CSI ROAUST RALIA

CSIRO LAND and WATER

Compaction of soils irrigated withbore water in central AustraliaW.W. Emerson and D.Weissman

Technical Report No. 12/97 (July 1997)

CONTENTS

Section

ABSTRACT

SUMMARY

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2.1

2.2

2.3

2.4

2.5

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3.1

3.2

3.3

3.4

3.5

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4.1

4.2

4.3

4.4

4.5

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INTRODUCTION

SOILS AND METHODS USED

sampling soils

Bulk density, pH, EC, CEC, clay and carbon content

Slaking and dispersion of soils in water

Water relations, penetration resistance of dense layers

Compacting soil

RESULTS

Amount and composition of clay in the soils

Soil pH, EC

Dispersion of soils in water

Dense layers

Soil compaction

DISCUSSION

Air-dry water content and clay content

Causes of dense layers

Surface crusting

Compaction in the field

Compaction and available water

RECOMMENDATIONS

ACKNOWLEDGEMENTS

REFERENCES

APPENDIX

LIST OF FIGURES

Figure

3.1 The relation between the air-dry water content of < 2 mm samplesand the percentage of clay present at four sites.

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3.2 The percentage of the clay present dispersed from 5 mm cubes ofsample no. 7 (AZRI) formed at different water contents and thenimmediately immersed in water.

3.3 A comparison of the particle size distribution at AZRI and AGF.

3.4 The increase in penetrometer resistance of sample no. 58 (AZRI) ondrying.

3.5 The compaction under uniaxial loading of no. 37 (TTF) at two watercontents (g/100 g) (Bars denote compaction in four layers).

3.6 The increase in bulk density with water content of cubes of sampleno. 7 (AZRI) formed at different water contents with and without oneper cent of gypsum added.

3.7 The settlement of short columns of air-dry, 0.25-5.6 mm diameterfractions of two soils on rapid wetting with water, CaCl2 or borewater and application of the overburden load (note both soils used indispersion class 2).

A.1 The percentage of clay present dispersed from air-dry aggregatesimmersed in water, estimated visually and from turbiditymeasurements.

LIST OF TABLES

Table Page

3 . 1 Exchangeable cations, and the percentage of the clay present (TableA. 1) dispersing from air-dry portions immersed in water. 6

3.2 The percentage of the clay present in c. 0.5g intact portions ofsamples dispersing into water after various treatments

A = N NaCl + H2O x 4; B = neutral 0.1 M Na4P2O7 + H2O x 4C =N H C l + H2O + 0 . 5N N a O H + H2O x 2 .

3.3 The average effect of the various treatments on the percentage of theclay present dispersed into water from initially dry portions ofsamples from TGF.

3.4 The water retention of intact portions of dense layers as comparedwith the same material forced through a 2 mm sieve.

A.1 Details of samples taken on 27-31 July, 1987. Sample number,location and depth; after air-drying, consistence, bulk density, watercontent, visual estimate of percentage of clay present dispersingwhen portions immersed in water; pH and EC, 1:5 soil/watersuspensions.

A.2 Particle size analyses of selected samples after adding dispersant andshaking vigorously and the relation between clay concentration(mg/l) in the suspensions and turbidity.

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ABSTRACT

Under irrigation, continuous, dense layers which are hard when dry, develop at shallowdepths in the sandy loam to loam soils. Based on results on samples from profiles atfour sites (AGF, AZRI, TGF, TTF), the soil properties found to be responsible for thisare:-

. a well graded particle size distributionl the presence of weakly bonded clay. no shrinkage on drying

Irrigation with bore water brings the soil into dispersion class 2 aiding compaction andcrusting.

The dry layers slake rapidly when immersed in water. Therefore the larger particles areweakly bonded together. No difference was found also using a ‘chemical hammer’ inthe strengths of the bonds between the clay particles in dense layers and underlying soil.

It is suggested that when soils are sheared under load, thin layers of clay are squeezedbetween the asperities of the non-clay particles. The hardness of the layers when dry isthen due to these clay bridges and not to silica precipitated from bore water.

The rapid rise in penetration resistance of the dense layers on drying reduces plant rootpenetration and decreases water use efficiency.

It has been shown that the bulk density of the soils at a depth of 0.l-0.2m can be made>1.8 Mg/m3 when

. wet - by heavy loading (traffic)

. moist - by shearing (simulating stresses at the depth of cultivation)

and that rapid wetting with fresh water plus the overburden, can collapse the soil to adensity of 1.75 Mg/m3

. when air-dry, if dispersive (class 2)l after working moist

Possible combative measures are :-

. add organic matterl alter the tillage systeml break up the dense layers when soft. add gypsum where the soil has become dispersive

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SUMMARY

Samples were taken down profiles at four sites, AGF, AZRI, TGF and AGF. Each profileincludes a dense layer at 0.1-0.2 m and another at depth. Bulk densities of these layersare mostly in the range 1.8-1.9 Mg/m3 except at AGF. The soils are neutral red earthswith a sandy loam surface soil, except at AZRI where it is a loam. Clay contentsgradually increase with depth. The composition of the clay down a profile is constant atany one site, but the AZRI clay is more reactive than the others. Graphs are givenrelating clay content to air dry water content.

The dispersion of all samples in water was assessed qualitatively using the AustralianStandard method (1980). Some of the samples which had been irrigated with borewater disperse from air-dry when immersed in water (dispersion class 2).

The bonding between the clay particles in some of the compacted layers was comparedwith that in the intervening softer soil using a ‘chemical hammer’ to stress the particles.The stress applied is an osmotic one induced by immersing soil first in water, and thenin various solutions, with an intervening water phase. The progressive amounts of claydiffusing out into the water are measured turbidimetrically. The aim is to extract all theclay in this way.

Once most of the exchangeable cations on the clay are replaced by Na-ions, there is nodifference in the strengths of the bonds between the clay in either shallow or deep denselayers or the intervening loose soil. Half or more of the clay present is weakly bondedand disperses into water either when the ESP of the soil is increased or the soil isworked when moist. It is suggested that about 6% of the total clay present in allsamples may be bonded by silica.

In order to simulate possible ways in which the near surface soil is made dense in thefield, selected soft soil samples were treated in three ways. To simulate traffic,pressures of up to 200 kPa were applied uniaxially for a short time to beds of soil atdifferent water contents. To simulate the combination of stresses that might be appliedfor example at the base of rototilled soil, moist soil was compacted three dimensionallyby making small cubes. To simulate flood wetting in the field, short columns of air dryaggegates were wet rapidly with water, synthetic bore (TTF) or M/100 CaC1 2 followedby the immediate application of a load equal to an overburden of 0.2 m of soil.

