Hoagland — 16 — Plant Nutrition 



The atoms are present as lattice ions. Occupying the 

 larger part of the lattice space are the oxygen and 

 hydroxyl ions, and in central positions lie smaller 

 silicon and aluminum ions, or sometimes magnesium 

 ions. The valence structure is balanced by adsorbed 

 basic ions, particularly calcium, magnesium, potassium 

 and sodium. Not only the extent, but also the kind 

 of surface becomes important. Several general types 

 of clay colloids have been distinguished on the basis 

 of X-ray patterns and also of chemical and dehydra- 

 tion methods. The two most definitely characterized 

 types are represented by montmorillonite and kaoli- 

 nite. A third clay mineral related to the micas has 

 been observed in some soil colloids. 



To recognize the types of colloids occurring in 

 various soils is both interesting and useful for many 

 reasons bearing on chemical and physical properties 

 of the soil. The kind of colloid may in part determine 

 the relation between colloid and soil solution and so 

 the fixation of chemical elements added to the soil in 

 the form of fertilizers or soil amendments, a matter 

 of high practical importance. 



Soil Acidity and Alkalinity in Relation to Plant 

 Growth : — In this limited survey I cannot pursue this 

 subject into its various ramifications. I shall rather re- 

 turn to several other questions raised by the soil solu- 

 tion studies. One of these was that of soil acidity and 

 alkalinity. At the beginning of the experiments in Cali- 

 fornia, soil investigators and plant physiologists were 

 not yet acquainted with physical-chemical concepts re- 

 flected in hydrogen ion or pB. values, in their applica- 

 tion to soil and plant systems. Many soils were de- 

 scribed simply as "sour" soils. A rather common idea 

 was that most plants of agricultural importance made 

 their best growth in a slightly alkaline medium and 

 that acidity was necessarily injurious. Sour or acid 

 soils were often improved by liming, a fact long evi- 

 dent from agricultural practice. 



