[Illustration: FIG. 162.--Roots of soy bean having tubercle-bearing
bacteria.]
248. Artificial Fertilizers. Plants need other foods besides
nitrogen, and they exhaust the soil not only of nitrogen, but also of
phosphorus and potash, since large quantities of these are necessary
for plant life. There are many other substances absorbed from the soil
by the plant, namely, iron, sodium, calcium, magnesium, but these are
used in smaller quantities and the supply in the soil does not readily
become exhausted.
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