Loading the soils when wet can produce high bulk densities. However, both the loadand water content required seem too high for this to be a general explanation. On theother hand the soil cubes attained densities of over 2.0 Mg/m3. Columns of class 2 soilcollapsed to densities of about 1.8 Mg/m3 when wet with water and the overburden loadapplied. The increase in density of these soils was much less with CaCl2 andintermediate with the bore water.

The hardness of the dense layers when air-dry is probably due to the presence of thinclay bridges around the points of contact of the sand. For the bridges to be strong, thepresence of appreciable clay easily dispersed by working the soil wet is probablyessential. It is also necessary that the clay content should not be so great as to causemacroscopic shrinkage on drying.

The maximum densites measured varies with the particle size distribution of the soils.Soils at AGF have a more uniform size distribution than at other sites, accounting for thelower measured bulk densities.

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Some measurements have been made of the increase in penetration resistance of densesoil as it dries out and also of the relationship between water content and matrix suction.It is deduced that dense layers may restrict root growth through the layers both in thewet state due to inadequate aeration and in moist soil due to a high resistance topenetration. The result may be a serious loss in the amount water in the soil profileavailable for plant growth.

Gypsum or other remedial treatment is needed on those soils at T G F, TTF and AGF

which air-dry, disperse when immersed in water. This is to counter the surface crustingand densification caused by rain when it occurs. Gypsum would also prevent thesubsequent dispersion and crusting of all the soils should it happen to rain immediatelyafter cultivating the soil moist. However, the addition of gypsum will not strengthenthe bonds sufficiently between the clay particles to avoid dense layers being formed atthe depth of cultivation of moist soil.

Field measurements are needed of the penetrometer resistance of intact dense layers.Also of changes in soil resistance caused by a given cultivation implement at a definedwater content. This would allow both the least damaging tillage system to be decided,as well the optimum time for ripping.

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1 INTRODUCTION

The growth of crops under irrigation on sandy loams in the Ti Tree area has beenhampered by the development of crusts and shallow dense layers. Miles and McEllister(1977) showed that this occurred in a virgin soil if flooded with bore water aftercultivation when the soil was moist. Crusts could be prevented by the addition ofgypsum and minimum cultivation. Baseden (unpub. 1970) suggested that the hardnessof the dense layers on drying might be due to cementation by silica dissolved in thebore water. All agreed that the increase in exchangeable sodium and pH of the soil dueto precipitation of residual bicarbonate in the bore water is likely to be a contributingfactor to the adverse structural conditions.

McLarty (1981) attempted to reproduce the crusting of Ti Tree soils by irrigating theclay loams of AZRI. Plots 1m2 were used and sodium bicarbonate added to the localbore water to make the residual bicarbonate concentration equal to that of Ti Tree.Crusts were formed but their penetration resistance was much less that that at Ti Tree.The addition of gypsum had little effect. One of the possible reasons suggested for thiswas the lower silicon content of the bore water, 38 ppm.

The present work on samples from dense layers tried to answer the followingquestions:-

. is precipitated silica a cause of their hardness?

. if not, what are the causes?

. what physical restraints to plant growth do they impose ?. what can be done about them?

To investigate possible strengthening of the bonds between the clay particles by silica a‘chemical hammer’ (Clapp and Emerson, 1965) was used. An osmotic repulsive stressis induced between the clay particles by immersing the soil in a dilute salt solution, herewater. Any bonding will reduce the tendency of the clay particles to be forced apart andhence reduce dispersion. Progressively more severe chemical pre-treatments are applieduntil the clay present disperses completely. For example Emerson and Foster (to bepublished) showed that the clay in a shallow duripan from near Coober Pedy dispersescompletely in water if the soil is extracted first with acid and then with alkali. Analysisof the extracts and other tests showed that this clay is held together by precipitatedsilica.

Preliminary qualitative tests quickly showed that bonding due to silica was unlikely tobe an important cause of the compacted layers. Nevertheless detailed quantitativemeasurements were still made of clay dispersion after chemical treatments. Three wayswere also considered in which shallow, dense layers might be achieved by compactiononly. The first tried to simulate the case where the surface soil has dried out for trafficwhile the layer at 0.l-0.2m is still at field capacity. The second simulated the kind ofthree dimensional compaction which might occur at the depth of cultivation whenrototilling moist soil. The last simulated the slumping caused by rapidly wetting air drysoil aggregates with water of low salt content under an overburden of 0.2 m of soil.

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2 SOILS AND METHODS USED

2.1 SAMPLING SOILS

In July, 1987 Mr. R.D.Bond of CSIRO Division of Soils accompanied by staff fromAZRI visited four irrigated sites, the Research Station itself, the Territory Grape Farm(TGF), Ti Tree Farm (TTF) and Arid Gold Farm (AGF). Fifty eight samples were takendown typical profiles including 14 from dense layers (Table A.1). Usually there is onesuch layer at a depth of 0.1-0.2 m and another deeper one at 0.5 m or more.

The samples were allowed to air-dry and their hardness assessed. Sub-samples forwater content were all taken on the same day.

2.2 BULK DENSITY, pH, EC, CEC, CLAY AND C CONTENT

The bulk density of compacted layers was measured by coating lumps with Saran resinand measuring the volume of water displaced. The pH and EC of 1:5 soil:watersuspensions were measured. Particle size analyses were made on representativesamples from each site. The soils were dispersed by adding calgon and NaOH and thenvigorous shaking for 10 minutes using a Spex mixer (Emerson, 1971; Norrish andTiller, 1976). Clay and silt in suspension were measured using a plummet balance. Onsome samples, clay was decanted off and the relation between clay concentration andturbidity measured using a Hach ratio turbidimeter. The particle size distributions of thesand fractions of a few samples were determined by dry sieving. The clay fraction oftwo samples was analysed by XRD.

Exchangeable cations and CEC were measured on a few samples using neutral NH4Cl.Samples of two shallow, dense layers were analysed for total C using a Leco furnace.

