SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 373 



are such that the reserves of essential nutrients are protected against loss by 

 their relative insolubility. 



It has been one of the salient objectives of soil chemists to clarify the 

 mechanisms by which unavailable nutrients become available to the plant. 

 This has been a substantial task because the chemistry of each of the essential 

 elements is different, and although there are certain similarities within groups 

 of these elements, each one has had to be tackled as a separate problem. With 

 some understanding of the transformations involved, the soil chemist has 

 attempted to develop relatively simple procedures that provide some measure 

 of the availability, and therefore the probable release rate, of the major 

 essential nutrients and has had some success in devising "quick" tests that 

 can be used to predict whether or not supplementing the supply by use of 

 a fertilizer would be economically advantageous. Wide use is now made of 

 such tests by farm advisers, fertilizer companies, etc. 



If the theory of dependence on the soil solution is inadequate to explain 

 nutrient uptake in the soil, what alternative is there? It is believed by soil 

 scientists that cations, or bases, such as calcium, magnesium, or potassium, 

 can be directly exchanged from the surface of soil particles to the surface of 

 a root in contact with it, without the necessity of being present as a salt in 

 the intervening soil solution. Essentially the difference is that in contact 

 exchange the base may be exchanged for hydrogen, whereas in the soil solu- 

 tion an accompanying anion must be present. The experimental proof of the 

 validity of the contact-exchange theory has presented many difficulties. It is 

 not a matter of proving one correct and the other incorrect. Both mechanisms 

 may be operative simultaneously; both may be in equilibrium. The system 

 may be complicated by the presence of micro-organisms, the surface of which 

 similarly may enter into exchange reactions, and the activities of which may 

 add products to the soil solution. 



The clay colloids. Soils are formed by the weathering of the parent rocks, 

 and the process of weathering involves extensive chemical changes. The 

 mineralogical composition of a soil may not be easy to determine, and the 

 proportion of recognizable minerals present, other than quartz, is often not 

 high. The clay component, arbitrarily taken as that fraction less than 2/x in 

 size, is colloidal in character, with a high ratio of surface to mass, and is 

 largely synthesized during the process of weathering. The type of clay depends 

 on the chemistry of the parent rock and the intensity of the weathering 

 processes. Clays are complex insoluble alumino-silicates, the structure of 

 which is built around tetrahedra of aluminum or silicon linked through oxygen 

 or to hydroxyls. They are micro-crystalline in the sense that there is regularity 

 of arrangement in planes, which regularity is not destroyed by isomorphic 

 substitution, consisting of partial replacement in the crystal lattice by other 

 elements. Clays are in effect insoluble acids bearing a negative charge which 

 can be neutralized by hydrogen ions or other cations. In a neutral soil the 



