330 HOW CROPS GROW. 
The Soil, on the other hand, is very variable in compo- 
sition and quality, and may be enriched and improved, or 
deteriorated and exhausted. 
From the Atmosphere the crop can derive no appreci- 
able quantity of those elements that are found in its Ash. 
In the Soil, however, from the waste of both plants and 
animals, may accumulate large supplies of all the elements 
of the Volatile part of Plants. Carbon, certainly in the 
form of carbonic acid, probably or possibly in the condi- 
tion of Humus (Vegetable Mould, Muck), may thus be 
put, as food, at the disposition of the plant. Nitrogen is 
chiefly furnished to crops by the soil. Nitrates are formed 
in the latter from various sources, and ammonia-salts, to- 
gether with certain proximate animal principles, viz., 
urea, guanin, tyrosin, uric acid and hippuric acid, likewise 
serve to supply nitrogen to vegetation and are ingredients 
of the best manures. It is, too, from the soil that the 
crop gathers all the Water it requires, which not only 
serves as the fluid medium of its chemical and structural 
metamorphoses, but likewise must be regarded as the ma- 
terial from which it mostly appropriates the Hydrogen 
and Oxygen of its solid components. 
§ 2. 
THE JUICES OF THE PLANT, THEIR NATURE AND 
MOVEMENTS. 
Very erroneous notions are entertained with regard to 
the nature and motion of sap. It is commonly taught that 
there are two regular and opposite currents of sap circu- 
lating in the plant. It is stated that the “crude sap” is 
taken up from the soil by the roots, ascends through the 
