368 NORMAN 



organic substances present in the root zone may not enter the plant, or that 

 there may not be interactions of nutritional importance between the surface 

 of roots and mineral particles, or that micro-organisms in the close vicinity of 

 roots are not involved in activities that affect plant growth. It has to be 

 concluded, therefore, that plant physiological studies in nutrient solutions 

 have certain limitations and that conclusions as to uptake mechanisms, etc., 

 drawn therefrom, though wholly applicable to such circumstances, may not 

 be completely valid or exclusively operate in the case of roots in soil. 



Plant requirements. Plants have the capacity of synthesizing a bewilder- 

 ing array of organic compounds, many of great complexity. Some of the 

 pathways of synthesis and interconversion are now becoming known. The 

 effectiveness of the total operation, the total dry matter synthesized, or the 

 size and fruitfulness of the plant, all are as dependent in ordinary circum- 

 stances on the quality of the soil and the nutritional and physical environ- 

 ment which it provides for the root system, as on factors such as light, carbon 

 dioxide supply, and equable temperatures that bear directly on the photosyn- 

 thetic areas of the plant. 



What does the soil provide that is essential to optimum plant growth? 

 First, inorganic nutrient ions; second, water; and third, oxygen. For optimum 

 growth an adequacy of each is necessary at all times; periods of inadequacy 

 cannot usually be compensated for later and are likely therefore to be re- 

 flected in a reduction of total seasonal growth. 



It is difficult to express plant requirements for nutrients on a meaningful 

 basis except in terms of soil area. In table 1 are given the quantities of the 



Table 1. Quantities of some nutrient elements in an acre planting of certain crops ^ 



N P K Ca Mg S 



Crop Yield lb. lb. lb. lb. lb. lb. 



Com 60 bu. 



Oats SO bu. 



Wheat 25 bu. 



Soybean 20 bu. 



Alfalfa 4 ton 



Sweet potatoes 300 bu. 



Cotton 1,5001b. 



Sugar beets 15 ton 76 10 50 22 7 5 



^ These figures are the amounts absorbed at different locations by these crops in 

 producing the yields indicated. 



major nutrients present in an acre planting of certain crops. As will be dis- 

 cussed later, these amounts may be greater than the minimum requirements, 



