44 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
elements derived from the soil combined. As a result of the small 
amount in the earth and the relatively large amount used by plants, 
nitrogen is one of the elements likely to become deficient in soils. 
Phosphorus :—Phosphorus in small quantities is a constituent of 
nearly all igneous or primitive rocks. It occurs chiefly as salts of 
phosphoric acid, namely, the phosphates, the most important of 
which is calcium phosphate. It is also an essential ingredient in all 
plants and animals forming a part of the nucleus of all living cells: 
It forms also an important part of the mineral matter of all bones. 
The amount of phosphorus in the earth’s crust is small, being esti¬ 
mated at .11%. The amount required in the growth of plants on 
the other hand is considerable. The phosphorus in the corn kernel 
is .3 per cent, the percentage in the seed of plants being somewhat 
above that for the plant as a whole. Phosphorus, like nitrogen, is 
an element that under cultivation becomes deficient in soils. 
Potassium :—Potassium is a constituent of many of the silicates 
and hence occurs like phosphorus in small amounts in the igneous 
rocks. The most important potassium minerals are the chlorides 
from which the commercial supply of potassium is derived. Potas¬ 
sium is relatively abundant in the crust of the earth, the estimated 
amount being 2.45%, or slightly more than magnesium. The amount 
required by the plant, however, is considerably more than of magne¬ 
sium, and potassium, like nitrogen and phosphorus, frequently be¬ 
comes deficient in soils. 
Sulphur :—Sulphur is found native and also combined to form 
sulphides, sulphates, and other combinations. Pyrite and gypsum 
are the most common sulphur minerals. The total amount of sul¬ 
phur in the earth’s crust is estimated to be .11 per cent, or the same 
as of phosphorus. Eecent investigations made by the Wisconsin 
Experiment Station indicate that the amount of sulphur required by 
plants is greater than has heretofore been supposed, and it is prob¬ 
able that under continuous cropping this element becomes deficient 
in soils.* 
PLANT FOOD TAKEN FROM THE WATER AND FROM THE \ 
ATMOSPHERE. 
Hydrogen :—Hydrogen occurs chiefly ns water, H 2 0, of which it 
forms about one-ninth part by weight. It is also a minor constituent 
*Sulphur Requirements of Farm Crops in Relation to the Soil and Air 
Supply, by E. B. Hart and W. H. Peterson. Research Bulletin No. 14, Wis. 
Exp. Station. 
