The Amount Of Water Needed By Plants

It takes a very large quantity of water to mature even an ordinary crop. Irrigation farmers appreciate this much more than farmers in humid regions, because they can see it in bulk. Hellriegel has determined the amount of water necessary for the growth of average crops of the following plants : clover, 400 tons per acre; potatoes, 400 tons; wheat, 350 tons; oats, 375 tons; corn, 300 tons; grapes, 375 tons. This does not take into account water that is constantly being evaporated from the soil in which the crop is growing; it considers only the water used by the plants themselves. At the Iowa Agricultural Experiment Station it was found that the loss of water in growing a ton of clover hay, including -what was used by the plants and what evaporated from the soil, was about 1560 tons, or enough to cover an acre 13.7 inches deep. The loss of water in growing one ton of air-dried corn fodder was 570 tons, or five inches per acre ; of one ton of oats, 1200 tons of water, or 11 inches per acre; of 200 bushels of potatoes, 582 tons of water, or 5.6 inches per acre.

The loss of water in growing one acre of pasturage was 3223 tons, which is equivalent to a rainfall of 28 inches per acre. These interesting figures emphasize what every good farmer already knows: that an abundant supply of water is even more essential to a large crop than an abundance of plant food, and that some crops make larger demands upon the soil reservoir than others. How Plants Drink. — It is not easy to see how it can take from 200 to 375 pounds of water to make one pound of dry plants unless one knows something of the way in which plants drink. Only a small amount of this water becomes a part of the structure of the plant. Some plants are very succulent; 94 per cent, of the strawberry fruit is sweetened water, 90 per cent, of the entire corn plant is water, and 80 per cent, of the entire potato plant is water. Even if the crop were 99 per cent, water this would account for only a small portion of the amount that is actually lost from the soil during its growth. Most of this enormous amount of water is lost by evaporation through the leaves. Contrary to the old notion, plants do not feed by sucking up tiny particles of soil. The plant food in the soil is first dissolved in soil water, as salt dissolves in water; this is then drawn up through the roots by a peculiar process of absorption called "osmosis."

The soil water drawn up by the roots contains very little plant food; it is so weak that we consider it pure water, and we drink it as it comes from tile drains or wells. Therefore the plant has to draw up a very large quantity of water in order to get sufficient food. After the plant has used the food in this very weak fertilizer solution, the pure water is exhaled through the pores of the leaves. Put a geranium, or other potted plant, under a glass jar and note how soon the inside of the jar becomes clouded with the moisture given off by the leaves. The soil in the pot may be covered with oil-cloth or coated with hot wax to prevent evaporation from it. A plant, then, is a pump; there is a cloud of invisible water vapour rising from every grass blade and every cotton leaf. The value of some plants as pumps compares quite favourably with the pumps we buy. Eucalyptus trees are sometimes used for draining malarial swamps; willows planted at the mouth of the sink drain keep the soil from getting soggy.