Alkaus In Coi^orado 
53 
the tubercle-producing organism is lacking in the soil and the plants 
do not get a sufficient supply of nitrogen. This statement is as com¬ 
mon now as almost any other one of every-day life. These organisms 
are simply little plants that thrive best on the roots of peas, clover, 
alfalfa, vetches, etc., and have the power to help the plants to use nitro¬ 
gen from the air in building up their nitrogen-containing parts. As 
this nitrogen, which these little plants help the peas to get, comes from 
the air, we say that they “fix” it. These little plants are supposed to 
need the help of the pea or the alfalfa to fix this nitrogen, but there 
are other plants growing in the soil that do not need help to use the ni¬ 
trogen of the air to build up their structures, provided they have a 
sufficient supply of everything else. In other words, if the conditions 
are favorable, they will take care of themselves as far as nitrogen is 
concerned. These plants multiply, build up their tissues and increase 
the total nitrogen in the soil. These plants are usually present in all 
of our soils; only a few samples have been found entirely free from 
them. If the conditions are exceptionally favorable, they will grow 
very freely and the amount of nitrogen fixed in a comparatively short 
time may be quite large. These plants use this nitrogen from the air 
to build the same kind of compounds that are built by other plants from 
the nitrogen of the soil and when the plants die, these compounds go 
through the same processes of change that the similar compounds 
built up by other plants undergo. The end product of these changes 
that interests us at the present is the nitric acid, nitrates, into which this 
nitrogen is finally converted. 
These nitrates are not colored, they are just as white as the “white 
alkalis” and the little plants that fix the nitrogen are also without color, 
but when there are nitrates enough present in the soil in which they are 
growing, whether it is natural or artificial, they color it eventually a 
deep, almost a black, brown. Matters in the natural soil are pretty well 
advanced when this takes place and this is the reason that samples taken 
from the brown spots prove to be rich in nitrates. The brown color 
is produced because the nitrates are present and serve as a good guide 
for us in judging whether the nitrates are there or not. Other things 
may cause the surface of the soil to become darker in some spots than 
others. Sodic carbonate gets its popular name of “black alkali” from 
the fact that it dissolves humus or eats the plant tissues and takes the 
products into solution with a very deep color, which leaves on the 
surface of the ground a black coating when it is evaporated to dryness. 
Calcic chlorid, one of the occasional constituents of alkali, causes the 
surface of the soil to be darker in spots where it chances to be because 
it takes water out of the air and keeps those spots more moist than the 
neighboring land. But humus and the calcic chlorid have nothing to 
do with the brown nitre spots. 
