Man's physical universe

xanabras

734 MAN'S PHYSICAL WELFARE

the Chilean nitrate beds and the various processes for the chemical

fixation of nitrogen, have already been discussed in Unit IX, Section 5.

One of the latest developments in the addition of nitrogen to the soil

is

the introduction of small quantities of ammonia to irrigation water,

from cylinders of liquid ammonia.

Cheap sources of nitrogen are

now available. It remains for the

farmers of the United States to

learn to use nitrogen-containing

fertilizers.

Why do plants require such relatively

large amounts of nitrogen?

The answer is that nitrogen compounds

furnish the nitrogen for the

proteins which plants manufacture.

These proteins are the basic

material from which the protoplasm

of every living cell is made.

Fig. 312. Soybean root showing

nodules which harbor nitrogen-fixing

bacteria. (Courtesy of the U. S. Department

of Agriculture.)

Phosphorus, Potassium, and Sulfur

Are Often Deficient in Soils.

Phosphorus is important in the

formation of new cells. Both phosphorus

and sulfur are present in

many plant proteins. Potassium

does not enter into the important plant products, but it seems to act as

a sort of helper. Thus the amount of nitrate absorbed by a plant will

depend upon the amount of potassium present.

The phosphorus, potassium, and sulfur resources of the United

States have already been discussed.

Elements Needed in Small Amounts

Are Very Important, but

Fortunately They Are Seldom

Deficient in the Soil.

Calcium serves to bind the plant

cells together, magnesium is a constituent

of chlorophyll, while iron

is required for its production. The

function of such elements as boron,

manganese, zinc, copper, molybdenum,

etc. are not well established,

w^'M

Fig. 313. Effect of potash deficiency

on ear formation. (Courtesy of

the American Potash Institute, Inc.)

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