54 Mineral Nutrition of Plants 



for crop growth will be carried forth with ever-increasing confidence 

 and success. 



SUMMARY 



Soils may be considered and studied from two standpoints. First, they 

 may be looked upon as being natural entities or bodies, just like rock 

 formations or rivers, or even plants and animals, and then studied 

 with respect to their origin, morphology, and classification without any 

 specific regard to their agricultural use. This kind of consideration of 

 soils constitutes the science of pedology, which provides a sound and 

 scientific basis for soil classification and mapping. Although pedology 

 is not concerned directly with soil fertilization and management, it 

 does point the way to the basic system of soil classification needed in 

 determining proper land use and crop adaptation. 



In the second place, soils may be looked upon solely from the stand- 

 point of being media for plant growth. As the title of this paper sug- 

 gests, most of the discussion has been given from this standpoint. 



It was pointed out that soils are not only three-phase systems in the 

 ordinary sense of the physical chemist, but have in addition a "living 

 phase" represented by bacteria and other microorganisms whose num- 

 ber in a thimbleful of fertile soil may exceed the number of people on 

 this earth. The soil is, indeed, an intricate dynamic system, and to 

 understand how a fertile soil may function as a well nigh perfect 

 medium for plant growth requires much study and considerable imagi- 

 nation. 



Not only must a good soil provide anchorage and aeration for plant 

 roots, store and deliver enormous amounts of water to growing plants, 

 but it must also serve as a frugal custodian of a dozen or more nutrient 

 elements. If a soil were not frugal with its resource of nutrient ele- 

 ments, it would soon become so depleted of these elements that destruc- 

 tion by erosion would be certain and rapid because a protective vegeta- 

 tive cover would then fail to grow. The manner in which a soil func- 

 tions as a frugal and judicious custodian of nutrient elements and how 

 the amounts of these in various forms should be considered and may 

 be determined is explained in considerable detail. The unfailing de- 

 pendability of a soil serving as a medium for plant growth is beauti- 

 fully summed up in verse by David Grayson as follows: 



