MOVEMENTS OF DISSOLVED SUBSTANC1 177 



held are not chemically changed. They remain in their 

 original form, attached to the surface of the solid as by a 

 magnet, and the amount of adsorption depends very largely 

 on the surface exposed. Hence, the smaller the soil grains 

 the greater the adsorption because of more surface exposed. 

 Another factor in adsorption is the eharaeter of the solid> 

 and of the substances in solution. Some solids have greater 

 attractive powers than others, and some dissolved substances 

 are more attracted than others. For example, humus, 

 hvdrated iron and aluminium oxides, and hvdrated silicates 

 or so-called zeolites, have greater adsorptive powers than 

 calcium carbonate and quartz. Potassium salts and phos- 

 phates are attracted more than sodium salts and nitrate-. 

 Plant foods held by adsorption on the surfaces of solids are 

 available to plants if the root hairs come in contact with 

 them. Substances absorbed chemicallv are not available 

 to plants until they are dissolved again, as may or may 

 not happen. It must be understood, however, that in no 

 case is the whole of a substance in solution absorbed either 

 chemically or physically. Xo chemical reaction is complete 

 in the soil. Only a part of the monocalcium phosphate, 

 for example, is precipitated as iron phosphate at any one 

 time, but enough of it may be rendered insoluble to affect 

 the growth of crops. Moreover, some of it is lost by leaching. 



141. Movements of Dissolved Substances. — Having dis- 

 cussed how plant foods are made soluble and how they are 

 retained in the soil after being made soluble, the question 

 naturally arises as to how the compounds in solution move 

 from place to place. That plant foods do move is obvious, 

 for thev are lost from soils in the drainage water and eventu- 

 ally are deposited in the ocean; and again they move up- 

 ward, for incrustations of salts which have been lifted by the 

 water appear on the surface of soils in arid countries. 



Movement of plant food in the soil takes place in two ways: 

 First, by the movement of water; second, by diffusion. 



(a) Movement by Water. — Water moves in the soil in 



two ways to affect substances in solution, by gravity and by 



capillarity, or more properly by surface tension. When the 



saturation point of soils is reached, water responds to the 



12 



