The chapters that follow will deal almost wholly with the problems above outlined as they present themselves in the construction of a rational system of farming without irrigation in countries of limited rainfall.

CHAPTER II

THE THEORETICAL BASIS OF DRY-FARMING

The confidence with which scientific investigators, familiar with the arid regions, have attacked the problems of dry-farming rests largely on the known relationship of the water requirements of plants to the natural precipitation of rain and snow.  It is a most elementary fact of plant physiology that no plant can live and grow unless it has at its disposal a sufficient amount of water.

The water used by plants is almost entirely taken from the soil by the minute root-hairs radiating from the roots.  The water thus taken into the plants is passed upward through the stem to the leaves, where it is finally evaporated.  There is, therefore, a more or less constant stream of water passing through the plant from the roots to the leaves.

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