32 AGRICULTURAL TEXT-BOOK. 



And, in the same manner, such manures act more rapidly in 

 soils rich in humus than in those chiefly consisting of sand. 



110. The different classes of soils are distinguished by grow 

 ing different classes of weeds ; and frequently by the existence 

 of different noxious insects. 



111. The capillary power of soils also differs. 



Capillary attraction or power, means the power by which a liquid 

 ascends in the interior of a capillary tube, or tube of small bore, 

 above the surface of the liquid which surrounds it. The phenomenon 

 occurs in solid bodies which are capable of being wetted. Thus, 

 when water is poured into the basin of a flower-pot, the soil gradu 

 ally sucks it in, and becomes moist even to the surface. The same 

 takes place in the soil in the open fields. The water from beneath 

 that contained in the subsoil is gradually sucked up to the surface. 

 Where water is present in excess, this capillary action keeps the soil 

 always moist and cold. 



Evaporation takes place from the surface of the land, and as 

 each atom of moisture is taken up into the atmosphere, its place 

 is supplied by another atom, communicated by the contact of 

 the particles of soil, the more superficial acting on the deeper 

 particles like so many pumps, to elevate the water, and supply 

 the loss. Thus a naturally porous soil may be kept injuriously 

 wet by an impervious sub-soil several feet below. 



Ordinary soils possess the power of separating, from solution 

 in water, the different earthy and alkaline substances presented 

 to them in manure. But these substances, necessary for the 

 growth of the plant, are rapidly carried, by rain and other causes? 

 down into the sub-soil, beyond the reach of roots. Were there 

 no counteracting influences, these substances would soon be lost, 

 and all soils become barren. But 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 sub-soil, through which it ascends. And as 

 this ascent and evaporation go on as long as the dry weather 

 continues, the saline matter accumulates about the roots of the 



