476 



Journal of Agricultural Research 



Vol. XII. No. 8 



period of 20 weeks. The increase was maintained for a period of 20 

 weeks, but the total number of organisms decreased greatly during this 

 time. 



Plainfield sand. — It was next arranged to determine the effect of 

 magnesium carbonate on the number of microorganisms in Plainfield 

 sand. The results are summarized in Table IV. 



Here again, magnesium carbonate caused a striking increase in the 

 number of soil microorganisms. The results are very similar to those 

 obtained with magnesium carbonate in Colby soil. An enormous 

 increase in the number of soil organisms was noted after i and 3 weeks. 

 The sudden drop in numbers after the 3-week period was due probably 

 to a mistake in weighing which resulted in a low moisture content. 

 After the 8- and 20-week periods, the gradual decrease in the number of 

 bacteria was observed, which agrees with the results obtained in Colby 

 silt loam (fig. 6). 



Table IV. — Influence of magnesium carbonate on the number of bacteria in Plainfield 



sand 



Treatment. 



Number of bacteria in i gm. of dry soil. 



After I 

 week. 



Rela- 

 tive. 



A f ter 2 

 weeks. 



Rela- 

 tive. 



Afters 

 weeks. 



Rela- 

 tive. 



After 8 

 weeks. 



Rela- 

 tive. 



After 20 

 weeks. 



Rela- 

 tive. 



None 



One-fourth mag- 

 nesium carbonate . . 



One-half magnesium 

 carbonate 



Full magnesium car- 

 bonate 



10, 300, 000 

 21, 100,000 

 13, 500,000 

 73,300,000 



100 

 203 

 131 

 7" 



3,500,000 

 S, 100,000 

 5,300,000 

 8,100,000 



100 5,500,000 

 145 10,400,000 

 15119, 300, 000 

 23o\sS, 300,000 



100 



190 



350 



1,060 



4, 700,000 

 5,800,000 

 10,300,006 

 16,000,000 



100 

 123 

 219 

 340 



2,450,000 

 3,560,000 

 5,620,000 

 6,530,000 



100 

 I4S 

 228 

 266 



The results secured with these two soil types show clearly that mag- 

 nesium carbonate in certain soils is a potent factor in the reproduction 

 of soil bacteria. 



iNIfLUENCE OF A MIXTURE OF CALCIUM AND MAGNESIUM CARBONATE ON 

 THE NUMBER OF BACTERIA IN SOIL 



In every case magnesium carbonate gave a much greater increase in 

 the number of bacteria than did calcium carbonate. Therefore the 

 question which suggests itself is, What effect will a mixture of calcium 

 and magnesium carbonates have on the soil flora? 



It was shown by many investigators, principally Loew (46) and his 

 associates, that a soil should contain calcium and magnesium in a certain 

 ratio in order to secure the best plant growth. On the other hand, 

 Lipman (38) made the following statement : 



In their behavior toward salts, bacteria differ in some respects from both plants 

 and animals and occupy a position by themselves. 



