DIVISION S-6—SOIL AND WATER MANAGEMENTAND CONSERVATION

Effect of Synthetic Conditioners on Soil Water Retention, Hydraulic Conductivity,Porosity, and Aggregation1

M. N. NIMAH, J. RYAN, AND M. A. CHAUDHRY2

ABSTRACTRecent attention has been focused on the use of synthetic soil con-

ditioners in the modification of soil water relationships especially in aridregions. Soils of extremes in texture—either sands or clays—presentproblems in this regard. In this laboratory study, Hygromull (a ureaformaldehyde) and Agrosil LR and Agrosil S (amorphous sodium hy-drosilicates) were evaluated on soils of different textures. Available watercontent was increased by Hygromull and Agrosil LR. Saturated hy-draulic conductivity of the clay soils was improved by Hygromull, whilethat of the sandy soil was reduced by Agrosil LR. Hygromull increasedporosity of all soils but, unlike Agrosil LR, had no effect on aggregation.In contrast, Agrosil S had no effect on any property studied. Notwith-standing the attributes of these conditioners, their acceptance in com-mercial farming in dry regions will depend on the outcome of field trialsand economic considerations.

Additional Index Words: sandy soils, clay soils, permeability, Hy-romull, Agrosil, soil amendments, soil conditioners

Nimah, M.N., J. Ryan, and M.A. Chaudhry. 1983. Effect of syntheticconditioners on soil water retention, hydraulic conductivity, porosity,and aggregation. Soil Sci. Soc. Am. J. 47:742-745.

1 Contribution from the Soils, Irrigation and Mechanization Dep.,Faculty of Agricultural and Food Sciences, American University ofBeirut, Beirut, Lebanon. Journal no. 575 B. Received 23 Apr. 1982.Approved 26 Oct. 1982.? Associate Professors and Graduate Student, respectively.

SOIL CONDITIONING implies improvement of the soil'sphysical properties, thus permitting more effective

utilization of soil and water resources. In recent years,considerable emphasis has been placed on the use of awide array of chemicals purporting to favorably influencesoil properties. Some materials are produced directly forsuch purposes, while others are by-products of industrialprocesses. A relatively recent review of the subject in-cludes materials used for soil conditioning, their mode ofaction, their use in soil and water management, and theirglobal use extent, both in agricultural and nonagricul-tural purposes (Gardner and Moldenhauer, 1975). Syn-thetic soil conditioners can be classified as soluble organicpolymers, emulsions, silicates, and foams.

Soluble materials undergo physico-chemical reactionswith soil constituents especially the clay fraction. Upondrying, an insoluble irreversible matrix is formed whichgenerally results in improved aggregation, porosity, andhydraulic conductivity (Schamp et al., 1975). Thus, suchmaterials are used to stabilize highway banks and con-struction sites, and to combat soil crusting, which impedesgermination, and both wind and water erosion. Othermaterials do not react with soil constituents but exert aneffect through modification of soil texture and by increas-ing soil water retention.

NIMAH ET AL.: EFFECT OF SYNTHETIC CONDITIONERS ON SOIL WATER, POROSITY, & AGGREGATION 743

Soil conditioners have potential importance in the aridand semi-arid regions of the world where there is a de-veloping awareness of the implications of soil erosion andinefficient water use. Such materials can favorably mod-ify soil water relationships especially retention and trans-mission (Hartman et al., 1976). Chemical soil condition-ers vary from well-defined inorganic salts and syntheticpolymers to products whose properties are less clearlyunderstood. While some studies have demonstrated pos-itive effects of soil conditioners, i.e., marked increases insoil water diffusivity with Krillium and polyvinyl alcohol(Kijne, 1967), others have not. For instance, in green-house and field studies with sandy soils cropped to turf-grass, McGuire et al. (1978) found that none of the con-ditioners studied, i.e., two bituminous emulsions, fivepolyacrylamides (PAM), and one polyvinyl alcohol(PVA), beneficially affected soil physical properties, cat-ion exchange capacity (CEC), or any turf grass param-eter. However, other studies such as that of Bolton et al.(1955) showed that soil conditioners (hydrolyzed poly-acrylonitrile and a modified vinyl acetate maleic acidcompound) improved aggregation and both total andnoncapillary pore space in a clay soil but had no effecton crop yield under field conditions.

