24 



Third. — It affords also a quicker escape of the water falling 

 thereon, thus shortening the time during which the soil is saturated 

 with moisture and increasing the time during which it is aerated 

 in a condition most favorable to plant growth. 



Fourth. — Plants rooted in a deep soil are better able to with- 

 stand drought, chiefly by reason of the fact that beyond certain 

 depths the evaporative power of sun and wind does not extend to 

 an effective degree, whereby a drought that may be ruinous where, 

 for want of drainage, the soil is shallow, may have little effect 

 upon a deep, well-drained soil. 



Fifth. — The capillarity of clayey soils — that is, the quality by 

 which they absorb and lift water from below and convey it up- 

 wards to the roots of plants (as a wick lifts oil from the body of a 

 lamp to the flame at its upper end) — is increased or quickened by 

 deeper cultivation and root action therein. 



Sixth. — Considerations of color, texture, material and exposure 

 or direction of slope tend to modify somewhat through evaporation 

 the physical character of soils as affected by drainage ; yet as a 

 rule a deeply drained soil is more uniform and constant in its 

 moisture conditions or humidity than is a shallow one ; and it may 

 be accepted as an axiom that variation in humidity decreases as 

 the depth or thickness of the aerated soil increases.* 



It has been usually held that a drained soil better withstands 

 drought, because it absorbs or receives air more freely into its 

 interstices or pores, and extracts therefrom by chemical and phys- 

 ical action the water which such air contains. This theory I deem 

 untenable and inadequate to explain the superior capacity of well- 

 drained soils to withstand drought. The true explanation is to be 

 found, I doubt not, in the greater penetration of the roots to depths 

 less affected by drought and in the increased power of the deeper 

 soil to obtain moisture by capillary attraction from below. 



The insufficiency of aeration alone to supply moisture in amounts 

 large enough to sustain ordinary vegetation is often shown by 

 drained peaty soils, which, though of the most porous and absorb- 

 ent nature, are not only incapable, during seasons of prolonged 

 drought, of acquiring moisture by aeration (absorption from the 

 air above) , but are equally incapable of receiving it by capillarity 

 (absorption from the water table below). The difficulty with such 

 soils is that the size of the pores is so large and their volume so 

 great that, while permitting the greatest possible degree of aera- 

 tion, they are incapable of lifting water to a sufficient height by 

 capillary attraction, and so remain dry. The same is true of soils 



* E. Wollny, Uuited States Experiment Station Record, Vol. VI., page S58, 



