28 
Colorado Experiment Station 
of the time in order to account for the conditions that we find. This 
explains how the spots grow and also why they become brown. Prof. 
Sackett has found that these organisms grow without producing any 
color when there is no nitrate present; at least, if they are grown for a 
few generations without any nitrates, they cease to produce any color, 
but if a certain amount of nitrate be added, they produce a dark-brown 
color. This is what happens in these “brown spots” and explains why 
we find the brown-colored spots rich in nitrates. The brown color tells, 
us beforehand that there is an excess of nitrates present. These or¬ 
ganisms taken directly from the soil may produce this brown color for 
a few generations without the addition of nitrates. 
“BROWN SPOTS” FREQUENTLY CONFUSED WITH “BLACK ALKALI”’ 
These “brown spots” have been confused with “black alkali” 
sodium carbonate, but the two things are entirely different in their 
nature and also differ in their color; the “black alkali” produces a dark 
almost black color on the surface of the soil. The hue of this is en¬ 
tirely different from that of the brown of the nitre spots. Of course, 
we meet with many brown places which do not owe their color to the 
brown coloring matter of these organisms, or yet to “black alkali”. We 
have no more reason to say that every brown spot owes its color to 
Azotobacter pigments than we have to say that all men are white. These 
Azotobacter may be present without producing this color. They make 
this color most readily when the nitrates have accumulated somewhat- 
The “black alkali” makes no color unless there are organic substances, 
particularly humus, present for it to dissolve. “Black alkali” may be 
present in injurious, perhaps fatal quantities, without being black at 
all. This is sometimes the case in Colorado because there is not humus 
enough present to show this characteristic of “black alkali”. Humus 
or humus-like substances may cause the soil to be dark, or even black, 
or go into solution and make the water brown. These are all well 
known facts. We even have a name for these humus-like substances. 
mattiere noire, or black matter, but this does not look like the brown 
spots, or the black of “black alkali”. These distinctions are easily rec¬ 
ognized in the field, more so than in the books. The presence of calcic 
chlorid in a soil may also make dark spots in the soil, but these do not 
necessarily have anything to do with either the “brown spots”, “black 
alkali” or the humus; these spots are just what they look like, wet spots, 
for the calcic chlorid takes up so much water that it becomes wet, eveni 
goes into solution, of its own accord. 
The “black alkali” in the soil up to quantities ten times greater 
than is necessary to kill ordinary crops will not prevent the formation 
of these nitrates; on the contrary, up to this amount, it favors the de¬ 
velopment of these nitrogen-fixing organisms, so these two may be. 
