1 70 SYSTEMS OF PERMANENT AGRICULTURE 



gram of carbohydrate (mannite) consumed, 8.92 milligrams of 

 nitrogen were fixed in 6.6 days with magnesium carbonate, and 

 5.80 milligrams in 4.6 days with calcium carbonate. Ashby says 

 (Journal of Agricultural Science, January, 1907, pages 46, 47) : 



"With magnesium carbonate there was 50 per cent more nitrogen fixed, and 

 a delay of two days in development. . . . One must conclude, therefore, the 

 magnesium carbonate not only neutralizes more effectually than calcium car- 

 bonate any trace of acidity due to foreign organisms in the early stages of 

 culture, but also prevents butyric fermentation; but at first it inhibits the 

 growth of Azotobacter itself. " 



Table 26 gives the results of an investigation concerning magne- 

 sium carbonate conducted with the assistance of the author's 

 students and associates at the University of Illinois. The experi- 

 ment bears upon two lines of inquiry (i) the value of magne- 

 sium carbonate for soil improvement, and (2) methods of correcting 

 this " alkali" when present in injurious amounts. 



Several series of 4-gallon pots were filled with the common 

 brown silt loam prairie soil from the University farm, and to five 

 of the six pots in each series was added magnesium carbonate in 

 amounts varying from .4 per cent to 2 per cent of the dry soil. 

 In addition, Series C and F received calcium sulfate in such an 

 amount as to maintain the ratio of MgO to CaO = 4 to 7, in ac- 

 cordance to Loew's advocated optimum ratio. 



After the crop of 1904 was harvested, the pots in Series F were 

 thoroughly leached in order to remove magnesium, more or less 

 of which was expected to react with the calcium sulfate, leaving 

 the harmless calcium carbonate. 



The data recorded in Table 26 show a distinct and persistent 

 benefit from the use of magnesium carbonate up to .8 per cent of 

 the soil, while with 1.2 per cent the plants are very seriously injured 

 and with 1.6 per cent they are usually so nearly killed as to produce 

 no grain, and they are practically all killed with 2 per cent of mag- 

 nesium carbonate. 



The application of i per cent of magnesium carbonate would 

 require 10 tons per acre for the surface 6 inches, but if the material 

 were applied and mixed with only the surface inch by a light har- 

 rowing, it would require only i^ tons per acre for i per cent. 

 Since pure dolomite would contain only 46 per cent of magnesium 