2.3 SLAKING, DISPERSION OF SOILS IN WATER

2 . 3 . 1 No chemical treatment

0.5g portions from all air-dry samples were immersed in 200 ml of distilled water andthe rate and coarseness of any slaking observed. Visual estimates were made of thepercentage of the clay present dispersing from the slaked fragments after 2 hr, 24 hr andlonger if further dispersion was still occurring. On five samples which showedappreciable dispersion in water, the dispersed soil was gently swirled into suspensionand an aliquot taken for a photomoter reading of the < 2 µm fraction. The suspensionwas then agitated using a Dawe soniprobe (100w) with a micro-tip for 1 minute. Thephotometer reading corresponding to <2 µm particles was then taken as before (Bakkeret al., 1973).

A few samples which showed little or no dispersion air-dry were wetted to about ‘fieldcapacity’, cubes formed (Loveday and Pyle, 1973) and their dispersion in water againchecked (Australian Standard, 1980). On one sample from AZRI (no.7) a completewater content/dispersion curve was made. In the last case, the turbidity due to any siltparticles present in the solution removed was measured as well as taking the usualreading for clay.

2 . 3 . 2 Chemically treated

Weighed air-dry portions (c. 0.5g) of hard layers and of the softer soil in between wereimmersed in 30 ml of N NaCl or 0.1 M Na4P2O7 adjusted to pH 7 contained in plasticvials. After 48 hr, these solutions were removed and replaced with water. 48 hr laterthe water was decanted off and any clay present was measured using the turbidimeter.It was usually necessary to dilute the suspension before allowing it to stand for the timeofsettlement corresponding to 2 µm The water was replaced 3 or 4 times until theturbidity due to clay was about 50 NTU, corresponding to 1-2% of the total clay presentin 0.5g of soil.

This process was followed at 24 hr intervals by adding and decanting N HCl, H2O,0.5N NaOH and H2O twice. All transfers of solution were carried out with great care tominimise any release of clay from the soil in the vial due to mechanical disturbance.The final step in order to get all the clay up was to sonify the suspension for one minuteusing the Dawe and a micro-tip.

2.4 WATER RELATIONS, PENETRATION RESISTANCE OF DENSE LAYERS

Three dense layers were chosen to cover the range of hardness found. Lumps of thesewith a volume of about 10 ml were wetted with water at 5 kPa suction. The suction atthe base of the lumps then reduced in stages to zero before draining the material at 1 kaand then 10 kPa. The water retention of the lumps was compared with that of air-drysamples of the soil passed through a 2 mm sieve and then wetted and drained in thesame way.

A large air-dry lump from sample no. 58 was shaped with a hacksaw, then weighed andits dimensions measured. It was first wetted with water at 5 kPa suction and then thesuction reduced to 1 kPa. The clod was weighed periodically to determine its wateruptake. When equilibrium had been reached, the dimensions of the clod were measuredagain. Duplicate measurements were also made using of the resistance to penetration ofthe clod to a depth of 5 mm. A 3 mm diameter cone with a semi-vertical angle of 30degrees was used coupled to an Amatek spring gauge. The clod was next drained atsuctions of 10 kPa and 40 kPa and the measurements repeated.

Duplicate sub-samples of 58 were poured into water in rings on a suction plate, the soilpuddled and then drained at 1 kPa suction. The same process of measuring the increasein penetration resistance with decrease in water content with applied suction was thenfollowed.

2.5 COMPACTING SOIL

Two samples of nos. 37 and 57 were selected which had air-dry water contents almostidentical to hard layers whose bulk density had been measured (nos 36 and 58, TableA.l). The samples also happened to be in dispersion class 2. < 2mm sub-sampleswere sprayed to a given water content and a weighed amount transferred to a 50 mmdiameter cylinder mounted on a hydraulic jack to give a depth of soil of about 30 mm.An increasing load was then applied in steps for a short time to the soil bed measuringthe progressive decrease in thickness (Emerson, 1978). The increase in densityobtained by compacting in four 7 mm layers instead was checked

A sample of a soil which only dispersed appreciably water after remoulding wet, no.7,was also sprayed with water to a given water content. As much wetted soil as possiblewas then forced into a cubical brass mould of side 5 mm using a small spatula, As eachcube was made it was weighed immediately. One was then immersed in water formeasurement of the degree of dispersion of the clay, the others oven dried. The

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dimensions of the dried aggregates were measured using a micrometer. Gypsum (oneper cent by weight) was added to another sample of the soil and the process repeated.

Sub-samples of nos. 37 and 57 were sieved between 0.25 mm and 5 mm diametersieves and then poured into 75 mm diameter brass cylinders to form columns 40 mmhigh. The soil was covered with a coarse filter paper. Water was then dripped onto thefilter paper at a rate of wetting equal to 120mm/hr. As well as water, M/100 CaCl2 andsynthetic TTF water were used. The last was made up to the analysis given by Milesand McEllister (1977), see also Table A.1. It was necessary to increase the partialpressure of CO2 above the atmospheric concentration before all the salts added woulddissolve.

On a soil which disperses when worked moist, serious slumping is likely to occur if thesoil is saturated in the field. This could occur if rain followed rotary cultivation. A <2mm fraction of sample no.7 was spray wetted till not quite sticky, then transferred to abrass cylinder. It was then wet rapidly as before with water and the overburden loadapplied.

3 RESULTS

3 . 1 AMOUNT AND COMPOSITION OF CLAY IN THE SOILS

There is a notable difference in the soil texture between AZRI and the other sites, areflection of the greater content of silt sized particles (Table A.2). McLarty (1981)pointed this out when comparing crusting at AZRI and TTF. In most profiles there is asteady increase with depth in the water content of the air-dry soil (Table A. 1). As theorganic matter content of the subsurface layers is small, this indicates a correspondingincrease in clay content. Plotting clay contents of the samples listed in Table A.2against water contents gives a series of straight lines (Fig. 3.1). The linear plotsindicate that the clay composition is approximately constant at each site.

Figure 3.1 The relation between the air-dry water content of <2 mm sample andthe percentage of clay present at four sites.

The ratio of clay concentration in suspension to the turbidity due to clay is also fairlyconstant at each site (last col. Table A.2). This justifies the approximation made later(section 3.3.3) of taking the ratio for a sample equal to that of the nearest known valuein the profile.

Water retention of air-dry soil is due mainly to hydration of the exchangeable cations.The total organic carbon contents of samples 7 and 58 are only 0.14% and 0.21% byweight. Therefore neglecting any contribution due to organic matter is neglected, thesmaller the slope of the water retention/clay content line (Fig.3.1) the greater the CEC.For example on this basis the clay at AZRI should have about twice the CEC of the clayat TGF. Chemical analysis confirms this (see Table 3.1). Examination by XRD of clay

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extracted from sample nos. 6 and 33, indicates that this difference arises from thegreater proportion of illite present as compared with kaolin.

