294 CAPILLARY PDWER OF THE SOIL. 



The tendency of the water to ascend, however, is not the same in all 

 soils. In those which, like sandy soils and such as contain much vege- 

 table matter, are open and porous, it probably ascends most freely, while 

 stiff clays will transmit it with less rapidity. No precise experiments, 

 however, have yet been made upon this subject, chiefly, I believe, be- 

 cause this property of the soil has not hitherto been considered of such 

 importance as it really is, to the general vegetation of the globe. Let us 

 attend a little to this point. 



I have already drawn your attention to the fact, that the specimens of 

 soil which are submitted to analysis generally contain very little saline 

 matter, and yet that in a crop reaped from the same soil a very consider- 

 able proportion exists. This I have attributed to the action of the 

 rains which dissolve out the soluble saline matter from the surface 

 soil, and as they sink, carry it with them into the subsoil; or from 

 sloping grounds, and during very heavy rains, partly wash it into the 

 brooks. Hence from the proportion of soluble matter present at any one 

 time in the surface soil, we cannot safely pronounce as to the quantity 

 which the whole soil is capable of yielding to the crop that may be grown 

 upon it. For when warm weather comes and the surface soil dries 

 rapidly, then by capillary action the water rises from beneath, bringing 

 with it the soluble substances that exist in the subsoil through which it 

 ascends. Successive portions of this water evaporate from the surface, 

 leaving their saline matter behind them. And as this ascent and eva- 

 poration go on as long as the dry weather continues, the saline matter 

 accumulates about the roots of the plants so as to put within their reach 

 an ample supply of every soluble substance which is not really defective 

 in the soil. I believe that in sandy soils, and generally in all light soils, 

 of which the particles are very fine, this capillary action is of great im- 

 portance, and is intimately connected with their power of producing 

 remunerating crops. They absorb the falling rains with great rapidity, 

 and these carry down the soluble matters as they descend — so that when 

 the soil becomes soaked, and the water begins to flow over its surface, 

 the saline matter being already buried deep, is in little danger of being 

 washed away. On the return of dry weather, the water re-ascends from 

 beneath and again diffuses the soluble ingredients through the upper soil. 



In climates such as ours, where rains and heavy dews frequently fall, 

 and where the soil is seldom exposed for any long period to hot summer 

 weather unaccompanied by rain, we rarely see the full effect of (his ca- 

 pillary action of the soil. But in warm climates, where rain seldom or 

 never falls, the ascent of water from beneath, where springs happen to 

 exist in the subsoil, goes on without intermission. And as each new 

 particle of water that ascends brings with it a particle, however small, 

 of saline matter (for such waters are never pure), which it leaves behind 

 when it rises into the air in the form of vapour, a crust, at first thin, but 

 thickening as time goes on, is gradually formed on the surface of the soil. 

 Such crusts are seen in the dry season — in India, in Egypt, and in many 

 parts of Africa and America. In hot, protracted summers they may be 

 seen on the surface of our own fields, but they disappear again* with the 

 first rains that fall. Not so where rains are unknown. And thus on the 

 arid plains of Peru, and on extensive tracts in Africa, a deposit of saline 

 matter, sometimes many feet in thickness, is met with on the surface of 



