By various methods it is possible to measure the water thus taken from the soil. While this process of taking water from the soil is going on within the plant, a certain amount of soil-moisture is also lost by direct evaporation from the soil surface. In dry-farm sections, soil-moisture is lost only by these two methods; for wherever the rainfall is sufficient to cause drainage from deep soils, humid conditions prevail.
Water for one pound dry matter
Many experiments have been conducted to determine the amount of water used in the production of one pound of dry plant substance. Generally, the method of the experiments has been to grow plants in large pots containing weighed quantities of soil. As needed, weighed amounts of water were added to the pots. To determine the loss of water, the pots were weighed at regular intervals of three days to one week. At harvest time, the weight of dry matter was carefully determined for each pot. Since the water lost by the pots was also known, the pounds of water used for the production of every pound of dry matter were readily calculated.
The first reliable experiments of the kind were undertaken under humid conditions in Germany and other European countries. From the mass of results, some have been selected and presented in the following table. The work was done by the famous German investigators, Wollny, Hellriegel, and Sorauer, in the early eighties of the last century. In every case, the numbers in the table represent the number of pounds of water used for the production of one pound of ripened dry substance:
Pounds Of Water For One Pound Of Dry Matter
Wollny Hellreigel Sorauer
Wheat 338 459
Oats 665 376 569
Barley 310 431
Rye 774 353 236
Corn 233
Buckwheat 646 363
Peas 416 273
Horsebeans 282
Red clover 310
Sunflowers 490
Millet 447
It is clear from the above results, obtained in Germany, that the amount of water required to produce a pound of dry matter is not the same for all plants, nor is it the same under all conditions for the same plant. In fact, as will be shown in a later chapter, the water requirements of any crop depend upon numerous factors, more or less controllable. The range of the above German results is from 233 to 774 pounds, with an average of about 419 pounds of water for each pound of dry matter produced.
During the late eighties and early nineties, King conducted experiments similar to the earlier German experiments, to determine the water requirements of crops under Wisconsin conditions. A summary of the results of these extensive and carefully conducted experiments is as follows:—
Oats 385
Barley 464
Corn 271
Peas 477
Clover 576
Potatoes 385
The figures in the above table, averaging about 446 pounds, indicate that very nearly the same quantity of water is required for the production of crops in Wisconsin as in Germany. The Wisconsin results tend to be somewhat higher than those obtained in Europe, but the difference is small.