3.2 SOIL pH, EC

The concentration of soluble salts in the crust under the drippers at AZRI was about0.6% by weight as estimated from its EC following Piper (1947). The 0.65-0.75 mlayer of this profile was the only sample found to contain calcium carbonate (TableA.1). At TGF in the more sandy soil, there has been no accumulation of salt due toirrigation. However, in the vineyard itself, there has been an increase in pH near adripper (cf.samples 57 and 58 and sample 16). At TTF there is a markedly acid layer atdepth (samples 29, 30). This shows that little water penetrated between the rows oftrees. At AGF, there seems to be a large increase in both salt content and pH under theirrigated dates as compared with the virgin site (cf. samples 44 and 49).

3.3 DISPERSION OF SOILS IN WATER

3 . 3 . 1 Air-dry soil, effect of bore water, ESP

Most of the soils show zero dispersion or only a trace. The exceptions are where soilhas been irrigated. At TGF and TTF, there is appreciable dispersion under the drippers(samples 57,37) and by previous irrigation under Sudax (sample 41). There was beena similar increase under the date palms at AGF (sample 44).

In Fig. A. 1, the visual values for the five samples showing appreciable dispersion areplotted against quantitative measurements of the percentages of the clay present actuallydispersed. The results indicate that the eyeball estimates are reasonably consistent. Theeye in fact is a very sensitive detector of dispersed clay. The eyeball estimates werethen converted using the calibration of Fig. A.1 to give the percentages of claydispersed listed in Table A. 1. Note, these percentages do not refer to the total claypresent in a soil. Dispersant and additional sonification are needed to achieve this value.Although dispersion is often only barely detectable, it is useful to note, because itusually means that the soil when worked wet will disperse severely.

Table 3.1 The composition of the exchangeable cations and CEC using neutralNH4Cl, the calculated ESP and percentage of the clay present (TableA.1) dispersed from air-dry aggregates immersed in water.

Sample Site Exchangeable cations CEC ESP % <2 µmno. fraction

Ca K dispersed

+ soil moist when sampled

Analysis for exchangeable cations on selected samples shows as expected that someexchange sites have to be occupied by Na-ions before appreciable dispersion occurs inwater (Table 3.1). However where this occurs, there is considerable difference in the

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amount of dispersion at a given ESP. Of particular interest is the much greaterdispersion and double the ESP of sample 57 as compared with sample 58. The sampleswere taken at the same depth, but 58 was outside the zone wetted by irrigation waterwhen the samples were taken. The more frequent wetting of 57 is also consistent with aslight increase in its pH as compared with 58 (Table A.1). A proportionately largerincrease in dispersion and pH was found between samples 44 and 45 (Table 3.1) fromthe same depth. Sample 44 was wet while 45 was beyond the limit of irrigation.

3 . 3 . 2 Remoulded wet soil

Quantitative measurements were made on one of the samples showing a trace ofdispersion, no. 7. The cubes made at different water contents were used, with theresults shown in Fig. 3.2 . The percentage of the total clay present dispersed increasesrapidly as the soil becomes wetter. The maximum value happens to be the same as thatdispersed into water by the osmotic stress generated after first immersing the soil inNaCl ( see next section).

Water contentFigure 3.2 The percentage of the clay present dispersed from 5 mm cubes of

sample no. 7 (AZRI) formed at different water contents and thenimmediately immersed in water.

The reduction in dispersion with time of standing was checked visually with cubes at awater content of 9.2% by wt. After 4 days, the amount of dispersed clay was reducedby about half. The increase in strength is much less than that observed previously for aheavy clay subsoil (Emerson 1978).

3 . 3 . 3 Chemically treated

The amount of clay present in each weighed portion of the samples listed in Table A.2was known from its particle size analysis. The clay contents of other samples werededuced by taking the known value for the nearest sample and multiplying by the ratioof the an-dry water contents of the two samples (see section 3.1). The total turbidity tobe expected due to suspended clay (volume of each extract x NTU) was then calculatedfrom the conversion factor for the sample itself or that of the nearest known sample asgiven in Table A.2.

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The mean value of the ratio of the total turbidity of the extracts to that calculated testedwas 0.985 ± 0.017 for the 36 samples. This is very satisfactory given the small weightof soil taken each time. However, the percentage of clay released from each portion by agiven chemical treatment was calculated on the basis of the actual total turbiditymeasured for that portion.

Table 3.2 The percentage of the clay present in c.0.5 g intact portions ofsamples dispersing into water after various chemical treatments.A = N NaCl + H2O x 4; B = neutral 0.1 M Na4P2O7 + H2O x 4;C = M HCl + H2O + 0.5 N NaOH + H2O x 2.

Site Sampleno.

Depth(m)

* H = hard, MH = moderately hard, W = weakly aggregated + soil moist when sampled

Consider the results for samples 57 and 58 first (Table 3.2). The samples came fromthe same depth, one hard and dry, one moist and soft. Once most of the exchangeablecations on the two samples (cf. Table 3.1) have been replaced with Na-ions usingNaCl, both portions dispersed to the same extent. Duplicate samples of 58 also showedgood reproducibilty. The only sample that gave answers at variance with those fromothers at the same site, was sample no.6. This sample did contain many ant holes.

The largest number of samples tested from one site came from TGF. The results for thissite have been averaged in Table 3.3, using the same letters to denote treatments as inTable 3.2. The results show clearly that the clay particles in both shallow and deep hardlayers and soil between are bonded together in the same way. About half the claypresent is dispersed after the NaCl treatment. Kaolin is the dominant clay mineralpresent. This clay may not disperse in water either due to the presence of positivelycharged edges or hydroxy aluminium ions on the exchange sites (Schofield andSamson, 1954). The sodium pyrophosphate treatment neutralises or reverses suchcharges and removes the aluminium ions (see discussion in Emerson, 1983). Thisreagent can also reverse the charges on any positively charged oxides present (Hingstonet al.1973). The result is that about a further 25% of the clay present is released inwater (cf. cols. A and B, Table 3.3). Alkali by itself should also remove any aluminiunions on exchange sites and reverse any positive charges. With an acid pre-treatment is

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has been found that clay bonded by silica can be released as well. Therefore not onlyshould treatments A + C and B + C release the same of clay as found (Table 3.3), butthe difference between B + C and C should indicate the amount of clay bonded bysilica. Norrish and Tiller (1976) have suggested that an acid alkali treatment may alsodissolve authigenic clay bonds, In any case, the difference, about 6% of the claypresent gives an upper limit to the proportion of clay which might be bonded by silica inall the soils, namely 6%.

