THE EFFECT OF SURFACE-ACTIVE COMPOUNDS ON THE SUPPRESSION OF WATER EVAPORATION

FROM SOILS (!)

P.D. MISTRY and M.E. BLOODWORTH (2)

ABSTRACT

The average water budget for the State of Texas is approximately 366, 600, 000 acre-feet annual income. Of this total amount, about 46 percent is transpired to the atmosphere through plants, while that rendered ineffective through evaporation (soil and free-water surfaces) is estimated to be near 40 percent. Therefore, the transfer of water to the atmosphere by these two processes accounts for an estimated 86 percent of the total water budget for the State of Texas and is of great concern — both economically and socially — to all segments of the population. Because of this all important problem concerning water, a study was undertaken to determine the effect of surface-active compounds on suppressing the evaporation of water from coarse-textured soils.

Laboratory investigations concerning the effect of selected surface-active compounds on moisture losses from both large and small-diameter soil columns are reported. The surface layer of each column was treated with an arbitrarily determined concentration of each of eight surfactants and then subjected to wetting and drying cycles. Soil moisture losses were determined periodically from each soil column.

The observed differences are discussed in light of the available information on the surfactants used. The data indicate that two of the compounds, a non-ionic surfactant and a fatty alcohol, were highly effective in reducing water evaporation from a coarse sand. Evaporation from a fine sandy loam soil was retarded effectively by irrigating with water containing a non-ionic compound. There was found to be a strong interaction between soil texture and effectiveness of the evaporation suppres­sors. The results should have wide appication to other states and countries.

INTRODUCTION

Economic as well as social aspects regarding the consumption of water for useful purposes have dictated the need for conserving the available water supply in various countries of the world. For example, of the total approximate income of 366, 600, 000 acre-feet of annual income of water for the state of Texas, about 46 percent is transferred to the atmosphere by transpiration through plants while evaporation from soil, as well as free water surfaces, accounts for 40 percent loss. Therefore, suppression of evaporation has assumed a significantly important place in the research activities dealing with the overall problem of water-use and conser­vation.

Surface-active compounds (3) are known to have been effective in reducing the evaporative losses from free water surfaces. However, studies dealing with the role of such compounds in suppressing evaporation from agriculturally important soils have been limited.

i1) This paper was presented at the X11I General Assembly of the International Union of Geodesy and Geophysics which was held in August, 1963, at Berkeley, California, and was a contribution of The Texas A. & M. University, College Station, Texas.

(2) Research Assistant, Department of Oceanography and Meteorology and Professor of Soil Physics, Department of Soil and Crop Sciences, respectively, The Texas A. & M. University, College Station, Texas. The senior author is now at the Institute of Agriculture, ANAND, Gujarat State, India.

(3) The term surface-active compound is generally used to designate any sub­stance whose presence in small amounts markedly alters the surface behavior of a given system (see SCHWARTZ and PERRY, 1959).

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Evaporation from soils is a complex process which is not only governed by atmospheric meteorological conditions but also is influenced greatly by the physical conditions prevailing in the soil. Reference may be made to the research studies by Gardner et. al. [1959], Cruse and Harbeck [1960], Bowers and HANKS [1961 ] QAYYUM and KEMPER [1962], and others, wherein the physical conditions of the soil have been shown to have influenced the movement of water within the soil profile and also from soil to the air. The surface-active compounds alter the interfacial energies in the soil-water continium and thus affect the physical properties of the soil [GROSSI and WOOLSEY, 1955]. It has been anticipated and suggested in some cases that such compounds might reduce evaporation from soils. Therefore, a study was initiated, as part of a larger research program concerned with the role of surfactants and their interactions in the dynamic soil-water system, to determine the effectiveness of certain selected compounds in reducing evaporation of water from coarse-textured soils. The present paper reports the results of this study.

MATERIALS AND EXPERIMENTAL PROCEDURE

A total of eight surface-active compounds, which comprised the non-ionic, anionic and cationic categories, was used in the study (table 1).

TABLE 1

The Category and Source of Surfactants Used in the Study

Surfactant Category Manufacturer

Aqua-Gro Arquad-2HT-T Deriphat-170C Ethofat-242/25 Ethoquad-C/25 G-271 Hexadecanol Neomerphin-N

Non-ionic Cationic Amphoteric Non-ionic Cationic Cationic Fatty Alcohol Anionic

Aquatrols Corp. of America Armour Chemical Division General Mills Armour Chemical Division Armour Chemical Division Atlas Power Co. Procter & Gamble Co. E.I. du Pont de Nemours & Co.