While most studies of soil conditioners have dealt withsoluble polymers and emulsions, the literature on silicatesand foams is scant. Agrosil and Hygromull are examplesof such materials. Buring and Prun (1974) showed thatthe silicate Agrosil LR increased water-holding capacityand decreased water percolation rate in sandy soils dueto a decrease in average pore size. In contrast, the find-ings of Khoury et al. (1978) showed that it increasedpore size. Similarly; under semiarid conditions, improvedcrop growth was reported with Agrosil LR and the acidicfoam—Agrosil S (Buring and Prun, 1974),—whereas, inthe field trials described by Khoury et al. (1978), nopositive effect was subsequently shown. In studies withthe foam material—Hygromull—Rasp (1972) showedthat it improved the water-holding capacity, increasedporosity, and reduced bulk density of a sandy and a loamsoil. Buchner et al. (1969) similarly reported improvedwater-holding capacity in light-textured soils and aera-tion in heavy clay soils. However, the observation byKhoury et al. (1978) that Hygromull increased pore di-ameter and reduced available water in a clay soil ques-tions the validity of these findings.

Because of the dearth of published information on sil-icate and foam-type synthetic conditioners, the unrecon-ciled reports dealing with Agrosil and Hygromull, andthe lack of integration between measured parameters inreported studies, an investigation was undertaken withthese materials to determine their influence on sandy andclay soil water retention, hydraulic conductivity, poros-ity, and aggregation.

MATERIALS AND METHODSSoils and Soil Conditioners

Soil samples were selected from three locations in Lebanon:Leba'a in the south; Furzol in the central Beka'a valley; andSheia'h near Beirut on the coast; some physical and chemicalproperties are given in Table 1. Samples were air-dried andpassed through a 2-mm sieve for this study.

The synthetic soil conditioners (produced by BASF, SpartRII-VA2, 6700 Ludwigshafen, Rhein, Germany) were Hygro-

Table 1—Some physical and chemical characteristics of the soils.

Soil Organiclocation Classification Sand Silt Clay CaCO3 carbon pH

80.0 2.8 17.2 0.0 0.3 8.0Sheia'h ArentLeba'a Rendollic

xerorthent 17.1 28.2 54.7 75.2 0.9 8.0Furzol Vertic

xerochrept 12.4 26.0 61.6 38.1 0.9 8.2

mull and two forms of Agrosil. Hygromull is a ureaformalde-hyde foam (flakes) with 30% C and 30% N and a bulk densityof 0.035 g/cm3. By virtue of its porous structure, Hygromull isclaimed to absorb 50 and 70% water on a volume basis whenincorporated into the soil under normal field conditions (Prunand Drach, 1977). Though water is absorbed slowly by thismaterial, it is believed to be released uniformly to the plant. Itis recommended for increasing water-holding capacity of sandysoils and improving aeration of clay soils. Agrosil LR is anamorphous sodium silicate having 45% Sio2, 8.7% P, a pH of7, and a bulk density of 0.76 g/cm3. Agrosil S contains 36%SiO2, 44% P, and 6% S; has a pH of 3.0 to 3.5; and a bulkdensity of 0.87 g/cm3. Upon dissolution in water, Agrosil formsa mixture of gels and sols (Prun, 1974; Buring and Prun, 1974).The larger moleculed silicate gels are honeycombed with cap-illary pores which retain water. Precipitation or formation ofgels influences pore size distribution and thus permeability.When Agrosil S is used in calcareous soils, an added effect inreducing soil compaction is postulated due to the evolution ofCC<2 upon reaction of this acidic material with soil carbonates.

ProcedureThe three soils were treated with the conditioners as follows:

Sheia'h soil—Hygromull and Agrosil LR; Furzol soil—Hygro-mull and Agrosil S; and the Leba'a soil—all three materials.Hygromull was added to the soil at the rate of 200 m3/ha whileboth Agrosil materials were added at 2 t/ha. The specific mea-surements were as follows:

Water Retention— Weighed amounts of Hygromull andAgrosil (LR or S) were mixed with 80 and 100 g of dry soil,respectively, and packed in PVC tube sections (2.5 cm in lengthand 6.5-cm internal diameter). Control samples consisted of100 g of dry soil alone. The soil was retained with muslin gauze.Samples were saturated for 2 h, then allowed to dry out overa 7-d period. The wetting-drying cycle was repeated six times.For determining water retention, the samples were placed on apressure membrane with the muslin removed and saturated for24 h. Pressures of 0.33 and 15 bar were applied for 48 h onsamples after 2, 4, and 6 wetting-drying cycles and of 1, 2, and5 bar after six wetting-drying cycles. Available water was con-sidered to be the difference in water contents determined at— '/3 and — 15 bar matric potential.

