258 



THE IRRIGATION AGE. 



the leaves droop, dry and wither, because then they are draw- 

 ing from their sap, living, so to speak, upon their own blood. 

 This evaporation in the plant is similar to the perspiration 

 constantly exuding from the skins of healthy animals and it 

 has added to it the mechanical evaporation which takes place 

 on the surface of all moist bodies when exposced to hot or 

 dry air. There can be no growth or health without it, hence, 

 it is often beneficial to wash or spray the leaves of plants and 

 trees to remove the dust or other clogging material that has 

 accumulated upon the leaves and "stopped perspiration." To 

 stop this leaf evaporation is to kill the plant as surely as was 

 killed the boy in the Roman pageant. His entire body was 

 covered with a thick coating of gum arabic, on which was 

 laid a layer of gold leaf, the intention being to have, him pose 

 as a golden statue. He died in a few hours and it was not 

 until the cause of his sudden death was investigated by scien- 

 tific men that it was discovered that the closing of the pores 

 of the skin, thereby preventing evaporation from its surface, 

 was the cause. On dry, dusty soils, where there is none, or 

 very little .rainfall, the accumulation of dew during the night 

 is generally sufficient to "trickle" along the leaves and carry 

 down the dust and other accumulations on the leaves which 

 interfere with evaporation. Sometimes the plant, as if aware 

 that there is a stoppage in its circulation, will throw out fresh, 

 new leaves to cure the defect, but this is done at the expense 

 of the root, tuber, or fruit. 



The amount of loss due to natural and mechanical evap- 

 oration from plants, of course, differs very greatly in the vari- 

 ous species of plants depending, in a great measure, on the 

 special Structure of the leaf, whether fine or coarse meshed, 

 large or small, lean or fleshy, the natural perspiration, how- 

 ever, always exceeding the mechanical. Both processes, more- 

 over, are more rapid under the influence of a warm, dry at- 

 mosphere aided by the direct rays of the sun. 



As showing the quantity of evaporation an experiment 

 was tried with an acre of maple trees containing 640 trees. 

 The calculation is not positively exact, but it is worth accept- 

 ing as a basis for other experiments on crops of all kinds and 

 may come somewhere near enabling the irrigator to determine 

 the quantity of water to be applied to the soil, whether there 

 is a water table within the reach of the surface or none at all. 



The evaporation was assumed to take place only during 

 a day of twelve hours and each of the 640 trees was esti- 

 mated as carrying 21,192 leaves. From an estimate based on 

 the quantity of evaporation from one tree containing the 

 number of leaves above specified, which were carefully 

 counted, the 640 trees evaporated from their leaves in twelve 

 hours 3,875 gallons of water, or 31,000 pounds. During ninety- 

 two twelve-hour days, the life of the maple leaf, the evap- 

 oration would amount to 2,852,000 pounds. During that period 

 the rainfall was 8,333 inches or 43.8 pounds to every square 

 foot of surface, equal, per acre of 43,560 square feet, to 

 1,890,504 pounds. The evaporation from the leaves of the 

 trees, therefore, exceeded that of the actual fall of rain by 

 nearly one million pounds. Whence did the surplus come? 

 Evidently from the water stored in the water table and drawn 

 up by the action of the roots of the trees. Where there is no 

 water table or ground water and the soil is dry "all the way 

 down," it is necessary to create one by irrigation and this is 

 ' not so difficult as might be imagined, for we must consider 

 that in the case of maple trees the roots may reach down into 

 the subsoil for fifty feet, and in the case of ordinary fruits, 

 vegetables, and cereals, a water table at that depth would be 

 wholly unnecessary even if generally impracticable. Soil sat- 

 uration at any depth beyond four feet with unlimited surface 

 cultivation is sufficient, although in the case of vines and 

 trees it should be much deeper. 



The above experiment with the maple trees although, 

 perhaps, of no practical value on account of its uncertainty, 

 being more or less guess, demonstrates two things, when there 

 is also taken into consideration the quantity of sap in plants 

 and the amount of salts held in solution in it. 



First How easily a soil may be exhausted by cutting and 

 removing plants and crops therefrom. 



Second As a direct corollary, through its diametric op- 

 posite, it shows how easily alkaline salts may be removed from 

 the soil by cutting and removing the plants and crops. These 

 alkali-consuming plants hold large quantities of the earth 

 salts in their sap in solution, the carbonates, sulphates, the 

 sodas, and potash, literally taken up out of the soil. Of course, 

 when removed a certain amount of alkali is removed with 

 them. This has been the experience with the "salt meadows" 

 in Germany and Holland, and in the United States, as has 



been already noted, and, in a small wav with the alkali lands 

 of the West where the experiment has been made. 



