No. 4.] HINTS ON LAND DRAINAGE. 373 



the elements and agents of plant nutrition which such waters 

 transfer from the atmosphere to the soil, and, as a further 

 consequence, the washing away of the soil by the flow over 

 its surface is reduced. 



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 dur- 

 ing which it is aerated in a condition most favorable to plant 

 growth. 



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

 withstand 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 upwards 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 thick- 

 ness of the aerated soil increases.* 



It has been usually held that a drained soil better with- 

 stands drought, because it absorbs or receives air more 

 freely into its interstices or pores, and extracts therefrom 

 by chemical and physical action the water which such air 

 contains. This theory I deem untenable and inadequate to 

 explain the superior capacity of well-drained soils to with- 

 stand drought. The true explanation is to be found, I doubt 

 not, in the greater penetration of the roots to depths less 



* E. Wollny, United States Experiment Station Record, Vol. VI., page 858. 



