THE SOILS OF FLORIDA. 
43 
particularly on Florida soils. It should be noted that the soils may 
contain sufficient calcium to serve as plant food and yet the soil 
remain acid, the calcium not being in a form available to correct 
acidity. In addition to the calcium incorporated in the organic 
structure of the plant, considerable additional amounts are some¬ 
times taken up and deposited in the coarser tissues of the plant. 
The purpose of the calcium so deposited is possibly to neutralize 
organic acids, that might otherwise be injurious to the plant. 
Iron :—Iron occurs free in nature to some extent and is a con¬ 
stituent of many minerals. In its various forms it is slightly more 
abundant than calcium, the estimated amount in the earth’s crust 
being 4.43 per cent. It is present in all soils and is the chief color¬ 
ing constituent. The most common and best known iron minerals 
are the oxides, limonite, hematite and magnetite; the carbonate, 
siderite; and the sulphides, pyrite, marcasite. There are also many 
silicates of which iron is a constituent. The amount of iron required 
by plants is insignificant as compared to the relatively large amount 
occurring in soils. 
Magnesium :—Magnesium, like calcium, is not found native, but 
as a mineral constituent is only a little less abundant than calcium, 
the estimated amount in the earth’s crust being 2.40 per cent. The 
most common mineral is the double carbonate of calcium and mag¬ 
nesium, dolomite. It is also a constituent of many silicates. Plants 
require many times more magnesium as a plant food than calcium. 
The supply in the earth, however, is such that the supply of magne¬ 
sium rarely becomes exhausted in soils. 
Nitrogen :—Nitrogen in the free or uncombined condition is a 
gas, and as such makes up three-fourths (75.5%) of the atmosphere. 
It is a very inert element and rarely combines with other elements. 
It is a constituent of the nitrates and the ammonium minerals. The 
nitrates are the most important minerals, but being readily soluble 
in water they accumulate in quantities only under exceptional con¬ 
ditions. The sodium nitrates of Chili are the most extensive known 
deposits. Nitrogen exists in the soil as nitrates and as nitric acid. 
It is also a constituent of organic matter from which source it be¬ 
comes available to plant growth through the action of bacteria. The 
legumes and a few other plants as previously mentioned, are able to 
get a part of their nitrogen directly from the air. All other plants 
derive nitrogen from the trace contained in the earth. Notwith¬ 
standing the scarcity of nitrogen in the soil the amount used by 
plants is considerable, the corn kernel containing as estimated by 
Professor Hopkins, 1.76%, or more than twice as much as all other 
