12 
COLORADO EXPERIMENT STATION 
tection from frost is secured when a crop is plowed into the land in 
the fall as when it is left standing throughout the winter. 
The winter of 1903-4 was very severe in the East and in some 
sections hundreds of orchard trees, particularly peach trees, were 
destroyed. Prof Green* made a survey of the injured orchards the 
following summer and found that injury occured only on impoverish¬ 
ed and .bare soils. A cover crop, sod, good growth of weeds, or stable 
manure afforded almost complete protection from the cold. And 
moreover, and what is more important for our purpose he found that 
where such materials had been plowed under recently, the protection 
was just as efficient. 
Popular writers on horticultural topics have woefully confused 
humus with decaying organic matter and have implied that all organic 
material is humus as soon as it is mixed with the soil and decay has 
set in. The fact is, humus is the final product of organic decay and as 
such has entirely different effects on soils than have organic materials 
which are undergoing the processes of decomposition. 
When green manure is plowed into the soil various low forms of 
plant life including fungi, yeasts and bacteria attack it thus inducing 
decay. Wraenkel “found in the cultivated soil of Liebefeld 5,750,000, 
in meadow land 9,400,000, in a manure pile 44,500,000 bacteria per 
cubic centimeter.” These figures seem high for so small a quantity of 
material, but taking the average size of a bacterium, a cubic centimeter 
might readily contain six hundred millions. 
Other forms of bacteria begin to multiply as soon as fermentation 
sets in. Different organisms have different and important functions to 
preform in promoting chemical activities in the soil; plant food elements 
are set free, changed and combined into substances which plants can 
use. No less than five different acids are generated by the processes 
of decay, carbonic acid being among the more important. * 1 2 Sackett 
found that clover taken in full bloom in June when ground and mixed 
with soil at the rate of 10 tons per acre, gave off, at the end of three 
days, 3 carbon dioxide corresponding to 3812 pounds per acre foot. 
This action continued through a period of three weeks, gradually 
diminishing however until at the end of that period very little of the gas 
was evolved. One hundred tons per acre of red clover treated in the 
same manner gave off after 12 days five tons of carbon dioxide per acre 
foot. This investigator also tested the solvent action of pure carbon 
dioxide on various materials. Pure ground bone meal was placed 
in a flask and carbon dioxide was allowed to pass through it. At the 
end of one hour 2 .ti% of the insoluble phosphoric acid had been 
made soluble. At the end of two hours 5.21% was made solub le. 
Ground phosphate rock treated in the same manner gave the follo wing 
*Green Bull. 157 Ohio Agri. Exp. Sta. 1904. 
1 Hilgard, Soils, p 143. 
2 Sackett, W. G.. unpublished notes. 
3 Carbon dioxide and carbonic acid gas are synonymous; when com¬ 
bined with water, carbonic acid is formed 
