66 NITRIFICATION AND DENITRIFICATION. 
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 of reduction of nitrogen compounds 
