paper but become neutral on addition of lime or calcium 



