Man's physical universe

xanabras

730 MAN'S PHYSICAL WELFARE

Plants Require Certain Minerals for Proper Growth.

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In Unit X, Scclion 1, it was pointed out that tiie chief raw materials

plant growth are the carbon dioxide taken from the air and the

water taken from the soil. If one were to plant seeds in a bed of straw,

excelsior, wood shavings, sawdust, sand, or other porous material and

keep this seed bed moist, the seeds would sprout and start to grow; but

the young plants would soon cease to develop.

It must be that plants

require something else in addition to carbon dioxide and water.

Modern experiments with hydroponics, i.e., soil-less gardening, in

which seeds are planted in seed beds of porous materials such as those

mentioned above, suspended above tanks of water containing dissolved

salts into which the roots extend, have shown that there are at least

eleven elements that are essential to plant growth, namely, nitrogen,

potassium, phosphorus, calcium, magnesium, sulfur, iron, manganese,

boron, zinc, and copper.

By careful control of the composition of the nutrient solutions and

careful regulation of the temperature and other conditions, huge yields

have been obtained; for example, the yield of a hydroponic crop of

tomatoes \vas at the rate of 100 tons per acre per year as compared with

15 tons for soil-grow-n crops; w^hile a hydroponic potato crop was 1500-

2000 bushels per acre as compared with 120 bushels for soil-grown

potatoes.

Hydroponics offers important possibilities for increased crop production

in localities where soil is scarce or in the overpopulated regions

of the earth. A profit of $2000 per year per acre was obtained by one

operator in growing tomatoes. The important lesson to be gained from

soil-less gardening is that much greater yields can be obtained from

growing plants in the soil if the proper minerals are provided in the

right amount.

Soils Differ Very Much in Composition.

Soils vary from rich black loam to dry and unproductive sand ; they

may be acid, basic, or saline; they may contain relatively small or large

amounts of organic matter (humus). The origin and nature of a given

sample of soil determine the extent to which it will provide the minerals

required for plant growth.

An analysis of any agricultural soil will show that it contains many

times the quantity of all the minerals, with the exception of nitrogen,

phosphorus, and potassium, that plants require. Fortunately, most

of this material is insoluble in water; otherwise it would soon be washed

out of the soil by rains and irrigation. On the other hand, this insoluble

material must dissolve before it can pass through the cell walls of plant

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