V-#UHBENT trends in irrigation, its develop­ment and growth in our country, and various technical aspects of the practice were discussed in a symposium on irri­gation water held at the AMERICAN CHEMICAL SOCIETY'S 115th national meet­ing by the Division of Water, Sewage, and Sanitation Chemistry.

Evidence indicates that irrigation was practiced in the United States in the basin of the Gila River in Arizona prior to the colonizing of this part of the continent in the 16th century. The southwestern United States was first settled primarily by explorers who came through Mexico from Spain, where irrigation had been in use from the time of the Moors in that country. Agricultural development the arid southwest was limited during t. 'ÎOO years prior to the middle of the l9th century, however; and it was not until the great gold rush of 1848 and its resulting large movement of population across the plains and mountains to the Great Basin and to the Pacific Coast that irrigation agriculture received its first great stimulus.

Our first 50 years of irrigation was a period of much conflict, litigation, and legislation concerning property rights to surface waters; and yet, at the same time, energy, ingenuity, and cooperation of the pioneers resulted in irrigation agriculture's being well established by the end of the 19th century.

Statistics show its growth since that time. In 1889, 3.5 million acres of land were under irrigation. Ten years later, total acreage had jumped to 7.5 million. The increase continued during the next 45 years, and, by 1945, total irrigated area in the 17 western states stood at 19.5 million acres. It is expected that the current irri­gation census will show further increase. Irrigation has extended into the eastern part of the country and some is reported

from each of the 48 states, the total of eastern states aggregating 1.1 million acres in 1945.

Prior to the turn of the current century, irrigation was undertaken by private capi­tal and individual or cooperative effort, with aid and encouragement from the state government in some cases. In 1902, when the U. S. Congress passed the Reclamation Act, the initiative for under­taking larger and interstate projects de­signed to conserve the surface waters in the West was transferred from the states to the Federal Government. This act has been extensively revised and amended, and it is now generally accepted that further irrigation development is a national affair.

As irrigation agriculture grew, the basic scope of the objectives underwent con­siderable changes. In earlier enterprises, the thought was simply to divert and dis­tribute available stream water where it was needed for crop cultivation. However, as the need developed for the construction of reservoirs for the storage of flood waters to equalize stream flow, the potentialities of hydroelectric power were recognized and the objectives of stream control grew steadily wider. Increased agricultural pro­ductivity was seen to be just one of the benefits realized, and the development of water power, mineral resources, wild life, recreation, and flood control, and the ac­companying new centers of social and in­dustrial life all became considerations in irrigation planning.

It is interesting to note that irrigation in the United States is not confined to the arid regions. Since it will become increas­ingly important in the East as agriculture becomes* more intensified and specialized, its application in the humid areas must be considered. In many of these areas the average annual rainfall is adequate for crop needs, büTlts seasonal distribution is

either unsatisfactory or undependable. Equipment for providing irrigation when needed serves as insurance against drought injury to rice, citrus, and certain vegetables and small crops in nonwestern states.

In the early development of agriculture, the primary concern was to find plenty of water, and little thought was given to quality, conservation, drainage, type of soil, and the other important factors taken into consideration in modern irrigation planning. It became evident, particularly in the arid sections of the West, that there are not sufficient quantities of satisfactory water available to irrigate all irrigable land, and the current trend is in the di­rection of the most effective and complete use of the water resources.

In the last quarter-century, increased effort has been made to use greater skill and care in selecting new land to be irri­gated—to find and use techniques and operations to prevent or remedy conditions that impair the productivity of land al­ready under irrigation, and to explore and adopt methods and expedients by which to avoid waste in the storage, distribution, and application of water to the land.

Quality of Water an Important Factor

During the first half century of irriga­tion in the United States, users were little concerned with the quality of water ap­plied to their farms. The composition and concentration of the dissolved constituents were either not known or not understood, and many irrigation projects ended in fail­ure. An orderly study of a number of sur­face waters was made by the U. S. Geo­logical Survey in 1905-07; and the analy­ses, which were useful at that time, have become increasingly important as a basis for reference of present-day conditions.

