184 THE PRINCIPLES OF FLORICULTURE 



influence within the soil. These inhibiting or unsanitary 

 conditions may be divided into two or three groups : 



"It is now pretty well recognized that in addition to 

 the beneficial substances that result from decay of roots 

 and other organic material in the soil, and from the 

 action of microscopic plants, there may also be produced 

 various poisonous or toxic substances which are injurious 

 to the growth of the plant. It appears that one of the 

 reasons why the continuous growth of a crop upon the 

 same soil results in reduced yields, is the accumulation of 

 the poisonous products of decay or secretion of the roots 

 of that plant. It is coming to be understood that the 

 products or toxic substances from one plant may not be 

 injurious or may be even beneficial to another plant, and, 

 therefore, in plant association this fact should be taken 

 into account. It is a topic that requires much further 

 investigation for the guidance of the practical man. 



" One of the functions of commercial fertilizers may 

 be to render less innocuous these toxic substances, for 

 it is known that fertilizer substances by promoting or 

 depressing oxidation and other changes render these 

 toxic substances less harmful. 



" It should always be known that the crop produced is 

 competing with many other plants in the soil. We well 

 recognize the importance of keeping down weeds which 

 will compete with the main plant for food and moisture. 

 But we have not given so much attention to the competi- 

 tion of the crop with the microscopic plants in the soil, 

 the bacteria and the fungi. Instead of being a dead 

 mass, the soil is teeming with life. In every grain of 

 soil there may be millions of bacteria and many kinds of 

 fungi and molds. These require the same food elements 

 that the higher plants use, and it may frequently happen 



