66 NITRIFICATION AND DENITRIFI CATION. 



manure is added to such soils the results are sometimes surprising 

 in causing an increased fertility far beyond that which might be 

 expected from the small amount of manure itself. 



For example, one frequently sees that an open pasture or meadow 

 supports a somewhat limited crop of grass, although nitrogen com- 

 pounds may be abundant enough in the soil. If cows are pastured 

 there 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 explained. 

 The effect lasts for a long time, and months afterward the oasis of 

 green may be seen in the pasture, gradually 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 by this excrement the nitrifying bacteria are stimulated, 

 and these in a short time begin 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 con- 

 verting the soil nitrogens into nitrates. That this is the whole ex- 

 planation in these cases is by no means sure, but it is certain that the 

 nitrifiers do unlock much nitrogen previously not in an available 

 condition. 



DENITRIFICATION. 



There is another group of microorganisms in soil and other 

 decaying masses acting in exactly the reverse direction from the 

 nitrifiers. Whereas nitrification oxidizes ammonia compounds and 

 nitrites, to form nitrates, denitrification takes the oxygen out of 

 nitrates, reducing them to nitrites and ammonia, and may even 

 reduce these to free nitrogen. Nitrification prepares plant foods, 

 but denitrification destroys them. The one process is useful, the 

 the other detrimental, to soils 



Three different types ol reduction of nitrogen compounds 



