THE IRRIGATION AGE. 



257 



Notes on Practical 

 Irrigation 



D. H. Anderson 



RELATIONS OF WATER TO THE SOIL. 



When a small portion of soil is thoroughly dried and then 

 spread out on a sheet of paper in the open air it will gradually 

 drink in watery vapor from the atmosphere and thus increase 

 its weight to a perceptible degree. In hot climates and during 

 dry seasons this property of absorption in the soil is of great 

 importance restoring, as it does, to the thirsty ground, and 

 bringing within reach of plants, a part of the moisture they 

 have so copiously exhaled during the day. Different soils pos- 

 sess this property in unequal degrees. During a night of 

 twelve hours, for it is at night that watery vapor is deposited 

 on the ground (evaporation from the soil occurring during the 

 day), 1,000 pounds of perfectly dry soil will absorb the follow- 

 ing quantities of moisture in pounds. 



Quartz sand 



Calcareous sand 2 



Loamy soil , 21 



Clay loams 25 



Pure clay 27 



Peaty soils and those rich in vegetable matters will ab- 

 sorb a much larger quantity from the atmosphere, sometimes 

 becoming "wet"' two inches deep, a surprising quantity of water 

 when the weight of it on an acre of ground is calculated. The 

 weight of dry and wet soils has already been given, and the 

 difference between the two will, of course, show the quantity 

 in weight of the moisture or water absorbed. The average 

 weight of dry soils is about 94 pounds, the average ordinary 

 v/et weight is 126 pounds, the difference, being 32 pounds, rep- 

 resents the average weight of water per cubic foot. Now, mul- 

 tiplying 43,560 square feet in the acre by 32, gives 1,393,920 

 pounds to the acre one foot deep, and dividing by 12 to ascer- 

 tain the weight of one inch, we have 116,100 pounds, or about 

 58 tons of water falling on an acre of ground in the shape of 

 dew in a single night. Of course that quantity represents the 

 highest possible absorptive quality in a heavily charged vege- 

 table soil. Other soils would receive a less quantity as will be 

 readily understood, but there is enough to be equivalent to 

 quite a smart shower and worth encouraging. 



In what are known as "dry" climates there is always 

 some moisture in the atmosphere which is deposited upon the 

 soil, for wherever there are oxygen and hydrogen there must 

 be moisture. But the quantities vary in climates as much as 

 they do in soils. Where there is evaporation from the soil 

 moisture during the day there is also a re-absorption of moist- 

 ure the soil at night and, with this fact in mind, it may be 

 laid down as an axiom : The tendency of water is to evap- 

 orate from the soil into the atmosphere during the day and 

 to fall back upon the soil during the night. To reduce the 

 idea to an axiom : A dry soil has an affinity for a moist at- 

 mosphere, and a dry atmosphere loves a moist soil. 



Saturation and Power to Retain Moisture. 

 The rain falls and is drunk in by the thirsty soil ; the dew 

 descends and is absorbed, and the waters of irrigation poured 

 upon the ground quickly disappear. But after much water 

 falls upon the earth the latter becomes saturated, can hold no 

 more, and the surplus runs off the surface or sinks down 

 through until it reaches the water table. This happens more 

 speedily in some soils than in others. Thus, 100 pounds of dry 

 soils, as here specified, will hold the quantity of water set op- 

 posite their respective names without dripping or pruning off. 



Quartz sand 25 pounds 



Calcareous sand 29 pounds 



Loamy soil 40 pounds 



Clay loam 50 pounds 



Pure clay 70 pounds 



But dry, peaty soils and adobe will absorb a much larger 

 proportion before becoming saturated to the dripping point; 

 sometimes such soils will absorb their own weight of water. 

 Arable soils generally will hold from forty to seventy per 

 cent of their weight of water. 



