1 64 AGRICULTURAL ECONOMICS 



broken by a roller. But they have no great water-holding capacity 

 or retentive power, and are not infrequently described by their cul- 

 tivators as hungry soils that cannot stand drought. 



Coarse sand is perhaps the most variable of all soil constituents 

 in amount, and, as its properties are in many ways the reverse of those 

 of clay, it exercises a very great effect in determining soil fertility. 

 Through its lack of cohesion it keeps the soil open and friable; in 

 moderate amounts it facilitates working, but in excess it increases 

 drainage and evaporation so much as to interfere seriously with the 

 water-holding capacity of the soil. Many good loams contain less 

 than 4 per cent and, in general, strong or tenacious soils contain less 

 coarse sand than one-half the quantity of clay present. As the 

 amount of coarse sand increases, the soils become less and less suited 

 to cultivation, till finally the sand-dune condition is reached. 



Vine gravel is not usually present to any great extent, and is of 

 importance only when the coarse sand is already dangerously high. 

 Stones, if uniformly scattered through a stiff soil, are on the whole 

 beneficial, because they facilitate tillage. Where they form a bed 

 underlying the soil they may do harm by causing overdrainage. 



Calcium carbonate. Calcium carbonate is often present in small 

 amounts only, but it plays a controlling part in soil fertility. It pro- 

 duces physical as well as chemical effects; it gives rise to the soluble 

 bicarbonate that flocculates clay, and thus physically improves the 

 soil texture. Two soils similar in constitution and general external 

 conditions, temperature, water supply, etc., have very different agri- 

 cultural value because of their different content of calcium carbonate, 

 one being readily cultivated while the other is wet and sticky and 

 suitable only to pasture land. 



The soil water. The soil retains by absorption and surface attrac- 

 tions some 10 to 20 per cent of its weight of water, distributed as films 

 over its particles. The rain water falling on the soil immediately 

 begins to soak in, but during its passage a certain amount is retained 

 on the surface of the particles and never drains away; it forms a 

 series of continuous films exhibiting all the special properties asso- 

 ciated with the surfaces of liquids. Thus, the water remains on the 

 particles against the force of gravity. Further, it tends to distribute 

 vcnly throughout a uniform mass of soil by moving from places 

 the curvature of the films is flat to places where the curvat in<- is 

 sharp. Kvaporation is continually reducing the thickness of the films, 

 and finally breaks them altogether, so that the soil becomes dry. 



