THE PHYSICAL PROPERTIES OF THE SOIL. 861 



in the case of easterly and southerly winds, which are generally dry.^ 

 [This, of course, is not universally true. — Ed.] 



During tlie rainy period the precipitation serves first to replace the 

 water evaporated during the preceding period — that is, to renioisten the 

 vsoil. The saturation of the soil takes place in different ways accord- 

 ing to the conductivity and water capacity of the soil. Fine grained 

 soils which conduct water slowly are only moistened to a slight depth, 

 and in consequence of this the water accumulates to a greater or 

 less extent on the surface in the case of level land, or flows away if the 

 ground is sloping. Soils of this kind store up during a copious precip- 

 itation of long duration more water than corresponds to their water 

 capacity, because the water percolates slowly. The greater the per- 

 meability of the soil the easier the water sinks and the thicker the 

 layer of soil which is moistened. A high degree of permeability, there- 

 fore, lessens the storage of water, since more or less of the rainfall 

 passes down into the depths of the soil. Soils of this kind are most 

 uniformly moistened when there is at each precipitation not more water 

 than the soil can retain. From this we see the importance of greater 

 frequency of ])recipitation for such soils. The need of frequent pre- 

 cii)itation decreases as the water capacity increases and permeability 

 decreases, because the water supplied to the soil is stored up to a 

 greater or less extent. In extreme cases — that is, with difficultly per- 

 meable soils — a less frequent but more copious sui)ply of water must be 

 regarded as more advantageous.^ 



In as far as the humidity depends on the texture of the soil, the 

 quantity of water stored up during the wet season corresiionds almost 

 exactly to the water capacity of the soil. When, on the other hand, 

 the differences in soil moisture are caused by those in evaporation, pre- 

 cipitation may cause an equalization of the quantities of water stored 

 up. Therefore the variation in soil moisture due to difference in char- 

 acter (evenness, color) of the surface, or to difference in covering, expo- 

 sure, and inclination, may during long rainy periods or the colder 

 seasons become more or less completely equalized. 



From the f\ict that the atmospheric precipitation serves first to 

 replace the water lost by evaporation during a period of dryness, and 

 that an excess sinks down or collects only when the soil is satnrated, 

 we must infer that the quantity of percolating water and the height of 

 the ground water can not be ascertained from the permeability of the 

 soil or quantity of precipitation alone. The general facts in this con- 

 nection are summarized below. 



THE QUANTITIES OF PEnCOLATING WATER. 



In soils destitute of plant life the quantity of percolating water varies 

 with the atmospheric supply, but in a ratio which depends on the phys- 



» J. A. Henselc, Forsch. Geb. aor. Phys., 16, p. 311. 

 «E. Wollny, Forsch. Gob. agr. Phys., 14, p. 138. Compare also p. 853. 

 17034-^No. 10 2 



