10 Diseases of Greenhouse Crops 



use. They must be softened and predigested, and 

 this work is done by the friendly micro-organisms. 

 The supply of plant food is therefore directly de- 

 pendent on the work of these minute scavengers. An 

 intimate relation exists between the higher and the 

 lower form of plant life, the one depending on the 

 other for sustenance. 



A. Number of Micro-organisms in Soil. In- 

 vestigations by Waksman * and others clearly show 

 that micro-organisms are present in soils everywhere 

 (see Table I and fig. 2, b). 



It should be remembered that differences in the 

 physical and chemical nature of the greenhouse soil, 

 the sort of fertilizers used and the amount of tem- 

 perature and moisture will all be important factors 

 in determining the number of micro-organisms pres- 

 ent. 



Nature and Function of a Healthy Soil Flora 



The function of a noimal soil is to provide avail- 

 able plant food. About 95 per cent of the weight of 

 a growing plant is made up of carbon, hydrogen, 

 oxygen and nitrogen. The remaining 5 per cent 

 constitutes the mineral or the non-combustible part 

 or ash of the plant. Carbon, hydrogen, and oxygen 

 are absorbed in the form of carbonic acid and water; 

 nitrogen is usually derived from nitrates produced 

 by micro-organisms out oi organic matter in the soil. 

 Neither the organic nor the mineral elements are in 



'Waksman, L. A., Soil Sdence, 3: 565-589, 1917. 