Table 3.3 The average effect of various treatments on the percentage of the claypresent dispersed into water from initially air-dry portions of TGFsamples.

Initial Averageconsistence depth (m)

Clay removed (%) by treatmentA A + C B B + C

The NaCl treatment tended to release a greater percentage of the clay present fromsamples taken at AZRI as compared with other sites (Table 3.2). This would be expectedfrom the smaller proportion of kaolin present.

3.4 DENSE LAYERS

3.4.1 Bulk density, particle size distribution

Samples of the dense layers are hard when air-dry but soften on wetting. When air-dry,the hardest layers (H) have bulk densities in the range 1.79-1.89 Mg/m3, expressed onthe weight of oven dry soil present while those classed as moderately hard (MH), in therange 1.65-1.75 Mg/m3 (Tables A.l, 3.2, 3.3). When unconfined portions areimmersed in water, water penetrates rapidly as shown by their complete slaking within afew seconds.

The values of the bulk density of the samples from AGF are notably smaller than fromthe other sites (Table A-1). The main reason for this is the greater uniformity of theparticles present. For example, as shown in Fig. 3.3, 70% of the particles in sampleno.49 from AGF have diameters between 50 and 200 µm. In constrast, sample no. 6from AZRI) is well graded, with only 25% of it between 50 and 200 µm diameter. Thecorresponding bulk densities are 1.62 and 1.82 Mg/m3 respectively. Note that themedian size of the particles is about the same.

3.4.2 Water retention

The bulk density of the air-dry portion of each dense layer used is given in Table 3.4,col. 1. Its maximum possible water content (col. 2) is then calculated from this bulkdensity, the measured density of the solids of 2.65 Mg/m3 and assuming there is noswelling. In practice, a maximum degree of saturation of the pore space achievable bywetting a dry soil is about 95% The corrected maximum water content is then close tothe measured water retention at 1 kPa suction ( col. 4). The same material broken downto pass a 2 mm sieve retains much more water at this suction, indicating that appreciableinteraggregate pores are being filled.

Considering now the water retention in equilibrium with a suction of 10 kPa, the watercontent of the densest layer is almost the same as that of material passed through a 2 mm

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sieve. In other words little water is retained between the particles in the sieved material.On the other hand the < 2mm sample of the least dense sample, no.55 , still retainsmuch more water. This reflects the greater comminution of the weaker material whenpassed through the 2 mm sieve.

Particle size (mm)Figure 3.3 A comparison of the particle size distribution at AZRI and AGF.

Table 3.4 The water retention of intact portions of dense layers as comparedwith the same material forced through a 2 mm diameter sieve.

Sampleno.

IntactBulk

density(Mg/m3)

Calc. max.water

uptake

Water retained (g/100g)at a suction of

1kPa 10 kPaIntact <2mm Intact <2mm

3 . 4 . 3 Penetrometer resistance

The bulk density of the large lump used from sample no. 58 was 1.82 Mg/m3,compared with the bulk density of the same material puddled of 1.75 Mg/m 3. Thereforefor the same water content, the puddled material should be weaker. The results supportthis when the samples are wet (Fig. 3.4). However the strength of the puddled materialappeared to increase more rapidly. This is likely to be because slight cracking occurredin the unconfined lump tested at a suction of 40 kPa. When air-dry, the penetrometerresistance of each material was beyond the limit of measurement i.e. > 60 bar.

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Figure 3.4 The increase in penetrometer resistance of sample no. 58 (AZRI) ondrying

3.5 SOIL COMPACTION

3. 5. 1 Loading uniaxially

The two samples nos. 37 and 57 gave very similar results. Only one curve is drawn infor each water content (Fig.3.5). Also shown is the relatively small effect ofcompacting in successive very thin layers. Even with the soil at a water contentappreciably greater than its value at 10 kPa suction (Table 3.4) a load of 125 kPa wasrequired before the field bulk density of 1.82 Mg/m3 was achieved. It was necesssary toapply the same pressure to sample no. 7 and at a water content of 13.9% by wt., beforethe bulk density of 1.82 Mg/m3 was reached. As can be seen from Fig. 3.5, the loadrequired at a water content close to ‘field capacity’ seems too high to be the mostimportant cause of the dense layers formed in the field. This applies to sample 7 also.

Load (kPa)

Figure 3.5 The compaction under uniaxial loading of no. 37 (TTF) at two watercontents (g/100 g) (Bars denote additional compaction in four layers).

12

3 . 5 . 2 Moulding cubes

Results for soil from sample no. 7, moistened and then compacted in a mould eitherwith or without added gypsum are plotted in Fig.3.6. There is some scatter of points,but the curves are very satisfactory considering the smallness of the cubes and that thepressure applied while filling the mould was not controlled. High densities are achievedwith the soil only just moist enough to work uniformly. For example, the field densityof, 1.82 Mg/m3 is reached at a water content of 4.3% by weight. The addition ofgypsum did not affect the density. Nevertheless, the marked similarity in the shape ofthe density/water content curve and the dispersion/water content curve (Fig. 3.2) isstriking and will be discussed later.

Of particular interest is the finding that the average volume of the dry cubes was 0.25%greater than that of the mould. In other words the cubes expand when pushed out of themould removing the lateral stresses exerted by the walls. The expansion was detectedhere because the soils studied show negligible shrinkage.

I I I I I I I

3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Water content (g/l100g)

Figure 3.6 The increase in bulk density with water content of cubes of sampleno. 7 (AZRI) formed at different water contents with and without oneper cent of gypsum added.

3 . 5 . 3 Rapid wetting

Results for samples 37 and 57 are plotted in Fig.3.7. The results are similar and onlysingle curves are drawn. Considering the results of wetting with distilled water first, thesamples slumped to a bulk density of about 1.55 Mg/m3 on wetting. Some dispersedclay was noted in the first drop suspended from the base of the column before wateraddition was stopped. Further compaction occurred on applying the overburden load,the bulk density increasing to 1.75 Mg/m3. Compaction was much reduced whereCaCl2 was applied. Using synthetic TGF bore water gave some additional compaction ascompared with using CaCl2 (Fig.3.5). These results are in line withe dispersion of the

13

aggregates. Air-dry portions of sample no. 37 for example immersed in bore water stillgave a visual dispersion value of 10% compared with 25% dispersed in water (TableA. 1) and zero for CaCl2.

