110 CHEMICAL EQUILIBRIUM AND ANTAGONISM 



reaction occurs the salts held in a mechanical manner are 

 probably within the inner circle of the capillary film where 

 very little movement is possible; consequently, unless 

 there is long-continued and excessive washing of the soil, 

 little of this salt is lost except by diffusion which is a very 



Fig. 15. Alkali Coming to the Surface where Seepage Water 

 from a Canal Comes to the Surface and Evaporates. The Canal 

 Runs through a Shale that is High in Soluble Salts. 



slow process in case the salts are not promptly removed 

 from the point of concentration. This adherence, or 

 adsorption, may account for the great quantities of salts 

 that are slowly yielded to water leaching through soils. 

 As more and more of the salts are given up to the solution 

 and carried away, the remaining portion is with greater 

 and greater difficulty yielded to the free, or percolating, 

 water. Because of the greater surface exposed, fine clays, 

 loams, and soils rich in organic matter hold the salts by 

 absorption more tenaciously than the coarser-grained sands. 

 Soils such as the clays, which are high in colloidal ma- 

 terial, are also affected by an interchange of ions. The 



