Page and Bod man 135 



SOME FACTORS ESSENTIAL FOR PLANT GROWTH 



The soil factors which are essential for normal plant growth are: (a) 

 a favorable soil reaction and an adequate supply of all the essential 

 nutrients, (b) a favorable supply of water, (c) adequate oxygen, (d) 

 favorable temperature, and (e) friability or looseness of the soil so that 

 roots are not restricted in their free growth and development. Of these 

 factors it is usually assumed that the supply of nutrients and the reac- 

 tion can be controlled through addition of fertilizers and lime, but 

 where the other factors are unfavorable, added nutrients may not be 

 effective and hence, in the broad sense of the term, can be considered 

 as being unavailable. The role of the water supply in affecting availa- 

 bility of nutrients will be the topic of another paper in this symposium 

 and will not be discussed here. 



In discussing the remaining topics, "availability" will be treated in 

 the sense indicated above, i.e., whenever plants fail to assimilate and 

 utilize nutrients, the nutrients will be considered as being unavailable 

 even though they may be in solution or appear to be readily available 

 from chemical tests. It is realized that this is an unorthodox usage of 

 the term "availability" but, as will be seen later, the effect of physical 

 properties of the soil on plant growth and utilization of nutrients may 

 be very significant. In the opinion of the authors too much reliance on 

 a strictly chemical definition of availability of plant nutrients is mislead- 

 ing and only through adoption of the broader definition and all that it 

 implies can we gain a proper understanding and interpretation of plant 

 growth and fertilizer response. 



When soil physical properties are unfavorable, nutrient availability 

 may be reduced in any or all of the following ways: by restriction of 

 root extension, thus limiting the volume of the soil which the roots 

 can penetrate and the number of particles with which root contact can 

 be made; by affecting the chemistry of the soil so that nutrients are no 

 longer in soluble form; by limiting the metabolic activity of roots so 

 that they can no longer function properly in assimilating the soluble 

 or exchangeable nutrients in the soil. 