Some of our natural streams are almost

990 C H E M I C A L A N D E N G I N E E R I N G N E W S

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F O R I R R l O A T I O N U S E : -- • •« : se^Wig:

A C&EN stoff review of a symposium revealing fhe progress of the past half century which has made well-planned and scien­tific irrigation a leading agricultural factor in the United States

JL-> pure as rain water, while others carry in solution a large number of dissolved constituents. These are classified into thuee groups: minor constituents, anions and cations, and total concentration of con­stituents.

Silica, nitrite, sulfide, phosphate, iron, aluminum, ammonia, hydrogen ion, or­ganic matter, and boron are included in the minor constituents. Excluding boron, they usually occur in low concentrations and are of little importance in irrigation practice. Of importance, however, is the fact that they characterize a water and are helpful in tracing the source of replenish­ment of ground water.

Boron is the tricky constituent. It oc­curs as borax, calcium borate, or LIS boric

acid in hot mineral springs; and although the concentration varies from mere traces to several hundred parts per million, it has been found in nil irrigation waters ex­amined to date. The element is essential to the normal growth of all plants, though the quantity required is small, and a de­ficiency of boron produces striking symp­toms in many plants. To show its versa­tility, however: Boron is toxic to certain plants when present in concentrations re­quired for optimum growth of others. Lemon trees show important injury when irrigated with water carrying 1 p.p.m. of boron, whereas alfalfa makes maximum growth with concentrations of from 1 to 2 p.p.m.

Since it is necessary to know the boron

concentration of a water in order to ap­praise its quality for irrigation use, limits for its concentration have been set up and are shown in Table I for sensitive, semi-tolerant, and tolerant crops. Sensitive plants include citrus, avocado, walnut, and deciduous fruit trees. Semitolerant plants are the vegetables, cereals, and cotton. Alfalfa, sugar beets, palms, and asparagus» are tolerant to boron.

Cations Form Bulk of Dissolved Material

The bulk of die dissolved material in irrigation water is accounted for by the cations ( calcium, magnesium, sodium, and potassium ) and the anions ( carbonate, bi­carbonate, sulfate, chloride, fluoride, and

SYMPOSIUM ON WATER FOR IRRIGATION USE Presented by the Division o /VVcter , Sewage, and Sanitation Chemistry at the 115th national meeting of the

American Che-jnkal Society, San Francisco, Calif., March 30, 1949

Introductory Remarks L / . Wilcox, Qual i ty of Irrigation Waters of the West . . Salinity Laboratory, U. S. Dept. of Agriculture

Trends of Irrigation Development in Hie United States .

C. S. ScoAWc/, Bureau of Plant

Industry, U. S. Dept. of Agricuf-fure (now retired)

Qual i ty of Irrigation Water L V. Wilcox, Salinity Laboratory, (/. S. Degot. of Agriculture

C. S. Howard, U. S. Geological Survey

Influence of Irrigation Waters on Soils . . C. A. Bower, Salinity Laboratory, U. S. Dept. of Agriculture

Effect of Irrigation W a t e r on Plants C . H. Wadleigh and H. E. Hayward,

Salinity Laboratory, U. S. Dept. of Agriculture

V O L U M E 2 9, N O . 11 . . . M A R C H 1 2 , 1 9 5 1 °91

nitrate). The cations to a great extent de­termine the physical as well as the chemi­cal properties of the soil, anil, in turn, the composition and concentration ot the soil solution are related to the quantity and quality ot irrigation water applied to the soil. The nature and concentration ot the cations in a water are significant in deter­mining the quality or the water for irriga­tion since the reactions of the cations with the soil determine the suitability of soil for agricultural use.

The bivalent cations, calcium and mag­nesium, tend to keep a soil permeable and in good tilth and are usually supplied in adequate quantities by most irrigation waters. Sodium produces the opposite effects. It is required in very limited amounts it at all for plant growth, al­though most plants take it up freely from saline soils. However, it seems merely to increase the osmotic concentration ot the cell sap and thus retard desiccation. In higher concentration, the sodium ion is toxic to plants. The "per cent sodium" is the ratio of sodium to the total cations in the irrigation water and determines the ad­verse effect of sodium on the soil.