A similar slumping was observed with the sample of no. 7 which had been spray wettedto a water content of 12.5% by wt. When wet rapidly with water, the soil columncollapsed to a density of 1.76 Mg/m3. Adding the overburden load had little additionaleffect.

Dripper0.2mm/min 0.3g/mm 2

Figure 3.7 Settlement of air-dry 0.25-5.6mm aggregates of sodic sandy loams on wetting

and subsequent loading.

14

4 DISCUSSION

4.1 AIR-DRY WATER CONTENT AND CLAY CONTENT

The linear relations found here between the air dry water content of soil samples andclay content and composition (Fig.3.1) could be useful in characterising the soil layersfrom different sites. The concept of an air-dry water content is necessarily vague.However, if samples are kept at an approximately constant temperature and fluctuationsin humidity are not too rapid, then relative values for samples taken on the same day areuseful, cheap measurements. If the water content of ‘standard samples were taken atthe same time, then the method could be made absolute.

4.2 CAUSES OF DENSE, HARD LAYERS

4 . 2 . 1 Silica

The hardness of the layers when dry appears to be due to their high density. Noevidence was found for additional bonding of the clay particles due to silica from thebore water. The bonding is the same in samples from Ti Tree where bore water withhigh silica content is used as from AZRI where the bore water used contains less thanhalf the amount. Also the bonding is similar in both shallow and deep dense layers. Atdepth, wetting with bore water will be infrequent.

When looking for additional bonds between soil particles as for example by silica(Brown and Mahler, 1988) it seems better to carry out physical tests first to check if theclay is restrained in any unusual way. If so, various extractants can be tried to make theclay disperse to the same extent as the pure clay minerals present. The successfulextractants can then be analysed.

4 . 2 . 2 Organic matter, particle size distribution

An obvious requirement for uniform high density is the absence of organic matterparticularly coarse organic matter, old root channels or with the present soils, channelsmade by ants. Conversely, if the overall organic matter content of the soils is increasedby growing a grass for example, then the consolidation due to a given load and itspenetration resistance can be reduced. Sands et al. (1979) have measured these effectsfor sands. Bodman and Constantin (1965) showed that the addition of 0.4% VAMA byweight significantly reduced the maximum bulk density of fine sand-silty clay mixturesachieved by kneading compaction.

The usual other requirement quoted for high bulk densities in sandy soils is a wellgraded sand matrix. An example of the latter is the work of Mullins andPanayiotopoulos (1984) on the density of mixtures of a paste of kaolinite with twosands, one well graded, one of uniform size. With 20% by wt. of kaolin added, thetwo mixtures gave densities of 2.1 Mg/m3 and 1.7 Mg/m3 respectively afterconsolidation under a pressure of 1.5 MPa. However, the importance of the particlesize distribution of the whole soil has not been stressed. This is mainly because onlythe density of binary mixtures of particles with two very different average particle sizescan be predicted easily.

15

4 . 2 . 2 Pressure plus shear

It has been suggested already that most of the layers of high densities found at shallowdepths result from compaction due to tillage implements rather than traffic. Bodmanand Rubin (1948) showed using a ring shear apparatus, that a combination of a shearforce and direct pressure on a soil bed can produce much higher densities than pressurealone. They reported for a sandy loam at a water content of 8.3 % by wt., shearingunder a pressure of 1.5 MPa as compared with applying a pressure only of 1.5 MPa,increased bulk density from 1.61 to 1.93 Mg/m3. The result for direct compression iscomparable with that in Fig. 3.5, taking into account the smaller water content. Watercontent density curves similar to those for the cubes of sample no. 7 (Fig.3.6) havebeen reported by Marshall (1959) and Henderson et al. (1988) for sandy loam subsoils,with a maximum density of 2.0 to 2.05 Mg/m3 at a water content of 9.0 to 9.5% by wt.Compaction in these cases was carried out using the standard Proctor method, which isa form of kneading compaction. The results with sample no.7 will now be used tosuggest a detailed mechanism by which dense, continuous layers are formed.

4 . 2 . 3 Bonding of clay and amount of clay

For most soil clays with divalent cations on the exchange sites, there is an energybarrier to the expansion of air-dry clay when immersed in water. In order for the clay todisperse, the clay particles have to be moved a certain distance apart by manipulation ofthe wet clay, which means a certain water content is required (Emerson, 1967). For asoil, the water content is reduced in proportion to its clay content (Emerson, ibid).Therefore sandy soils can be very prone to dispersion in this way. There will also be arange of energy barriers between the clay particles. Then as the water content isincreased progressively more will become dispersive, as shown for sample no. 7 (Fig.3.2). If clay particles disperse on immersion in water, then it also means that in thewet, worked clay, prior to immersion, the main force holding the clay particles togetheris the surface tension force. It is suggested that when such moist soil is sheared in themould to form cubes, the stresses exerted are sufficiently strong to bring the asperitiesof the sand grains into contact. At the same time, the clay around the points of contactis also brought into a potentially dispersive state. With subsequent drying, the clayparticles are gradually brought closer together by the surface tension force in the annularrings of water around the points of contact. With the intrinsic attractive forces betweenthe clay particles being weak, the clay particles will tend. to remain in a semi parallelarrangement. Eventually on air-drying, the clay forms thin bridges around the points ofcontact.

For the development of continuous dense layers in the field it is necessary to havesufficient clay present to form the bridges yet not sufficient as to allow shrinkage cracksto form on drying. The present soils show negligible shrinkage is negligible. A limitcould be set on the amount of clay between in the contact zone between the largerparticles if the clay were extracted and the swelling of orientated flakes measured(Emerson, 1964).

4 . 2 . 4 Clay bridges

For a given amount of clay between the sand grains, the strength of the postulatedbridges would be expected to increase with the bulk density of the clay in the thin layersforming the bridges. In practice, the finer the particle size of the clay, the greater thebulk density. On flakes of pure Ca-clays, 0.5-1 mm thick for example, the bulk densityof a fine grained illite was measured at 1.82 Mg/m3 as compared with 1.40 Mg/m3 for acoarse kaolinite (Emerson,1964). This is mainly ascribed to improved face to faceorientation of the dry clay. In kaolinite edge to face flocculation may be favoured(Schofield and Samson, 1954). If the repulsive forces between the clay particles areincreased as for example by introducing Na-ions onto the exchange sites (samples no.37 and 57) then orientation should be further improved. Conversely, the addition of

16

gypsum might reduce the strength of clay bridges in this way, even if as shown here, itdoes not result in a decrease in overall density. On the basis of this discussion it wouldbe predicted that drying sand clay mixtures with a flocculated kaolinite would produce aweaker material than mixing the same amount of illite with the sand.