The selection of chemicals was quite arbitrary and was primarily dictated by their availability in the laboratory and represents no bias towards any particular manu­facturer. Additional data concerning the above compounds have been discussed by COVEY and BLOODWORTH [1961]. Also, none of these chemicals is reported to be toxic to plants when used in the concentrations (0.10 percent) reported in this study.

Application of the compounds to the soil was achieved in different ways, depending upon the nature of the compound. Those which were easily dispersed in water were mixed by volume to the desired concentration and added as an irrigation application. Others were mixed and added directly in the desired concentration with the 0-3 inch depth of dry-soil. The concentration of each surfactant was 0.1 percent, and the depth of treatment was 3 inches in each case.

Porous materials used in the study were mortar-sand and an Amarillo fine sandy loam soil (4). To determine the influence of the chemicals on evaporation, the sand

(4) The mortar-sand was characterized by the following physical analysis (parti-

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and soil materials were air-dried, passed through a 1-mm mesh sieve and packed as uniformly as possible into galvanized iron cylinders which were 8 inches in diameter and 22 inches in height. The cylinders were provided with a drain-plug at the bottom. They were first filled with 0.25 inch diameter gravel to a depth of 2 inches. After application of the treatments, enough water was added to each cylinder from the top to bring the soil to saturation. All cylinders were allowed to remain in this condition for 24 hours before draining. After draining, all cylinders were weighed on a platform-type balance which weighed to the nearest 1-gram. Successive weighings every alternate day gave the accumulative water-loss from each cylinder. Each treatment was subjected to three wetting-drying cycles, with each cycle extending over a peroid of 5 weeks.

The experiment was conducted in a constant temperature room (27° ± 1 °C). The relative humidity was not controlled, but hygrothermograph measurements indicated a range between 30 and 40 percent. Air circulation in the room was main­tained. Radiation within the room was fairly uniform and of moderate intensity, except during the period when the lights were on for weighing the cylinders.

The experiment was a randomized block with 3 replications for each treatment. The moisture content was determined gravi metrically at the conclusion of the

experiment for both mortar-sand and soil. In case of the soil, fiberglass soil moisture units, with thermistors, were used to observe the distribution of moisture and tem­perature at the 0-1 inch and 0-6 inch soil depths.

It was observed during the experiment that many of the cylinders showed signs of heavy crust formation at the surface when the surface-soil approached dryness. Likewise, there was a slight tendency for the surface soil to separate from the side of the cylinder due to contraction on drying and the approximate separation was about 1 mm. No attempt was made to remove the crust from the surface, nor was there any allowance made in the accumulative evaporation figures for the separation of surface soil from the sides of the cylinder, as these features were common to almost all the cylinders.

The use of chemical compounds in reducing evaporation, from a practical standpoint, will be ineffective if water infiltration is impaired. Therefore, infiltration studies were conducted with the Amarillo fine sandy loam soil.

To determine the effect of the chemical treatments on water infiltration, the soil cylinders, at the end of the third wetting cycle, were used as " constant-level "infiltro-meters. A piece of cheese cloth was placed on the soil surface and water was poured slowly through a sprinkler on the cloth so as not to disturb the soil surface. When the level of water was 2 inches from the surface, the cheese cloth was removed and the penetration of water during fixed time intervals was measured with a hook-gauge. Additional water was added as soon as the level dropped 0.5 inch. Infiltration rates were measured for all treatments in triplicate.

To study further the effect of a non-ionic surfactant, Aqua-Gro, on suppression of evaporation from a coarse-textured soil such as the Amarillo fine sandy loam, an experiment was conducted in which concentrations of 0.01, 0.10 and 1.0 percent of the non-ionic surfactant were used in water that was applied to the soil. The design was that of a randomized block with 3 replications. Each column of soil was 2 inches in diameter by 11 inches in length and was packed to a bulk density of 1.58 g/cm3

prior to the first irrigation. The different surfactant concentrations were applied during the course of the initial irrigation.

An application of 280 ml of the chemically-treated water was added to each soil column. After draining for 24 hours, the bottom of each cylinder was sealed-off

ticle-size distribution) : > 1 mm - 10.7 percent; 1 to '/i mm - 39.5 percent; 1/2 to 1li mm - 25.4 percent; X]A to 0.1 mm - 23.7 percent; < 0.10 mm - 0.60 percent. The Amarillo fine sandy loam soil contained 72 percent sand, 16 percent silt, and 12 per­cent clay.

