CHAP. XIL] Protection of the Soil 271 



The more porous the soil, the larger the individual particles, 

 and the greater the extent to which stones occur throughout 

 the finer earth, the higher is the general conductivity of the 

 soil. 



Soils which are retentive of moisture, like clay and stiff loams, 

 remain cold and inactive in spring. Porous, sandy, and gravelly 

 soils become easily warmed, but are liable to the danger of 

 radiating their warmth too rapidly, and are thus apt to sink 

 below the freezing-point at the most critical times in spring, 

 and again in the autumn to a less important extent. 



Even a small admixture of moisture in sandy soils makes 

 them less apt to respond closely to sudden diurnal and noc- 

 turnal changes in the atmospheric temperature ; hence any 

 sylvicultural measures likely to increase the amount of moisture 

 in light porous soils are beneficial. And in the same way what- 

 ever tends to remove anything approaching to excessive moisture 

 in heavy land likewise stimulates the root-systems ramifying 

 throughout it to a more active imbibition of nutrients ; for the 

 rootlets are stimulated into activity far more by the soil-tem- 

 perature than they possibly could be merely by the atmospheric 

 warmth that is conducted down the stem. Apart from such 

 extreme cases of wetness as make drainage advisable to a 

 greater or less extent, it will be at once recognizable what 

 an important influence may be exerted both on light and on 

 heavy soils by an admixture of humus. Humus is strongly 

 hygroscopic in absorbing and retaining moisture It acts as 

 a non-conductor, not only in keeping out sporadic warmth from 

 the lower layers of soil early in spring before the warm changes 

 in the weather become settled, but also in retaining it well into 

 the autumn when diurnal variations again become marked. 

 It is, however, needless to recapitulate its good qualities, as 

 they have already been commented on in each of the previous 

 sections of the present Chapter. A good admixture of humus 

 in the upper layers of soil, and a soil-covering of foliage under- 

 going the process of humification, will therefore, by tending 