4.3 SURFACE CRUSTING

Attention has been mainly concentrated so far as to how the shallow layers might havebecome compacted. The surface soil itself when dry should slake less than theimmediate subsoil because of its higher organic matter content. However the sameorganic matter can also make it easier to disperse the soil in water if first worked wet,e.g. as found with a red earth at Katherine (Emerson and Smith, 1970). It is likelytherefore that cultivated, moist surface soil will slump and crust badly if rapidly wet byfresh water, in a similar way as shown here for sample no.7 when wet.

In the present work, only wetting with water of near zero kinetic energy has beenconsidered. In practice, the impact of drops from rain or from sprays using bore waterwill cause additional stresses to be applied. The further breakup will cause more severecrusting on drying. The addition of gypsum may reduce the strength of the crust in asimilar way to that discussed for clay bridges.

4.4 COMPACTION IN THE FIELD

4 . 3 . 1 Shallow layers

These red earths will be particularly susceptible to compaction by traffic. In the hotclimate, a surface mulch will rapidly develop making the soil easily trafficable eventhough the water content of the immediate subsoil remains high. Direct heavy loadingby traffic (Fig. 3.5) could achieve the bulk densities measured on some samples.

However the more general cause of the high bulk densities is likely to be the combinedvertical and horizontal shear stresses produced during cultivation. Working a moist soilwith an implement such as a rototiller would leave the top 0.15m in a friable condition.However, at the depth of working, the underlying soil would be compacted by stresseswith strong vertical as well as horizonal components. The situation may be comparableto that when filling the mould to form a cube. The actual water content required toachieve a density of over 1.8 Mg/m3 will depend on the magnitude of the appliedstresses. The water content may be reduced if the soil is dispersive as nos. 37 or 57.

As demonstrated, slumping of the immediate subsurface layers can also occur as a resultof rapid wetting due to heavy rain. For self dispersive samples, such as 37 and 57, thewetting process itself is sufficient. For samples such as no. 7, recent prior cultivation isrequired. There is also an indication of some additional slumping of samples 37 and 57when wet with bore water containing residual carbonate as opposed to a CaCl2 solution.This is supported by the fact that the visual dispersion of an air-dry portion of no. 37was only reduced from 25% in water to 10% in synthetic bore water as compared withzero for CaCl2.

4 . 4 . 2 Deep layers

A possible explanation might be that the weight of the wet overburden produced by theoccasional storm would be sufficient to cause the particular layer to collapse. Thisseems very unlikely in that a more gradual increase in density with depth would beexpected. The abrupt changes in pH values in particular down the profiles at Ti TreeFarm (Table A.1) indicate that the landscape is depositional. It seems most likely that

17

the layers represent old land surfaces which have been compacted naturally by rain andanimals.

4.5 COMPACTION AND AVAILABLE WATER

There are two physical conditions which can limit the ability of plant to extract waterfrom sandy soils. The soil can become so wet that aeration is inadequate for plant rootsto grow. As the soil dries out, it can become too strong for plant roots to penetrate.This may occur well before the conventional ‘wilting point’ is reached. The twolimiting conditions are usually expressed in the form that 10% of the total soil volumeshould be air-filled (Marshall 1959) and that the penetrometer resistance of the soilshould not be more than 25 bar (Taylor et al., 1966). Taking the value for the highestbulk density measured here of 1.89 Mg/m3 (sample 33), the soil has to dry to a watercontent of 10% by weight before aeration is adequate. This compares with a watercontent of 9% by weight at 10 kPa suction draining (Table 3.3).

It is only possible to get an approximate value for the lower limit of available waterhere. As discussed already it is believed that the values measured for the unconfinedclod are too small at the dry end. The data for the uniformly puddled sample with adensity of 1.75 Mg/m3 may be used to calculate a value for the same material compactedto a density of 1.89 Mg/m3 using a regression formula of Henderson et al. (1988).This is based on laboratory results for the penetration resistance of a 12.9 mm cone intocompacted samples of sandplain subsoils. Using a value of a=8, the increase in densitywould be expected to double penetration resistance at the same water content. Apenetration resistance of 25 bar would then be expected at about 9.0% by weight (Fig.3.4). The water content for this sample at the conventional wilting point (15 bar) is3.5% by weight. The calculations while approximate, do show that root growth couldbe restricted to a very narrow range of water content. This would affect not only theamount of available water in the layer itself, but also the rate at which plants couldextract water from the underlying soil. There is a need for direct measurements ofpenetrometer resistance on intact layers.

18

5 RECOMMENDATIONS

1 Soi ls

To understand the physical properties of irrigated soils in the area it is useful torecognise three soil types:-

A Z RI -Others -

heavier texture, dispersion class 3 (Australian Standard 1980)dispersion class 2 (where bore water containing residual carbonate has beenused

- dispersion class 3

Previous field experience should be re-evaluated in the light of thesedivisions plus the three mechanisms of compaction discussed in the paper.

2 Compaction, field

Existing -

Mechanism -

Prevention -

Penetrometer resistance/water content curves need to be establishedfor intact layers. Bulk densities should also be measured. The effectof compaction on available water can then be evaluatedA cone penetrometer with a diameter of 6-10mm would be ideal forthe work.

The variation in penetrometer resistance with depth and water contentneeds to be measured after tillage with various implements particularlya rototiller.

Experiments need to be carried out

adding organic manure such as grape marcgrowing a structure forming crop as part of a rotationminimising tillage practices such as rototilling which impart a largecomponent of vertical stress to the subsoil

Amelioration - Rip, and for dispersion class 2 soils, add gypsum

3 Addition of Gypsum

Further laboratory work is needed to evaluate its effectiveness by determining:-

• is soil cohesion reduced significantly even though overall densification is notprevented?

• is the surface crusting of dispersion class 2 soil significantly reduced after exposureto sprays delivering bore water containing residual carbonate ?

19

6 ACKNOWLEDGEMENTS

This work was made possible by the enthusiasm of Mr. F.V. McEllister, Department ofPrimary Production, who also arranged the initial funding. He and his staff thencombined with Mr.R.D.Bond of the Division of Soils to take appropriate soil samples.Initial discussion of the problems involved with Mr.Bond were much appreciated.

The CEC and sand analyses reported were carried out by Mr. T.A. Beech and the XRDanalyses by Mr. G.G.Riley, both members of the Division of Soils, Adelaide.

