PHOSPHORUS I21 
indicated by the huge limestone caves whose great spaces show how 
the limestone has been dissolved by waters which held carbon di- 
oxid in solution. All such dissolved lime finds its way to the ocean 
where it supplies marine animals and plants with the lime for their 
shells, and where it is also laid down in the deposits of lime mate- 
rial that may, in later ages, form new limestone rocks. But, aside 
from this future possibility, the bacterial agencies of the earth’s 
surface are constantly dissolving the limestones and adding them 
to the soil to be subsequently carried away by drainage. 
PHOSPHORUS 
Vegetation needs only very small amounts of phosphorus, but 
these small amounts are requisite to the production of good crops, 
as has been many times appreciated by the farmer who finds de- 
cidedly increased crops following the application of phosphate fer- 
tilizers to the soil. There are many substances containing phos- 
phorus which may be used to supply the amount needed by the soil. 
They are: (1) Mineral compounds, of which the chief are ground 
phosphate rock (floats), superphosphates and a by-product of steel 
manufacture called Thomas slag. (2) Organic compounds. A 
considerable quantity of phosphorus is contained in the humus, 
likewise in bone, which is used as a fertilizer chiefly for its phos- 
phorus. The solid part of barnyard manure contains some phos- 
phorus, and a variety of other sources, are also utilized—ground 
fish, tankage, castor pomace, and the like. ‘The phosphorus in some 
of these substances is readily soluble in water, and this must always 
be the case before it can be utilized by plants. 
Apparently the solution of the phosphates is dependent upon 
bacterial action. It is easy to understand how the phosphorus 
from organic sources is rendered available through the agency of 
the soil organisms. As these bacteria decompose the various 
organic products in the soil, the phosphorus contained in them is 
set free from its combinations. Bone, for example, is vigorously 
attacked by the bacteria, and is in time completely disintegrated, 