Potassium is often present in natural waters in concentrations not exceeding a few tenths of an equivalent per million. The reaction witli the soil is similar to that of sodium, but the effects are not so harm­ful. It is essential to plant growth, being one of the three major plant food elements.

The effect of the various anions are not so easily recognized. The pH is deter­mined chiefly by carbonate and bicarbon­ate, which buffer the water. Sulfate is limited in concentration to the solubility of gypsum, 2634 p.p.m., in presence of excess calcium. Chloride is usually the principal anion in very saline waters and nitrate is usually present only in traces.

To estimate the quality of a water for use under average conditions as related to soil, permeability, drainage, quantity of irrigation water used, rainfall, climate, and crops, it is necessary to know three char­acteristics: total concentration, either in terms of electrical conductivity units (micromhos per centimeter at 25° C. ) or dissolved solids, in parts per million; the soluble sodium percentage; and the boron concentration. Knowing these values, and after first checking Table I, one may classify the water by reference to a second table or, more conveniently, a diagram such as that shown in Fig. 1.

Table I. Permissible Limits (p.p.m.) of Boron in Irrigation Water

Grade Excellent Good Permissible Doubtful Unsuitable

Sensitive Semitolerant Tolerant Crops

0.00-0 .33 0.33-0.67 0.67-1 .00 1.00-1.25 over 1.25

Crops Crops 0.00 0.67 0.00 1.00 0 .67-1 .33 1.00 2.00 1.33-2.00 2.00 3.00 2.00 2.50 3.00 3.75 over 2.50 over 3.75

Partial analyses of several surface waters used for irrigation are shown in Table II. From use of the table and diagram, it is seen that the particular samples from the Sacramento and Platte Rivers are of ex-

1 5 0 2 8 0 5 0 8

1120 1170 1330

1 8 2 2 3 2 5 0 3 9 6 0

0.05 0.03 1.78 0.20 0.16 0.20

Table II. Partial Chemical Analyses of Representative Surface Waters

E l e c t r i c a l C o n d u c t i v i t y

i M i c r o m h o s X Vi B o r o n , R i v e r S o u r e r 1 0 a t 2 5 e C . ) S o d i u m p . p . m .

S a c r a m e n t o , Cal i f . N o r t h P l a t t e , N e b . C a c h e C r e e k , Cal i f . R i o G r a n d e . T e x . C o l o r a d o , Ari / . . G i l a , Ar i z .

cellent quality, while that from Cache Creek is of questionable quality because of high boron concentration. The Rio Grande and Colorado River samples are good to permissible and are satisfactory irrigation supplies on well drained soils. Because of high soluble sodium percentage, Cila River water is classed as permissible to doubtful.

Theory of Irrigation and Salt Balance The direct object of irrigation is to re­

plenish the root-zone reservoir of the soil to support crop growth. If the water used were pure, the input could be adjusted to this end alone. The roots of the crop plants, which penetrate and occupy the upper horizon of the soil, could then ab­sorb water and certain dissolved nutrients. However, this is not the full story. First of all, the water generally contains dis­solved salts, many of which are harmful. Secondly, of the water held by the soil in the root zone, only a part, often less than half, is available to plants. T h e unavail­able water is held to the soil particles by forces greater than exist in the plant roots.

The plant's selectively absorbing chiefly the water and only a small proportion of the dissolved constituents results in an in­crease in the concentration of dissolved salts in the soil solution. The increase in concentration is nearly as much as would follow the loss of water by evaporation. The residual and stronger soil solution may be diluted again by rain or by another irrigation, but the dissolved salts brought

Figure 1. Diagram aid in interpret­ing the analysis of irrigation water

gj^O—__T~ ~ T

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I

-L h h h

L k L L J_E*CELLENT J TO L GOOD

1 . 1 1

—-<LL1 ' ' D O u b t r u i

^»s^^ TO ^ k , ^ uisu.neLE

PERMISSIBLE TO

DOuBTfui

GOOO T O

PERMISSIBLE

1 . 1 ...

1 1 •' UNSUITABLE

DOUBTFUL T O

UNSUITABLE

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SOO i.OOO I.SOO 2,000 2.SOO 3.O0O TOTAL CONCENTRATION AS ELECTRICAL CONDUCTIVITY

in with each irrigation add an increment to the potential concentration.