Approval for publication has been given by the current Director of AZRI, Dr D. Tabrett.

20

7 REFERENCES

Australian Standard (1980). Determination of Emerson class number of a soil.1289.C8.1

Bakker, A.C., Emerson, W.W. and Oades, J.M. (1973). The comparative effects ofexchangeable calcium, magnesium and sodium on some physical properties ofred brown earths. Aust. J. Soil Res. 11, 143- 150.

Baseden, S. (1970). Soil crusting under irrigation in the Alice Springs Region.Unpub. Rep. Dept. Primary Industry, N.T.

Bodman, G.B. and Constantin, G.K. (1965). Influence of particle size distribution insoil compaction. Hilgardia, 36, 567-591.

Bodman, G.B. and Rubin, J. (1948). Soil puddling. Soil Sci. Soc. Am. Proc. 13,27-36.

Brown, T.H. and Mahler, R.L. (1988). Relationships between soluble silica and plowpans in Palouse silt loams 145, 359-364.

Clapp, C.E. and Emerson, W.W. (1965). The effect of periodate oxidation on thestrength of soil crumbs. Soil Sci. Soc. Am. Proc. 29, 127-134.

Emerson, W.W. (1964). The slaking of soil crumbs as influenced by clay mineralcomposition. Aust. J. Soil Res. 2, 211-217.

Emerson, W.W. (1967). A classification of soil aggregates based on their coherence inwater. Aust. J. Soil Res. 5, 47-57

Emerson, W.W. (1971). Determination of the content of clay sized particles in soils. J.Soil Sci. 22, 50-59

Emerson, W.W. (1978).Aggregate classification and hydraulic conductivity ofcompacted soils. Ch. 30 in ‘Modification of Soil Structure’ pp.239-262. JohnWiley & Son, London.

Emerson, W.W. (1983). Inter-particle bonding. Ch.31, In ‘Soils: an AustralianViewpoint’ . pp.477-498. CSIRO, Melbourne/Academic Press, London.

Emerson, W.W. and Smith, B.H. (1970). Magnesium, organic matter and soilstructure. Nature 228,453-454.

Henderson, C., Levett, A., and Lisle, D. (1988). The effects of soil water content anadbulk density on the compactibility and soil penetration resistance on someWestern Australian sandy soils. Aust. J. Soil Res. 26,391-400.

Hingston, F.J., Posner, A.M. and Quirk, J.P. (1972). Anion adsorption by goethiteand gibbsite. J. Soil Sci. 23, 177-192.

Loveday, J. and Pyle, J. (1973). The Emerson dispersion test and its relationship tohydraulic conductivity. Aust. J. Soil Res. 12, 87-96.

Marshall, T.J. (1959). Relations between Water and Soil. Tech. Comm. no. 50.C’Wealth Agric. Bur. England.

21

McLarty, F. (1981). Surface crusting and soil amelioration when using marginalquality bore water. Thesis, B.Agr.Sci., LaTrobe Univ.

Miles, M.R. and McEllister, F.V. (1977). Tea Tree ground water basin. Tech. Bull.no.21. Dept. N.T., Animal and Agric.Branch.

Mullins, C.E. and Panayiotopoulos, K.P. (1984). Compaction and shrinkage of sand-kaolin mixtures. Soil and Tillage Res. 4,191-198.

Norrish, K. and Tiller, K.G. (1976). Subplasticity in Australian soils. V. Factorsinvolved and techniques of dispersion. Aust. J. Soil Res. 14, 273-290.

Piper, C.S. (1947). Soil land Plant Analysis. Univ. Adelaide.

Sands, R, Greacen, E.L. and Gerard, C.J. (1979). Compaction of sandy soils inradiata pine forests. I. A penetrometer study. Aust. J. Soil Res. 17, 101-114.

Schofield, R.K. and Samson, H.R. (1954). Flocculation of kaolinite due to theattraction of oppositely charged crystal faces. Disc. Faraday Soc. 18, 135-145.

Skempton, A.W. (1986). Standard penetration test procedures and the effects in sandsof overburden presure, relative density, particle size and ageing andoverconsolidation. Geotechnique, 36, 425-447.

Taylor, H.M., Roberson, G.M. and Parker, J.J. (1966). Soil strength-root penetrationrelations for medium- to coarse-textured materials. Soil Sci. 102, 18-22.

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APPENDIX

Table A-l. Details of samples taken 27- 31 July 1987; Site, bore water analysis,sample number and depth (m); after air drying, consistence, bulkdensity (Mg/m3), water content (g/100g), percentage of clay presentin soil dispersed when portions immersed in water (visual estimate);pH, EC of a 1:5 soil/water suspension (* indicates completeflocculation of the suspension; + soil moist when sampled).

No. Depth Consistence Density Water %<2µm pH(m) ( M g / m3) Content dispersed

ARID ZONE RESEARCH INSTITUTE (AZRI)

Bore water: Bore no. 3, analysis 1981, 920 ppm, SAR = 3,pH 7.1, no residual bicarbonate (RSC)

Agricultural Block (drip irrigation)1 0-.07 crust 1.75 04 .l-.15 soft 1.67 2Meteorological Station (cultivated rotary hoed, before spray irrigation)5 0-.1 soft 1.36 10

TERRITORY GRAPE FARM (TGF)

7.48.0

7.57.57.87.98.6

7.3 147*7.7 125*

23

No. Depth Consistence Density Water %<2µm pH

(m) ( M g / m3) Content dispersed

TI TREE FARM (TTF)

Bore water: Analysis reported by Miles and McEllister (1977) for the Ti Tree basinaquifer, 794 ppm, SAR 4.8, pH 7.5 , 2.4 meq/l residual sodium carbonate.

Experimental siteTrees irrigated using micro-jets .Samples taken between rows just outside boundary of

4018

106*83*

612828

110*

733372

10437

1225645

98124

35123

151311

152930

24

Table A.2 Particle size analysis of selected samples after adding dispersant andshaking vigorously and the relation between clay concentration(mg/l) in the suspensions and turbidity.

Sampleno.

Clay Silt Fine Coarsesand sand

Clay conc.for 100 NTU

6781112

19 1822 2324 1058 14

49 16 1

1720

149

111611

AGF71 12

10412112893

86

Figure A.1 The relation of the visual estimate of the percentage of clay dispersedfrom c.0.5g air-dry portions immersed in 200 ml of water to thevalue obtained from the light absorption of the suspensions beforeand after treating with ultrasonics for 1 minute.