SOME GENERAL CONSIDERATIONS 3 



for two reasons : first, because of the complicated nature 

 of its two general constituents ; and secondly, because of 

 the action and interaction of these constituents with each 

 other. 



2. Factors for plant growth. — The growth and devel- 

 opment of a plant are largely the result of two sets of 

 factors, the internal and the external. The former de- 

 pends on the nature of the plant itself, the latter on its 

 environment. The external factors of plant growth under 

 normal conditions may be classified as follows: (1) me- 

 chanical support, (2) air, (3) heat, (4) light, (5) water, and 

 (6) food. With the exception of light, the soil supplies, 

 either wholly or in part, all the conditions named. As 

 a mere mass of ground-up rock with which are mixed 

 varying quantities of decayed organic matter, the soil 

 acts as a medium for root development and thereby pro- 

 vides a foothold for the plant. Air, heat, and water are 

 supplied as a consequence of the inherent physical condi- 

 tion of a soil. The circulation of water serves to bring 

 food into solution for absorption by the rootlets. Thus 

 the two prime fimctions of the soil are realized — the 

 supplying of plant-food and of a foothold for plant life- 



3. Plant-food elements.^ — While the physical condition 

 of the soil has tremendous influence on plant growth, the 

 food elements must first be considered, since their avail- 

 ability is so closely related to the factors that function 

 in soil formation. Ten elements are usually considered 

 as absolutely necessary for plant growth. They may be 

 classified as follows: — 



1 For a complete discussion of the plant-food elements as re- 

 lated to the plant, see Russell, E. J. Soil Conditions and Plant 

 Growth, Chapter II, pp. 30-46, New York City. 2d edition, 

 1915. 



