36 The Colorado Experiment Station. 
of bone was made available; in another, .46 per cent of the phos¬ 
phate of phosphate rock was changed into a soluble form. In neither 
case was any acid present. Similar phenomena have been reported 
by Stoklasa, and Koch and Kroeber. Although our data on this line 
of investigation is very meager, there is reason for believing that 
the conversion is brought about through enzyme action. 
The marked deficiency of phosphoric acid in acid soils is further 
evidence that soil acids are directly concerned in liberating phos¬ 
phoric acid from its mineral compounds. At the same time that soil 
micro-organisms are breaking down organic substances and liberat¬ 
ing acids to dissolve the phosphate, they, themselves, are appropriat¬ 
ing small quantities of phosphoric acid for their own use. Like the 
higher plants, we find phosphates present in their tiny cells in very 
appreciable quantities. The ash of bacteria and fungi frequently 
contains more than 50 per cent, of phosphate. 
Plants obtain their phosphorus from the soil as soluble phos¬ 
phate, and animals, in turn, feed upon these plants. Both contain 
phosphates in their tissues, but in forms too complex to be of serv¬ 
ice to future generations of plants until they are simplified. This 
is accomplished in time by bacterial decomposition, and the complex 
phosphates are again returned to the soil. Thus, the phosphorus 
cycle in Nature is completed. 
Potassium. 
The principal source of potassium in our soils is the potash 
feldspar which occurs in quantity in the country rock. Through 
countless ages, these great masses of granite have been crumbling 
and disintegrating, due to the action of heat and cold, freezing and 
thawing, wind and rain, until today the finely divided particles con¬ 
stitute a large part of the soil. Orthoclase, the potash bearing feld¬ 
spar* is a silicate of potash and alumina. However, under the 
agencies of decomposition, its identity is lost in the formation of po¬ 
tash zeolites, hydrated silicates of alumina and potash. It is in this 
form that the potash is found in the soil. Such a compound is far 
too complex to be of any immediate benefit to plant life unless some 
easy means of getting it into solution can be found. Here, as with 
phosphorus, the organic acids and carbon dioxide produced during 
the decomposition of organic material by soil bacteria attack the 
potash zeolites and set free the potassium, as potassium carbonate, 
which is readily soluble in water. The nitric acid formed during 
nitrification and the sulphuric acid resulting from the oxidation of 
hydrogen sulphide together with various organic acids may also 
combine with potassium to form potassium nitrate, potassium sul- 