61

with plastic to prevent the further loss of water. All columns were weighed at 8:0 0 a. m. each day for periods ranging from 7 to 10 days. Three subsequent irrigations with untreated water were applied in amounts of 200 ml/column, and the same weighing procedure was followed as indicated for the initial drying cycle of the soil columns.

Following the fourth drying cycle, soil samples were taken in 1-inch increments to a depth of 5 inches; one composite sample was taken for the 5-11 inch depth. Soil moisture percentages were obtained for all the samples and plotted as a function of depth.

Drying of the soil columns was accelerated by using 150 watt infrared heat lamps that were mounted 24 inches above the randomized mass of soil columns.

RESULTS AND DISCUSSION

A. Evaporation from large soil columns

Effectiveness of the surface-active compounds on suppressing evaporation during each of the wetting-drying cycles for mortar-sand and the Amarillo fine sandy loam soil is shown in Table 2. The values represent the percentage reduction on the basis of no-treatment value as 100 percent, during the entire wetting cycle with an average duration of 35 days. Absence of numbers, except in the check, indicated increased evaporation over the control.

TABLE 2

Effectiveness of Surface-Active Compounds on Suppressing Evaporation from Sand and Soil Columns for 35-Day Runs

Surfactant

Aqua-Gro Arquad-2HT-T Deriphat-170C Ethofat-242/25 Ethoquad C/25 G-271 Hexadecanol Neomerphin-N Check

Percent Reduction in Evaporation

Mortar San

First wetting

44.4 33.3 44.4

-22.2 11.1 33.3

-—

Second wetting

30.0 21.2

8.7 ---

30.0 --

d

Third wetting

43,4 37.7

--

37.7 -

52.8 -~~

Amarillo Fine Sandy Loam Soil

First wetting

71.9 4.9

45.2 55.7 -

10.1 6.2

28.2 —

Second wetting

50.7 -31.4 38.1 16.7 7.0 1.5

24.3 —

Third wetting

36.7 --

13.9 7.2 9.5 -

11.3 —

The accumulative evaporation from the large container of mortar-sand and Amarillo fine sandy loam soil, as affected by the different treatments, is shown in Figures 1 and 2. For ease of comparison, the accumulative evaporation for the control treatment is shown in each figure.

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250

200

150

100

50

0

-

, 1

i 1

i •6 l

CHECK A E T H 0 F A T - 2 4 2 / ;

AQUA-GRO

. ^ " - " ^

^ ^ ^ ^ ^ ^ ^

/ \ ^ ^ - - -"""" / X ^ , - ' "

I I I , 1 , 1 , 1 , 1 , 1 10

-DAYS

B CHECK

ETHOQUAD C/25

G-271 ARQUAD-2HI -T

10 12 TIME — DAYS

200 -

100

CHECK DERIPHAT - 170 C NEOMERPHIN-N HEXADECANOL

8 10 12 14 TIME — DAYS

Fig. 1 — Evaporation of water from large mortar-sand columns that had been treated with different surface-active compounds. The compounds were : [A] non-ionic surfactants; [B] cationic surfactants; [C] amphoteric (Deriphat-170C); anionic surfactant (Neomerphin-N); and a fatty alcohol (hexadecanoD- All accumulative values are averages for 3 evaporative containers.

63

a 500

CHECK ETHOFAT- 242 /25

10 12 14 TIME — DAYS

13 CHECK ETHOQUAD-C/25 G - 271

ARQUAD-2HT-T

5 500-

o 100 -

CHECK

— • — DERIPHAT- 170 C NEOMERPHIN-N HEXADECANOL

10 12

TIME — DAYS

Fig. 2 — Evaporation from Amarillo fine sandy loam soil columns which were treated with different surface-active compounds. The compounds were : [A] non-ionic surfactants; [B] cationic surfactants; [C] amphoteric (Deriphat-170C); anionic surfactant (Neomerphin-N); and a fatty alcohol (hexadecanoD-

64

It may be observed from Table 2 and Figures 1 and 2 that, in general, the non-ionic surfactants were more effective in reducing evaporation. However, the extent of reduction, as well as the effectiveness of the compounds in reducing evaporation, were affected by the texture of the porous material. For example, an individual compound was found to be effective in the coarse mortar-sand but was ineffective in suppressing evaporation from the soil. The reverse also was found to be true. Another obvious feature of the behavior of the non-ionic compounds was the fact that they usually became less effective in reducing the evaporation with each successive wetting-drying cycle.