This power of retaining water renders such a soil valu- 

 able in dry climates. But the more water the soil contains in 

 its pores the greater the evaporation and the colder it is likely 

 to be. Indeed, evaporation is a source of cold, sometimes to so 

 great a degree that ice will be formed. In very hot regions in 

 India where ice is inacessible it is customary to place small, 

 shallow saucers filled with water on the ground after night- 

 fall, and they are gathered in the morning before sunrise, the 

 water being converted into ice by the rapid evaporation from 

 the soil during the night. Our modern ice machines owe their 

 efficacy for making ice to the rapid evaporation of ammonia 

 under pressure. Ether, chloroform, alcohol, and numerous 

 other substances, produce a sensation of cold when rubbed on 

 the skin, which is not due to anything in those substances, 

 but wholly to their rapid evaporation or volatility. The pres- 

 ence of a saturation of water in the soil, however, excludes 

 the air in a great degree and thus is injurious to plants, whose 

 roots must have air as well as moisture, hence the necessity 

 for drainage where there is a liability to saturation. 

 Earth Vapor Constantly Rising. 



Unless rain or dew is falling or the air is saturated with 

 moisture, watery vapor is constantly arising from the surface 

 of the earth. The fields, after the heaviest rains and floods, 

 gradually become dry, and this takes place more rapidly in 

 some fields or parts of fields than in others, in fact, wet and 

 dry patches of ground may be seen on the same field, indicat- 

 ing a heavy or light soil. Generally speaking, those soils 

 capable of containing the largest portion of the rain that falls 

 also retain it with greater obstinacy and require a longer time 

 to dry. The same thing happens when the land is irrigated. 

 Thus, sand will become as dry in one hour as pure clay in 

 three, or peat in four hours. 



There is one fact every irrigator should constantly bear 

 in mind and that is : Water saturation of the soil is never 

 necessary to plant life ; it is, in fact, positively injurious except 

 in the case of aquatic plants. A long time ago men, seeing 

 rice growing luxuriantly in swamps, imagined that plant would 

 not grow anywhere else, and, accordingly, rice culture meant 

 a swamp. But it was discovered that rice would grow better 

 and produce a larger and richer crop in arable soil generally, 

 and now it is cultivated with astonishing success the same as 

 wheat, barley, or any other cereal, except for a short period 

 of flooding. 



Nature, through heavy rains and other water sources, con- 

 verts the soil into a storage reservoir by establishing a water 

 table beneath the surface from which the water vaporizing up 

 constantly moistens- the growing stratum of the soil, decom- 

 poses and dissolves the salts which are necessary to plant life, 

 and is itself decomposed by the principle of life in the plant 

 and its elements, oxygen, hydrogen, and nitrogen, utilized in 

 the interior of the plant itself. Where there is no natural 

 supply of water for this storage purpose irrigation must copy 

 nature and provide one, or at least furnish an adequate supply 

 of moisture for solvent purposes. When that has been done 

 everything has been done that should be done. 



A familiar illustration of the action of moisture may be 

 witnessed in the slaking of lime in the open air without the 

 direct application of water. The same transformation takes 

 place in the case of all the other soluble mineral salts when 

 in the presence of moisture. This transformation effected, 

 the plant thrives, and, to give it an excess of dissolving liquid 

 is to float off the material needed by the plant and thus de- 

 prive it of its nourishment. It is like feeding an infant on 

 thin, weak soup instead of nourishing bouillon and expecting 

 it to thrive. 



Evaporation From Plants. 



The tendency of plants is to exhale or perspire moisture 

 as well as the soil. The flow of the sap is constant from the 

 roots to the leaves to receive oxygen and carbonic acid and 

 back again to the roots ; like the circulation of the blood in 

 animals it travels in a circuit. When the sap reaches the 

 leaves it parts with a portion of its water, and in some plants 

 the quantity is very considerable. An experiment with a sun- 

 flower, three and one-half feet high, disclosed the fact that its 

 leaves lost during twelve hours of one day, 30, and of an- 

 other, 20 ounces of water, while during a warm night, without 

 dew, it lost only three ounces, and, on a dewy night, lost none. 



All this evaporation or exhalation of water from the 

 leaves of plants is supplied by the moisture in the soil, for 

 plants generally do not drink in water through their leaves 

 but through their roots, and when the escape of water from 

 the leaves is more rapid than the supply from the roots 