This condition explains the chief need and justification for the drainage of irri­gated land. The old belief that drainage would not be necessary if the volume of input could be limited to crop require­ments is no longer given credence. The aim of drainage design is not only to re­move surplus water, whether surface or subsoil, but also to remove from the irri­gated area approximately as much of the harmful salt as is brought in by the water.

The dissolved constituents in irrigation water may markedly change the proper­ties of soil as a medium for plant growth. The changes may be either beneficial or detrimental, but, more often, the applied water creates adverse soil properties and lower soil productivity, and in many cases formerly productive soils have had to be abandoned.

The effects of irrigation waters upon soils may be grouped in three classes: effects upon the concentration and com­position of the soil solution; effects upon the composition of the absorbed cations; and effects upon soil structure and per­meability.

Irrigation waters contain from 0.1 to 5 tons of dissolved salt per acre foot; under arid conditions, the concentration of the soil solution may increase sufficiently to impair soil productivity. The addition of salts to the soil solution increases its osmotic pressure and thus lowers the thermodynamic free energy of the soil water. It is thus necessary to drain or leach the soil periodically to prevent the abnormal accumulation of salt in the soil.

An equilibrium exists between the sol­uble and exchangeable cations in soils. When normal soils are irrigated, sodium often becomes the dominant cation in the soil solution rather than the beneficial calcium and magnesium. High sodium waters have a detrimental effect upon the physical conditions of the soil since the soil is dispersed and permeability lowered. On the other hand, calcium and magne­sium promote soil flocculation and the de­velopment of good structure.

When applied to soil technology, the term "structure" essentially means the ar­rangement of soil particles. Soils with good structures are friable, porous, and readily permeable to water. The structure of a soil as well as its permeability can be materially influenced and changed by irri­gation waters which affect the composi­tion of the absorbed cations and the con­centration of the soil solution. Sodium-saturated soils swell markedly and tend to disperse when wet, while calcium and magnesium soils remain flocculated and do not swell to an appreciable extent.

Certain sandy soils of California have been known to develop low infiltration rates within a single growing season be­cause of the use of irrigation waters with low salt content but high sodium percent­ages. One explanation seems to be that the formation of a thin, surface layer of dispersed soil lowers the infiltration rates. This is prevented by lowering the sodium

992 C H E M I C A L A N D E N G I N E E R I N G N E W S

PLAMT S T A L K S I K , PLANT STAIK

[ ^ ] Less than 1.0

Q j 1.0 to 2.0

SPECIHC CONDUCTANCE OF EXTRACT FROM SATURATEO SOIL IN MILLIMHOS PER CENTIMETER

Figure 2. Salt distribution under furrow-irrigated cotton. Soil originally salinized to 0.2*%

| | less than 6.5%

f i ] 6.5 to 7.0%

B 7.0 to 8.0%

H I 8.0 to 10%

i l l 10 to 12%

EU Over 12%

Figure 3 . Moisture distribution under cotton plants in comparison with salt distribution given in Figure 2

percentage of the water to approximately 50 through the addition of gypsum.

Effect on Plant Growth Inasmuch as water is applied to increase

the productivity in crops, the essential question boils down to: What is the effect of the quality of irrigation water on the growth of plants? Since the quality alters the status of the soil in which a plant is growing, it follows that the* growth of the plant should be influenced.

An accumulation of soluble salt in the soil solution is observed by the develop­ment of white alkali or soil salinity. Plant growth is suppressed by saline soils as a result of the increased osmotic pressure of the soil solution which seriously de­creases the physiological availability of soil moisture to the plant, and through an accumulation of certain ions in the soil solution which may have a toxic effect upon plant physiological processes.

