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- Soil Rhizo- 



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sphere sphere sphere 

 Potatoes Oats Wheat 



Soil Rhizo- 

 sphere 

 Soybean 



FuiURE 17. Effect of plant roots on the (list rilxit ion of antafionistic actinoniycctcs in soil at an early 

 stage of plant growth (Reproduced from: Houatt, J. W., Lechevalier, M., and Waksman, 8. A. Antib. 

 Chemoth. 1: 190, 1951). 



the spring (18 to 2o per cent). He also con- 

 chiiled that the addition of fresh organic resi- 

 dues in the fall was largely responsible for 

 the increase in the number of actinomycetes. 

 Their presence in forest soils and on the roots 

 of grasses and leguminous plants was con- 

 sidered as further evidence that they play 

 an important role in the decomposition of 

 plant residues. These observations were con- 

 firmed by Conn in 1916, who reported that 

 actinomycetes make up as much as 40 per 

 cent of the microbial population in soils rich 

 in plant roots, as compared to the population 

 of cultivated soils, where the numbers of 

 actinomycetes were only about 21 per cent. 

 Extensive studies on the actinomycete popu- 

 lation of the soil were also made by Miinter. 



Krainsky obtained much lower numbers, 

 howe\'er, because the synthetic media he 

 used permitted the growth of only 20,800 

 actinomycetes per gram of soil. Although 

 valuable for the recognition of a greater \a- 

 riety of species, these special media do not 

 allow the development of so great a number 

 of total colonies of actinomycetes as do or- 

 ganic media. 



Waksman and Curtis also reported that 



soils contain large numbers of actinomycetes. 

 Although the actual numbers diminished 

 with depth of soil, they increased in propor- 

 tion to the bacteria. At the surface of certain 

 soils, actinomycetes, as measured by the 

 number of colonies produced on agar plates, 

 made up 9 to 15 per cent of the total popu- 

 lation, or a total of 743,000 to 933,000 per 

 gram of soil. At a depth of 30 inches, the 

 numbers dropped to 240,000 per gram, but 

 the percentage rose to about 66. In Califor- 

 nia soils, the numbers varied from 380,000 

 to 1,890,000 per gram, and the percentage 

 of the total population from 19 to 45. 



The total and relative numbers of actino- 

 mycetes in a number of American soils, as 

 determined by the use of egg-albinnen agar, 

 are shown in Table 4. Acid soils, waterlogged 

 soils, and soils poor in organic matter con- 

 tained the lowest numbers of actinomycetes. 

 Heavy soils and organic matter-rich soils 

 contained the highest numbers. 



Gillespie and Waksman and Joffe found 

 that the critical degree of acidity for the 

 growth of the majority of actinomycetes in 

 the soil is pH 4.8 to 5.0, the optimum reac- 

 tion being pll 7.0 to 8.0. Jensen (1928) iso- 