Hydraulic Conductivity— Samples (300 g) of the three soils,with and without the conditioners, were placed in 15 by 6.5-cm PVC columns. Soil loss was prevented by a metal screenand muslin at the bottom of the columns. Treatments were intriplicate. Samples were saturated with distilled water and putthrough wetting-drying cycles as described above. Permeabilitywas determined with a constant head permeater after 2, 4, and6 cycles. Determination of flow rate began after 15, 30, and 60min with the Sheia'h, Leba'a, and Furzol soils, respectively.Hydraulic conductivity was calculated using Darcy's formula:

K=(Q/AT)-(H/L), [1]where K = hydraulic conductivity (cm/h), Q = volume ofwater (cm3) passed through soil in time; T = time (h); L =length of soil column (cm); H = total head (cm); and A =cross-sectional area of the column (cm2). Where data were ana-lyzed statistically and had significant F values, Duncan's Mul-tiple Range Test was applied.

744 SOIL SCI. SOC. AM. J., VOL. 47, 1983

UntreatedAgrosil LRHygromull

16o>

14 o

12

10

8 ££

-15 -10 -5 -2 -1Matric potential (bars)

Fig. 1—Water retention as a function of matric potential after six wet-ting-drying cycles by the sandy Sbeia'h soil previously treated withsoil conditioners.

Soil Physical Measurements—After 48 h from the finalpermeability measurement, bulk density was determined by thecore method (Blake, 1965) while porosity was calculated fromthe measured bulk density, assuming particle density of 2.65g/cm3. Water-stable aggregates were determined by the ag-gregate-size distribution with wet sieving (Richards, 1954).

RESULTS AND DISCUSSIONAvailable water and permeability data for the three

soils treated with the conditioners Hygromull, AgrosilLR, or Agrosil S are presented in Table 2. Interpretationof the available water data is facilitated by depictingwater content after six wetting-drying cycles at the en-tire range of matric potentials considered for the Sheia'h(Fig. 1), Leba'a (Fig. 2), and Furzol (Fig. 3) soils.

Available water was influenced differentially by thesoil conditioners depending on soil properties, notablytexture. Hygromull increased water retention by about125% in the sandy Sheia'h soil and about 25 to 30% forthe clay soils—Leba'a and Furzol. However, this effectwas not observed with the sandy soil at the first deter-

Table 2—Influence of soil conditioners on available water andpermeability of three soils after varying

wetting-drying cycles.

No. of wetting-drying cycles

Treatment

ControlHygromullAgrosil LR

ControlHygromullAgrosil LRAgrosil S

ControlHygromullAgrosil S

2 4 6

Available water

3.93.65.8

8.79.8

10.49.2

3.67.46.8

- % ——————Sheia'h*

4.0 4.39.2 9.76.0 6.4

Leba'a*9.2 9.4

11.7 11.910.8 11.19.6 9.7

Furzol*6.9 7.18.6 9.06.9 7.2

2 4 6

Permeability

18.4 b21.0 a7.9 e

5.3 b6.3 a4.1 c5.0 b

0.3 c0.9 b0.3 c

12.9 cd 10.7 d14.9 c 12.2 d6.9 e 6.5 e

3.9 c 3.8 c5.1 b 4.8 b3.6 c 3.5 c3.9 c 3.6 c

0.4 c 0.4 c1.0 ab 1.1 a0.3 c 0.4 c

i Agrosil LR

Agrosil S

i Untreated' Hygromull

-15 -5Matric potential (bars)

30

28

26 <uE

24

22

20 5

18

16

-2 -1 0

' Within any soil different letters denote significant differences at the 5%level of probability according to Duncan's Multiple Range Test.

Fig. 2—Water retention as a function of matric potential after six wet-ting-drying cycles by the Leba'a clay soil previously treated with soilconditioners.

mination, i.e., two wetting-drying cycles, and was lesspronounced with the clay soils. This observation supportsthe contention that Hygromull slowly absorbs water andis not immediately effective when applied to the soil (Prunand Drach, 1977). While Agrosil LR performed similarlyto Hygromull in the Leba's clay soil, it was not as effec-tive as Hygromull in the sandy Sheia'h soil. The acidicform of Agrosil, i.e., S, appeared to have no significanteffect on water retention where it was used with the twoclay soils.

