4 2 FLORIDA GEOLOGICAL SURVEY—FOURTH ANNUAL REPORT. 
liam Kamsey as quoted by Professor Hopkins. Professor Hopkins* 
estimate of the amount of each of these elements in a kernel of corn 
is added to give an idea of the amount of each element demanded 
in plant growth. 
While this table indicates approximately the average amount of 
each of the essential plant elements in the crust of the earth, it does 
not show the amount of each present in soils. The average in soils 
may be above or below this general average, depending upon the 
constituents themselves and the conditions under which the soils 
have accumulated. 
PLANT FOOD TAKEN FROM THE SOIL. 
Of the ten elements essential to plant growth six are derived 
solely, and a seventh chiefly from the soil. The others are taken by 
the plant either from the atmosphere or from water. The elements 
taken entirely from the soil are calcium, iron, magnesium, phospho¬ 
rus, potassium, and sulphur. Nitrogen is taken chiefly from the soil, 
although the legumes and some other plants are able to take a 
part of their nitrogen from the air. The three remaining essential 
elements, carbon, oxygen, and hydrogen, are taken directly from the 
air and the water, the carbon dioxide gas of the atmosphere and 
water absorbed through the roots being the sources of supply. As 
regards the amount of materials, the carbon, oxygen and hydrogen 
taken from the air and water make up approximately 95 per cent, of 
the bulk of plant structure by weight, the seven elements taken 
from the soil combined making up only about 5 per cent. Although 
required in such relatively small quantities these elements are none 
the less necessary, and if any one of the seven is lacking or deficient 
or not available, the productiveness of the soil is thereby reduced. 
Calcium :—Calcium is an abundant element in the earth’s crust, 
the estimated amount being 3.43 per cent. It is not found free or 
uncombined, but is a constituent of many minerals, the most com¬ 
mon of which are the carbonates and sulphates, limestone and gyp¬ 
sum. The amount of calcium demanded by plants is, as will be seen 
from the table, exceedingly small as compared with the large 
amounts existing in the earth. Except in the cultivation of legumes, 
it is rarely the case that calcium needs to be added to soils as a plant 
food. It is, however, not infrequently needful as a soil treatment, 
being required particularly in muck or other sour soils to neutralize 
the acids, which if not neutralized will act as a deterrent to many 
plants. For this latter purpose this element in the form of a ground 
limestone, lime or air slaked lime, is coming to be extensively used, 