The degree to which increasing concen­trations of neutral salts in the irrigation water affect the growth of various legu­minous forage crops can be understood through the consideration of four experi­mental treatments in which 0, 2500, 5000, and 7500 p.p.m. of a 50-50 mixture of NaCl and CaCLs was added to the water used for irrigating red clover, strawberry clover, big trefoil, and birdsfoot trefoil. As the soil salinity was increased through this range, the relative yield of red clover dropped from 100% at 0 p.p.m. to 28.4% at 2500 p.p.m., to 7.0r/r at 5000 p,p.m., and to 1.7% at 7500 p.p.m. Over the same test range, strawberry clover dropped from 100% yield to 3 9 . 1 % , 19.3%, and 6 .1%; big trefoil from 100% to 33.0%, 16 .2%, and 2.7%; and birdsfoot trefoil from 100% to 58 .3%, 44 .1%, and 35.3%. It is thus seen that, as the degree of soil salinity is increased, there is a correspond­ing decrease in plant growth, but the de­gree of growth reduction is more serious for some plants than for others.

Evidence indicates that the degree of growth inhibition of plants growing in saline soils is to a degree determined by the osmotic pressure of the soil solution. In experiments with red kidney^bean

plants in water cultures, with the osmotic-pressure o f the solutions adjusted to vary­ing levels with different salts, it was found that growth was essentially independent of the kind of salt added but decreased linearly with increase in osmotic pressure of the solution. Rate of entry of water into com roots was also found to b e related to the osmotic pressure* of various solutions but virtually independent of the nature of solutes used in the study. These facts emphasize that a decrease in free energy of water b y the presence of salt decrease« the ease with which a plant obtains water from the substrate—or, in other words, dissolved material in the soil solution de­creases the physiological availability of water. In considering the effect of osmotic pressure upon the growth of plants, it is necessary to take into account the affect of environmental factors of climate, differ­ences in artificial culture conditions and soil conditions, and variations in the irri­gation cycle, or the degree of soil mois­ture depletion developed before additional water is added.

Another interesting consideration is the fact that there is considerable variation in salt distribution in soils as a result of movement of water in the soil. Fig. 2 shows the distribution of salt in a plot of soil that had been originally salinized with 0.2% salt and then irrigated with non-saline water.

The salt moved out of the soil zone underneath the furrows and up into the ridges of soil. As a result, the soil beneath the furrows became practically nonsaline, while soil at the top o f the ridges became highly saline, some containing as much as 5% salt.

The salt distribution had a marked effect on the removal o f water by the roots of these plants from the soil. Instead of removing water nearest to the stalk, the roots were removing trie water from under­neath the furrows, a s shown in Fig. 3. Other experiments liave shown that as the content of salt in soil i s increased, there is an increase of residual moisture in the soil, even though the osmotic pressure of the soil solution was rather uniform for various levels.

It is thus evident that when irrigation water or other conditions in the soil being irrigated are effective in increasing the solute content of the soil solution, the osmotic pressure of the soil solution may depress growth response to plants through decreased availability of the water. How­ever, high osmotic pressure is not the only factor; the various ions, Na*, Ca4*, Mg**, CI", SO, . and HCCV, commonly found in irrigation water may accumulate suffi­ciently within the soil to become specific­ally toxic to the plant.

Current Status of Irrigation Agriculture After a century of irrigation experience

in the United States, we have over 20 mil­lion acres of irrigated land contributing to the support of 150 million people. Pro­ductivity is impaired on a substantial part of this acreage, possibly as much as one third of it, because of excess salinity in the soil, poor physical conditions of the soil due to excess sodium, or lack of an ade­quate supply of irrigation water.

The use of surface waters is governed by a reasonably adequate body of laws and administrative agencies, and progress is being made in regard to proper use of ground water.

Though there is still much to be done in way of increasing efficiency of distribut­ing water and applying it to land, engi­neering work in the planning, design, and construction of works for the storage and diversion of surface water has been highly developed.

Improvements have been made in the selection of lands to be irrigated; and, in the cases of reduced productivity of lands already under irrigation, remedial meas­ures have been developed. The need of obtaining accurate information on the broad problem of salinity—including in­vestigations of the quality of water, the concept of salt balance, the re-use of drainage waters, and the final disposition of the residual salt burden of streams and underground waters—is widely recognized.

It is generally accepted that irrigation agriculture is important and worth while, not only to the arid West but to the economy of the entire nation.

V O L U M E 2 9, N O . 1 1 M A R C H 12 , 1 9 5 1 993