The cationic surfactants were found to be inconsistent in their behavior towards the reduction of evaporation. As compared with non-ionic surfactants, the cationics appear to be less effective in suppressing evaporation. They are indicated to be more effective in sand than in soil. Also, a definite decline in the effectiveness with successive wetting cycles is obvious in the case of the treatments with soil; no such trend is apparent in sand treatments. This behavior is attributed largely to the cation exchange capacity of the clay fraction within the soil.

The anionic compound was observed to have no suppression effect on water-evaporation with sand; however, its influence in reducing evaporation from soil was greater than that of the cationic compounds. This behavior is in conformity with the observations of Covey and Bloodworth (1961) who observed that the capillary rise of dilute anionic surfactant solutions in dry soils was slower than that of pure water. The decline in the effectiveness with successive wetting cycles did exist but was not uniform. A marked decline was observed during the third wetting cycle.

Among the other surfactants studies, the results of hexadecanol are noteworthy in view of its recent successful role in reducing evaporation from free water surfaces (CRUSE and HARBECK, 1960), (La Mer, 1962) and (MEINKE and BLOODWORTH, 1962).

While hexadecanol was found to be highly effective in reducing water evaporation from the sand, its response in reducing evaporation from the Amarillo fine sandy loam soil was poor. This is in contrast with recent results reported by MALLIK (1962) who found hexadecanol to be effective in reducing evaporation from Poona black soil to the extent of 30 percent. Also, Roberts [1961] has reported that a mixture of hexa- and octa- decanol reduced transpiration in plants to the extent of about 40 percent.

Pore-size distribution has an important role in evaporation of soil moisture. The present findings with the fatty alcohol can be explained largely on the basis of pore-size distribution. An abundance of macro-pores in sand apparently allows the formation of a mono-molecular film on the surface of water which is held in the non-capillary pores and, therefore, retards the evaporation of water by diffusion. However, the absence of an abundance of the large macro-pores in case of the soil does not permit the formation of monomolecular films; consequently, the fatty alcohol remains ineffective in suppressing evaporation from the soil mass. This argument, although physically tenable at this time, needs to be substantiated by further experimental evidence.

The amphoteric surfactant Deriphat was equally effective, initially, and was next in order to the most effective non-ionic compound in both sand and in soil. Its influence did not persist with successive wetting cycles and completely vanished during the third cycle. The texture of the soil was of some concern in this regard, as the effectiveness of the compound during the second wetting cycle was considerably more pronounced in the soil than in the sand. However, the exact mode of action is unknown at this time.

65

B. Residual moisture in large sand columns

The moisture content with depth, at the end of the third drying cycle and as measured by the gravimetric method, is given in Table 3 for the mortar sand and soil.

TABLE 3

Moisture Content with Depth at the End of the third drying Cycle

Treatment

Aqua-Gro Arquad-2HT-T Deriphat-170C Ethofat-242/25 Ethoquad-C/25 G-271 Hexadecanol Neomernhin-N Check

Percentage Moisture

Mortar Sand

Surface

0.38 0.37 0.42 0.44 0.36 0.37 0.39 0.43 0.41

6-inch depth

2.22 4.49 5.54 5.29 4.74 4.38 4.39 4.10 3.75

12-inch depth

4.42 7.53 9.26

13.64 7.46 7.23 7.40 6.30 5.55

Amarillo Fine Sandy Loam Soil

Surface

0.43 0.41 0.52 0.49 0.41 0.39 0.42 0.50 0.49

6-inch depth

3.15 5.32 6.49 6.83 5.37 5.48 4.79 4.87 4.10

12-inch depth

12.17 13.15 11.29 14.05 10.59 11.73 10.07 11.18 10.12

The values shown in Table 3 are averages of 2 samples from each of the three replications for each treatment. In case of the soil, additional samples were drawn for moisture determinations from the 16-inch depth. The soil was found to contain some free-water for each treatment.

The use of fiberglass moisture units for determining the day to daj variations in moisture content during each drying cycle did not prove advantageous. Variation in the moisture content was not easily discernible due to the inconsistency of the readings.

The moisture data is not, however, readily reconcilable with the evaporation data without taking into account the influence of the compounds on water infiltration.

C. Water infiltration curves

The infiltration curves for the different treatments with soil are shown in Figures 3 through 5.