When water is poured into the saucer or sole of a flower- 

 pot filled with earth the soil gradually sucks it up and be- 

 comes moist even to -the surface. . This is what is known as 

 ''capillary action," and exists in all porous bodies to a greater 

 or less extent. A sponge is a well known instance of this 

 power, and if the small end of a piece of hard chalk is held 

 in water the entire mass soon becomes saturated. The experi- 

 ment with the flower-pot, however, represents the action in 

 the soil, the water from beneath that contained in the sub- 

 soilis gradually sucked up to the surface. It is one of the 

 operations of the laws of nature which maintains all things 

 in constant motion to preserve their life and vitality, for, if 

 permitted to remain at rest without motion, they sicken and 

 die, afterward putrefying as happens even with water which 

 becomes stagnant, that is, ceases to be in motion. 



In climates where there is winter, or even a moderate de- 

 gree of cold weather, this capillary action ceases and the 

 tendency of the water is to "soak" downward, and it is not 

 until warm weather that capillary action begins and the water 

 commences "soaking" upward toward the surface. In a warm, 

 or hot climate, this action is constant and it also takes place 

 whenever the soil is parched or dry. 



This suspension of capillary action in winter, or cold 

 weather, furnishes a strong point in favor of winter irriga- 

 tion, which really takes the place of the autumn and spring 

 rains, and of the snow that slowly melts and its waters car- 

 ried down into the soil to the water table ready to begin an 

 upward movement when the weather becomes warm and the 

 surface soil dry. 



The dryer the soil and the hotter the atmosphere, the 

 more rapid is the rising of the water to the surface by capil- 

 lary attraction, and, as the water ascends, it carries along with 

 it the saline matters dissolved by it and, reaching the sur- 

 face, evaporates, leaving the salts it carried behind. It is this 

 capillary action which has incrusted our own lands with alka- 

 lis of all kinds ; it is the same in India, Egypt, South Africa, 

 and elsewhere. On the arid plains of Peru, and on extensive 

 tracts in South Africa, alkali deposits, several feet in thick- 

 ness, are sometimes met with, all of which are caused by the 

 capillary action of water bringing up to the surface the salts 

 in the subsoil. So it is that the enormous beds of nitrate of 

 soda in Peru and those of the carbonate of soda in Colombia 

 were created; and in our own black and white alkali and 

 sodium bad lands capillary action may be blamed for their 

 condition. It must not be forgotten that wherever there is 

 seepage there is also capillary action, for that power is ex- 

 ercised in every direction. It does not matter which end of 

 the sponge or piece of chalk is held to the water, both be- 

 come saturated. It may be said that capillary action is a 

 violation of the law of gravity, or, rather, is a law of itself 

 acting independently. 



This tendency of water to ascend to the surface of the 

 earth is not the same in all soils. It is less rapid in stiff clays 

 and more rapid in sandy and open, porous soils generally, 

 and it is of especial importance in relation to the position of 

 the water table in the soil when considered as a source of 

 water supply or shallow rooting plants. Gravity draws the 

 water downward toward a water table, and in a dry subsoil 

 it is capillary attraction that impels it down. But when the 

 water in the surface soil is less than that below an upward 

 movement begins as though nature were desirous of main- 

 taining an equilibrium which, scientifically speaking, it always 

 does, or attempts to do. However, there is a zone of capillary 

 action, a space between the water table and the surface, in 

 which moisture rises and with it carries food substances to 

 the roots of plants. Where the water itself rises it means 

 more than capillary attraction, it means a rise of the water 

 table through additions from some new water supply or sat- 

 uration of the soil, in which case plants are injured vitally 

 and drainage must come to the rescue. It is the rise of the 

 water table that is to be feared in irrigation. The reason is 

 because the rise of alkaline solutions is greater than in the 

 case of pure water. Thus, a 50 per cent solution of sodium 

 chloride (common salt) and sodium sulphate will rise faster 

 than pure water, and a much stronger concentration of soda 

 carbonate will rise still faster. Hence the necessity of pre- 

 venting soil saturation and the maintaining of a zone of cap- 

 illary action, in which the roots of plants may be fed by ma- 

 terial furnished through that action when they would be killed 

 if saturation were permitted to overcome it. 



