PLANT NUTRITION 257 



Nitrification is an aerobic process, the work of autotrophic bacteria 

 whose energy is obtained from tlie oxidation in this change. The 

 minimal, optimal, and maximal temperatures for the process are about 

 5°C., 25 to 35°C., and 55°C., respectively. Although nitrification 

 proceeds most rapidly in a neutral or a slightly acid medium, it is now 

 known that it can occur, though more slowly, in soils that are defi- 

 nitely on the acid side of neutrality. The presence of moisture is, of 

 course, essential for cell microbial actions. 



Soil Colloids 



Soil particles are classified arbitrarily according to size. An easily 

 remembered classification is that made by the International Society 

 of Soil Science. 



Separate Diameter Limits, mm. 



The most important of the four groups from the standpoint of soil 

 chemistry as related to plant growth is the clay fraction. It is this 

 fraction which contains the soil colloids. 



One of the most interesting properties of a fertile soil is its ability to 

 hold plant nutrients with a force strong enough to prevent rapid loss 

 of these nutrients by leaching, yet weak enough to enable plants to 

 remove them. This property is largely dependent on the presence and 

 properties of soil colloids. Colloidal particles also have an important 

 influence on the physical structure, the water-holding capacity, and 

 the buffer action of soils. Since these factors play an important part in 

 determining the fertility of a soil, the properties of the colloidal frac- 

 tion must be related to soil fertility. Soil colloids are composed of both 

 organic and inorganic materials. 



Organic colloids. Colloidal humus is negatively charged and highly 

 hydrated. It usually exists in the soil in the form of a gel which is 

 mixed with, and coats, inorganic particles. The combined effects of 

 its charge and hydration make it quite stable. As in the case of in- 

 organic colloids, it will adsorb positive ions which can function in base 

 exchange. 



Inorganic colloids. The inorganic fraction of soil colloids is com- 

 posed principally of oxides of silicon, aluminum, and iron, with 