It is evident from the infiltration curves that the non-ionic compounds increased the amount and the rate of infiltration at the initial stage; the anionic, as well as the amphoteric compounds also were observed to have caused a slight increase in the rate of initial infiltration. The cationic compounds and hexadecanol showed a slight reduction in infiltration. An exact explanation for the varied effects of the surfactants on water infiltration is not at hand but presumably rests largely in the physico-chemical interactions of the compounds with the clay fraction of the soil. The obser­vations are, however, in agreement with those reported in the literature (BOWERS

66

and HANKS, 1961), (COVEY and BLOODWORTH, 1961). It was found also during the infiltration studies that the soil surfaces of each cylinder treated with the cationic surfactants behaved initially as if they were almost sealed at the top. This hydro­phobic behavior was the most pronounced in case of the soil treated with Arquad 2HT-T.

, 2 0

Ï O 1 15

1 H» 1

5

0

\-\ -\

4 \

-

\ \ W\

. 1 1 1 1 1

CHECK

AQUA-GRO E T H 0 F A T - 2 4 2 / 2 5

1 , 1 , ! , I 60 80 100

TIME - MINUTES

Fig. 3 — Water infiltration rates for Amarillo fine sandy soil co lumns following three 20-day wetting-drying cycles and which had been treated previously with non-ionic surfactants.

CHECK ARQUAD- 2H1-T ETH0QUAD-C/2& G - 271

60 80 100 TIME — MINUTES

Fig. 4 — Water infiltration rates for Amarillo fine sandy loam soil columns following three 20-day wetting-drying cycles and which had been treated previously with cationic surfactants.

67

25 r

0 20 40 60 80 100 120 140 160

T IME — MINUTES

Fig. 5 — Water infiltration rates for Amarillo fine sandy loam soil columns which had been treated previously with amphoteric and anionic surfactants and a fatty alcohol.

TIME — DAYS

Fig. 6 — Accumulative water losses from Amarillo fine sandy loam soil columns following the first irrigation containing the non-ionic surfactant (Aqua-Gro). Percentage reductions in evaporation over the control are shown on the right side between the curves.

68

120

100

tn z < a: o I 80

w tn O

a: Ui

< 60

* UJ

É 40

o o

2 0

0 0 2 4 6 8 10

TIME - DAYS

Fig. 7 — Accumulative water losses from Amarillo fine sandy loam soil columns following the fourth irrigation. Percentage reductions in evaporation over the control are shown on the right side and between the curves.

D. Water evaporation from small-diameter columns

Accumulative water losses and residual moisture by increments of soil-depth, as affected by three concentrations of the non-ionic surfactant, Aqua-Gro, are illustrated in Figures 6, 7 and 8. It is indicated conclusively that the 1 percent concen­tration was still effective in reducing evaporation 3 irrigations (or leachings) after the initial application. Furthermore, the soil moisture data of Figure 8 show that a considerable amount of water remained in the soil below the 4-inch depth at both the 0.1 and 1.0 percent concentrations. In fact, free water was found in all samples taken from the columns which had been treated with the 1.0 percent concentration (5). Since the non-ionic compound used has been shown to be non-toxic to crop plants, it is conceivable from these data that approximately 36 percent more water could have been made available for use by plants. If conducted under field conditions, the data may not be so revealing as indicated in the laboratory study; however, it does indicate that such compounds may offer promise when used under certain, selected conditions.

As indicated previously in the study concerning the large containers of sand,

(5) The 1/10 and 15 bar moisture percentages, as obtained from suction measurements, were 14 percent and 4.5 percent, respectively.

69

CHECK

0 . 0 1 % CONCENTRATION

0 . 1 0 %

1.00 %

1 4 . 9 %

an exact explanation as to why the 0.01 percent concentration of the non-ionic caused an excessive amount of evaporation over the check( Figure 7) cannot be given at this time. A similar effect has been found with other compounds at this laboratory and also has been reported by Bowers and Hanks (1961).

SOIL MOISTURE - PERCENT

Fig. 8 — Effect of different non-ionic (Aqua-Gro) surfactant concentrations on water evaporation from Amarillo fine sandy loam soil columns as indicated by the moisture remaining by depth-increments after 8 days of drying (see Figure 7).

SUMMARY

The use of non-ionic, cationic, anionic and amphoteric surface-active compounds and a long chain fatty alcohol in suppressing evaporation from coarse textured agricultural soils was investigated. The restriction of the study to coarse-textured soils was based on the consideration that a large area of the arid and semi-arid zones of the world, where the reduction in evaporation has a special significance, have coarse-textured soils.

