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THE BIOLOGICAL ENVIRONMENT 

 OF ROOTS 



A, G. JSorman 



UNIVERSITY OF MICHIGAN 



Soil-plant relationships are properly recognized to be highly complex. 

 Because of interactions between factors, the investigator can rarely 

 design rigorous experiments in the classical manner, in which all factors 

 but one are maintained constant, while the one under study is varied. 

 Moreover, problems of soil-plant relationships are usually approached 

 on a disciplinary basis, which may mean that the investigator myopi- 

 cally concentrates on the group of factors closest to his own interests, 

 ignoring all others. Each discipline may bring to bear on the problem 

 all the resources available to it, which is good, but because some of the 

 variables are not independent variables, there is a risk of oversimplifi- 

 cation. Furthermore, all the factors to be considered in soil-plant rela- 

 tionships are not of equal weight when evaluated in terms of total 

 plant growth, or yield of a particular plant part, as is customary. 



Some components of the soil system, though readily recognizable 

 as being variables in soils, may be relatively unimportant in causing 

 demonstrable effects on plant yield or plant welfare. Much of soil 

 biology, superficially at least, would appear to fall into this category. 

 Certainly the weight to be given to some of the biological variables in 

 the soil is not a matter of general agreement. The soil physicist has no 

 difficulty in demonstrating a direct relationship between the supply of 

 water or oxygen and plant growth. The physiologist can show direct 

 relationships between the supply of available major nutrients and 

 plant growth. Because the soil biologist cannot set up similar experi- 

 ments with soil organisms as factors affecting the soil as a medium for 

 plant growth, the significance of the presence of organisms is ques- 



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