24 Mineral Nutrition of Plants 



soil scientist should have at least a working familiarity and knowledge 

 in all of these special fields. On the other hand, many agronomists and 

 horticulturists are interested in soil science solely from the standpoint 

 of knowing how soils may serve as a medium for plant growth, and 

 how they must be managed so as to produce satisfactory crops and at 

 the same time be conserved for all time for this purpose. Although this 

 paper is devoted primarily to matters of special interest to the agron- 

 omist, horticulturist, and plant physiologist, a brief discussion of soils 

 from the pedologic viewpoint seems appropriate. 



SOIL AS VIEWED BY THE PEDOLOGIST 



The so-called science of pedology is the basic science of soils which 

 deals with the origin, evolution, morphology, and classification of soils 

 without any reference necessarily to their use for crop production. Soils 

 are considered to be natural bodies or entities in themselves, deserving 

 study as such, just as plants are to the botanist who often studies them 

 without thinking of their economic value or use. This does not mean 

 that the pedologist in pursuit of his vocation does not add to the eco- 

 nomic welfare of society. Quite the contrary. By learning as much as 

 possible about soils just as soils, a sound basic foundation is provided 

 for their best use in the same way that the botanist provides this in the 

 case of plants. 



In order to determine the main characteristics of a soil so that it may 

 be properly classified and evaluated, more than surface examination is 

 needed; it is, in fact, necessary to expose the soil for observation and 

 study from top to bottom. The pedologist calls this exposure a soil 

 profile, which is simply a vertical cross-sectional view of a soil, extend- 

 ing from the surface to a depth, usually, of three or four feet, or to bed- 

 rock. It is the view one may see along a road cut, such as that pictured 

 in Figure i, where it will be noted in particular that the soil has three 

 layers, called horizons by the pedologist. 



It may well be asked — Why does a soil have layers or horizons 

 which differ in characteristics? Why is it not uniform throughout? A 

 consideration of how a soil forms and develops will suggest answers. 



Let us suppose that an area of bare rock has become exposed to the 

 action of air and water with accompanying ever-changing temperatures 



