Lecture 1 — 13 — Survey 



ciate. A reserve of basic ions is held by the colloids, 

 and these ions can in general be readily displaced by 

 hydrogen ions biologically produced, from carbonic 

 acid given off by roots or from acids formed by micro- 

 biological processes, among which nitrification is of 

 primary consequence. The group of California soils 

 investigated were found to have their colloids saturated 

 with calcium, magnesium and potassium mainly; cal- 

 cium and magnesium predominated; of these two 

 calcium was first in quantitative order. The solid 

 phase of alkaline soils thus readily yields bases to the 

 soil solution when hydrogen ions become available for 

 ion exchange. But soils of humid regions are fre- 

 quently acid. This is not merely a matter of a certain 

 hydrogen ion concentration in the soil solution. Of 

 deeper import is the fact that in highly acid soils 

 much calcium and magnesium have been displaced 

 from the colloids by hydrogen ions and then leached 

 out of the soil. 



There is a third type of soil condition with refer- 

 ence to the exchange status of the soil colloids and 

 their interrelations with the soil solution which has 

 had extraordinary importance in arid regions. I have 

 in mind those soil conditions loosely grouped under 

 the term "alkali" soils. These are soils in which large 

 quantities of sodium salts are present, or have been 

 present in the past history of the soil. If alkali soils 

 are also highly alkaline, they are often referred to as 

 "black alkali" soils, because of the accompaniment of 

 a black crust of organic matter. About the origin of 

 alkali conditions I can only mention that high water 

 tables, or irrigation waters high in salt, are chiefly 

 responsible. Frequently, the black alkali soils are 

 very diflScult to reclaim. On leaching out excess salt, 

 which tends to maintain a flocculated state in the clay 

 colloids, the soils soon tend to become highly imper- 

 meable to water. Indeed some soils of this kind once 

 seemed to present a rather hopeless case. The real 

 nature of the difficulty was not fully appreciated. 