The water retention curves suggested that not only didthese materials influence total available water but thatretention varied with the conditioners depending on thepressure potential applied to the soils. For instance, at—1/3 bar or field capacity in the sandy soil (Fig.l), theHygromull and the Agrosil treated soil held 15.4 and13.5% water, respectively, while the control soil held only10.5% water. However, as the tension was increased from1 to 15 bar, the order of water retention was Agrosil LR,control, and Hygromull. The pattern of water release wassuch that there was little, if any, difference between con-tents at — 2 and — 15 bar matric potentials for eithertreatment. The results question the claims of Fritz (1976)that Hygromull releases retained water uniformly up to— 15 bar matric potential, and rather confirm the find-ings of Rasp (1972) who showed no difference in waterretention between pF 2.54 and 4.7 (i.e., about —0.35 and— 5 bar potential) in Hygromull-treated soils. The pat-terns of water extraction were relatively similar for thetwo clay soils (Fig. 1 and 2). In both cases water releasetended to increase relative to the control and the Agrosiltreatment as the water tension increased.

Differences between the water retention curves can beexplained on the basis of properties of the individual soilconditioners. In view of Hygromull's porous structure,water retained by capillarity can be easily removed atlow water tension, i.e., higher potentials, whereas ad-sorbed water is released at lower potentials as postulatedby Hillel (1971), Adsorption of water by the Agrosil would

NIMAH ET AL.: EFFECT OF SYNTHETIC CONDITIONERS ON SOIL WATER, POROSITY, & AGGREGATION 745

UntreatedAgrosil LRHygromull

-15

34

32

30.

-1

265

Table 3—Influence of soil conditions on bulk density, porosity,and aggregation of three soils after six wetting-drying cycles.

24

-2-1 0-5Matric potential (bars)

Fig. 3—Water retention as a function of matric potential after six wet-ting-drying cycles by the Furzol clay soil previously treated with soilconditioners.

then explain its comparatively higher retention values athigher soil tensions.

The process of wetting and drying in the absence ofthe soil conditioners had a marked effect in consistentlyincreasing the amount of water retained by the three soilswith a concomitant reduction in permeability in two soils.The exception was the Furzol clay soil with its shrink-swell characteristics and its extremely low initial perme-ability values. The most plausible explanation is slakingof soil aggregates and blockage of macropores with thedetached finer particles (Black, 1968). This could not beverified in this study since an assessment was made ofaggregation only after the final wetting-drying cycle.

The effect of the conditioners on permeability was var-iable. Except after the first two wetting-drying cycles,Hygromull had no significant effect on permeability ofthe sandy Sheia'h soil. This effect was expected in viewof the high permeability rates for this soil and the factthat Hygromull occurs as discrete flakes within the soilhaving no influence on aggregation (Table 3). However,it significantly improved permeability in both clay soils.This effect is again explained by consideration of thephysical parameters in Table 3. The decrease in bulkdensity was accompanied by a marked increase in po-rosity both factors which contribute to improved perme-ability.

In contrast to Hygromull, the Agrosil materials hadeither no effect or a negative effect on permeability. Thedecrease in permeability with Agrosil LR in the sandysoil and in the Leba'a clay soil was due, presumably, toaggregation of the finer clay particle by its gel-like actionand a consequent reduction in macropore size as previ-ously shown by During and Prun (1974). However, totalporosity or bulk density were not affected. This aggre-gating effect was most pronounced in the sandy soil inwhich permeability was significantly reduced. By com-parison with the untreated control soils, Agrosil S hadno effect on permeability, bulk density, or porosity, andonly had a slight aggregating effect if any. Thus, noneof the beneficial effects postulated for this material incalcareous soils were observed.

This study refutes some of the commercial claims re-

Treatment

ControlHygromullAgrosil LR

ControlHygromullAgrosil LRAgrosil S

ControlHygromullAgrosil S

Bulk density Porosity Aggregates ( < 50 /im)g/cma

1.461.201.41

1.161.031.151.16

1.251.081.27

Sheia'h44.954.746.8

Leba'a56.261.156.656.2

Furzol52.859.252.1

- % — ———— ———

17.817.843.1

77.276.682.179.6

69.168.673.7

garding these relatively new conditioners. Also it may beconcluded from the present study that, of the materialstested, Hygromull has greatest potential for commercialuse since it was comparatively effective with both sandyand clay soils. However, the volume of this low densitymaterial and the possibility of it being blown away bywind may be adverse factors. It may have greatest usein potted ornamental plants and in greenhouses whereproduct value may justify its cost. While Agrosil LR islikely to be of some benefit in sandy soils, the acidic Sform of this material is ineffective and warrants no fur-ther testing.