TRANSFORMATIONS OF CARBON AND NITROGEN. 1 05 



found in water, in sewage and in ordinary soil. They do not 

 appear to be present in the soil of forests or, at all events, do 

 not here produce nitrates, for these compounds are absent 

 from forest soil. This is attributed partly to the acid nature 

 of such soil, for the nitrifying bacteria will not grow in an acid 

 medium. It frequently happens, too, that the nitrifying bac- 

 teria are not present in meadows, and that the formation of 

 nitrates is here slow or wanting. These bacteria are, how- 

 ever, very abundant in ordinary manure, and one of the ad- 

 vantages of adding manure to certain soils seems to be rather 

 from the bacteria thus inoculated in the soil than the actual 

 plant food in the manure. For example, one frequently sees 

 that an open pasture or meadow supports a somewhat limited 

 crop of grass, although nitrogen compounds may be abundant 

 enough in the soil. If cows are pastured here it is common 

 to find plots of brilliant green, vigorously growing vegetation, 

 surrounding the droppings of the cow excrement. Now this 

 may be due in part to the food contained in the excrement, 

 which is utilized by the plant, but it is not wholly thus ex- 

 plained. The effect lasts for a long time, and months after- 

 wards the oasis of green may be seen in the pasture, gradu- 

 ally increasing in size until it reaches far beyond what must 

 have been the limits of the direct effect of the plant food in the 

 excrement. The explanation seems to be that this excrement 

 contains bacteria in abundance, and these in a short time be- 

 gin the work of converting the soil nitrogens into nitrates. 

 Their influence continues to extend through the soil as they 

 multiply and act upon a wider and wider circle, so that 

 an increased vegetation may continue for a long time under 

 the influence of these nitrifying bacteria which are constantly 

 converting the soil nitrogens into nitrates. The bacteria 

 brought into the soil in these cases have proved of much more 

 usefulness than the actual plant food which \vas in the excre- 

 ment. 

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