306 SOILS. 



the greatest cooling- influence upon the soil, and next to these 

 the dense herbaceous crops, such as clover, and the legumes 

 generally. 



Influence of the Nature of the Soil-Material. Aside from 

 the surface condition, the nature of the material itself exerts a 

 certain influence, not only upon the rate of introduction of 

 heat, but also upon the amount taken up. Thus quartz sand 

 having the highest density (greatest weight per cubic foot) 

 and also the highest capacity for heat among the usual mineral 

 soil-ingredients, will, mass for mass, experience a smaller rise 

 of temperature than would clay or loam soil, of less density 

 or volume-weight, and also of lower heat-capacity. While 

 this holds good theoretically, differences corresponding to 

 this consideration rarely occur in nature, for the reason that 

 the much greater influence of the mechanical condition of the 

 soil mostly overbalances these effects. Thus \Yollny has 

 shown that while quartz is a better heat-conductor than clay, 

 quartz cobbles or gravel will materially increase the tem- 

 perature of the soil in which they are imbedded. Yet com- 

 pact clay is a better conductor of heat than loose sand; hence 

 the latter, when exposed to the intense heat of the summer 

 sun in the desert, becomes intensely hot on the surface, yet al- 

 lows of the existence of abundant moisture at a depth of ten 

 or twelve inches; while clay in the same region, being usually 

 in a compacted condition, will show a lower surface-tempera- 

 ture and will be warmer and drier at a depth at which sand 

 will still retain abundant moisture, and be comparatively cool 

 (See chap. 13. p. 257.) So much indeed depends upon the 

 state of mechanical division and flocculation in which the sev- 

 eral soils may happen to be, that a hard-and-fast statement in 

 regard to their relations to heat cannot and should not be 

 given, as it would only lead to disappointments and practical 

 mistakes; the more as in all cases the moisture-condition ex- 

 erts an influence predominating by far over that of the dry 

 material itself, and this moisture-condition is subject to rapid 

 changes, owing to intrinsic differences in the several classes 

 of soils. Wollny states as the result of his experiments, that 

 in summer sandy soils are warmest; then humous, lime and 

 loam soils ; while in winter humous soils are warmest, then 

 loams; and sand coldest. 