In an experiment conducted under controlled temperature conditions, large soil columns were treated with eight surfactants with each having a concentration of 0.1 percent. They were mixed to a depth of 3 inches in mortar-sand and a fine sandy loam soil, and all soil columns were subjected to evaporation losses. The treated soils and their containers were weighed on scales at regular intervals, and the losses were then compared with those from the untreated soil columns. The greatest average-reduction in evaporation from the mortar-sand for three 20-day periods was provided by Arquad 2HT-T, hexadecanol and the Aqua-Gro, a non-ionic compound. All three effected a reduction of 29 percent. A reduction of 51 percent was obtained for similar periods by the non-ionic compound when applied to an Amarillo fine sandy loam soil.

70

A second laboratory study using soil columns 2-inches in diameter by 11 inches in length showed that a non-ionic surfactant, when applied in concentrations of 0.10 and 1.0 percent, reduced evaporation by 17 and 42 percent, respectively The wetting-drying cycles consisted of 4, with each being for a period ranging between 8 and 10 days.

Infiltration rates under relatively constant heads were measured for soils treated with the surfactants following 3 wetting-drying cycles. Infiltration rates were affected little by the chemical compounds after 2 hours.

The data revealed that non-ionic surfactants were the most influential in reducing evaporation without reducing infiltration. The cationic compounds varied among themselves in their influence on evaporation reduction. They also reduced slightly the initial infiltration. Anionic and amphoteric surfactants had no influence on sand, but they did reduce evaporation and infiltration from soil. The fatty alcohol exhibited a reverse tendency in that it reduced evaporation from sand but not effectively from soil.

Texture of the porous media played an important role in the action of the sur­factants in reducing evaporation. The physico-chemical interactions, such as the anion-cation exchange and change in contact-angle for the soil-water-surfactant complex, were not investigated but are planned for future studies. Some research has indicated that they do have a significant influence on the action of surfactants in reducing evaporation.

ACKNOWLEDGEMENTS

The authors wish to express their thanks to Mr. J.P. Law, Jr., Research Assistant in Soil Physics, for his help and interest in the work. Special thanks also are due Mr. David Kindt for his assistance in packing the soil columns, weighing the cylinders, and collection of data during the course of the study. Appreciation is expressed also to the commercial organizationes for furnisting chemicales for use in these and other studies along this line.

REFERENCES

BOWERS S.A. and HANKS, R.J., Effect of DDAC on evaporation and infiltration of soil moisture. Soil Sci., 92 (5) 340-46, 1961.

COVEY, W.G. and BLOODWORTH, M.E.,Some effects of surfactants on agricultural soils. Pub. No. MP-529, Texas Agr. Exp. Sta., Agricultural and Mechanical College of Texas, College Station, Texas, 1961.

CRUSE, R.R. and HARBECK, G.E., Evaporation control research, 1955-58, Geol. Sur. Water Supply Paper 1480, U.S. Geological Survey, 1960.

GARDNER, W.R., MAYHUGH, M.S. GOERTZEN, J.O. and BOWER, C.A. Effect of electrolyte concentration and exchangable sodium percentage on diffusivity of water in soil, Soil. Sci., 88 (5), 270-274, 1959.

GROSSI, F.X. and WOOLSEY, J.L. Effect of fatty quaternary ammonium salts on physical properties of certain soils, Ind. Eng. Chem., 47, 2253-58, 1955.

LA MER, VICTOR K., Retardation of Evaporation by Monolayers - Transport Processes, Academic Press, New York and London, 1962.

MALLIK, A.K., Control of soil evaporation, Jour, of Met. and Geophy., 13 (3), 429-30, 1962.

MEINKE, W.W. and BLOODWORTH, M.E. Water evaporation studies under controlled climatic conditions, Texas Eng. Exp. Sta. News, 13 (2), 11-20, 1962.

ROBERTS, W.J. Reduction of transpiration, Jour, of Geophys. Res., 68 (10), 3309-12, 1961.

QAYYUM, M.A. and KEMPER, W. D. Salt concentration gradients in soil and their effects on moisture movement and evaporation, Soil Sci., 93 (5), 333-42, 1962.

SCHWARTZ, A.M. and J.W. PERRY, Surface-active Agents - Their Chemistry and Terminology, Interscience Publishers, Inc., New York, N. Y., 1959.

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