Algae in Some Colorado Soils. 
35 
and Phormidium tenue; the olive-colored colonies are Phormidium 
tenue. Here and there are interlacing masses of dark-olive or 
black threads, visible to the naked eye; these are Microcoleus vag- 
inatus. 
DISCUSSION. 
It is well known that many different kinds of algae inhabit 
the soil. As a rule, it is generally understood that such a soil is 
necessarily muddy or very moist. In such cases the algal growth 
is visible to the naked eye, forming on the soil a characteristic plant 
mass. The soils from which the foregoing 22 samples were taken 
were, with the exception of No. 17, just ordinary cultivated soils, 
with a varying water content. The samples were representative 
of soils in rather widely separated localities in Colorado. At the 
time of collection, during October, 1911, no algae were noticeable 
on the soil surface; furthermore, one would not ordinarily think 
of such soils as being moist enough to support an' algal flora. And 
yet, cultures from these soils, with but two exceptions, samples Nos. 
3 and 17, revealed the presence in them of a considerable number 
of species of algae and a healthy development of these. 
It is unquestionably true that during favorable seasons of the 
year, there is developed in certain of our soils a rich growth of 
algae. This is probably confined to the surface layers. To what 
depth algae extend will depend largely upon the texture of the soil, 
its ventilation and methods of cultivation. It is probably true, how¬ 
ever, that the top crust of^soil, the first inch or less, is usually too 
dry to favor algae. Irrigation may play a part in determining the 
distribution of soil algae. Whether or not our unirrigated soils 
possess an algal flora remains to be found out. But it can be readi¬ 
ly understood how the turning of water on to an unirrigated area 
would introduce from the streams an abundance of algae. Although 
evidence is still insufficient, it is within the bounds of reason to 
believe, from these preliminary investigations, that all of our or¬ 
dinary cultivated soils, especially those under irrigation, are far 
richer in algae than is usually supposed to be the case. More than 
this, we venture to assert that soil algae play a far more important 
role in soil fertility than is generally believed. Unquestionably, the 
organic matter furnished by soil algae must be reckoned with as an 
important source of energy for the nitrogen fixing organisms. 
SUMMARY. 
Algae occur abundantly in many cultivated soils of Colorado. 
Twentv-one different species of algae were found in the soils ex¬ 
amined. 
With but two exceptions, all the species found belong to the blue- 
green algae (Cyanophyceae.) 
The family Nostocaceae is best represented. 
There is a predominance of forms possessing thick, gelatinous 
sheaths. 
