72 NITRIFICATION AND DENITRIFICATION 
produce good crops, the plants seeming to be insufficiently supplied 
with nitrogen in spite of its abundance. These barren soils will 
not yield good crops unless supplied with a considerable amount of 
nitrogen as a fertilizer. Upon an open hillside or a meadow we 
may find the land very poor for supporting vegetation, and yet its 
soil, when analyzed, may yield a considerable quantity of nitrogen. 
In such a soil the nitrogen is simply locked up in the humus in a 
form useless to plants At the end of decomposition, a large part 
of the nitrogen may be held in a form not available for ordinary 
vegetation, so that plants growing in such soil will be nitrogen- 
starved, although growing in the midst of plenty of nitrogen 
compounds. Such soils might become highly fertile if some 
agency for unlocking these nitrogenous compounds could free the 
nitrogen from its stable relations, thus producing compounds of a 
nature to be assimilated by plants. If a comparatively small 
amount of 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 compounds 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 con- 
tained 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, in part, 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 nitrogen into 
nitrates. Their influence continues to extend through the soil 
as they multiply and act upon a wider and wider circle, so that 
