206 



MODIFICATION OF THE SOIL POPULATION 



emphasize the advantage of the conditions existing in such soils as 

 compared with conditions in raw-humus soils. Cultural treat- 

 ments and appUcation of fertihzers are seldom practical in forests, 

 but, where the forest land is cleared for other cropping systems, 

 more elaborate practices are justified. To bring about the decom- 

 position of the organic matter in the raw-humus soils, the addition 

 of CaCOs and cultivation are often recommended. 



Influence of Green Manures. — The addition of green 

 manures to soil brings about a considerable modification in the 

 soil population. Table 48 shows that organic matter exerts a 



TABLE 48 



Influence of Organic Materials, With and Without NaNOs, on the 



Development of Microorganisms in the Soil (from Waksman 



AND Starkey) 



Treatment of soil 



Number of Organisms 14 Days 

 After Adding the Organic 



Matter 



Bacteria and 

 actinomyces 



Untreated 



. 5 per cent straw 



. 5 per cent straw + . 025 per cent NaNOs . 



. 5 per cent alfalfa 



. 5 per cent alfalfa + . 025 per cent NaNOa 



87,300 

 136,000 

 233,000 

 297,000 

 247,000 



8,875,000 

 34,200,000 

 35,700,000 

 74,600,000 

 73,400,000 



marked effect not only upon the bacteria and actinomyces but also 

 upon the fungi. When fresh, undecomposed plant material is 

 introduced into the soil, the Mucorales generally develop first, 

 and are later followed by various species of Penicillium, Asper- 

 gillus, Fusarium, Trichoderma, and others. The increase in bac- 

 terial growth is closely followed by a rapid development of numer- 

 ous protozoa. Some of the members of this diverse population 

 live on the organic substances originally contained in the plant 

 material which is added to the soil; some exist on intermediary 

 products of decomposition, while others feed upon the microbial 

 inhabitants themselves. 



