252 SOIL CONDITIONS AND PLANT GROWTH 



1. The population at any given moment is as large as the 

 conditions allow, and is limited by presence of a harmful factor 

 or by lack of some essential factor such as energy supply, food 

 supply, air, water, or temperature. Improvement in the supply 

 of the limiting factor, or removal of the harmful factor, allows 

 an increase in numbers of the populaton. 



2. Competition therefore is an important factor in de- 

 termining the relative numbers of the various groups : any 

 increase in the numbers of one group may lead to a decrease 

 in the numbers of others having similar requirements. Changes 

 in numbers of any one group, therefore, are not necessarily 

 sharply related to changes in external conditions, and when 

 the external conditions are brought back to their original level 

 the numbers of any one group may not return to what they 

 were before. 



3. Owing to the great differences in size the numbers of 

 the various groups do not allow a ready basis of comparison. 

 The most suitable basis for quantitative comparisons between 

 one group and another is their respective energy requirements : 

 these are comparable. Bacteria, fungi, protozoa, and nematodes 

 are all consumers : algae only are producers of energy materials. 



The relationships of the soil population to the growing 

 plant are as follows : 



1. The energy supplies in the soil come from the residues 

 of green plants. Since energy supply is probably the most 

 important factor limiting the numbers of the soil population 

 it follows that the soil population is to this extent dependent 

 on the plant. 



2. But the plant is also dependent on the soil population 

 since the soil population in its search for energy supplies 

 decomposes so much protein substance with formation of 

 ammonia and of nitrate as to leave over a surplus for the 

 plant. Further, certain members of the population also de- 

 compose materials such as straw, etc., which possess definite 

 structure and have undesirable physical effects on the soil, 

 while other members decompose toxins such as phenol, which 



they accumulated would be harmful to the plant. 



