tation procedure (Jackson, 1956) was used after the other treatments to ..... Meyrick, Elizabeth Pansier, Barbara Cosnett, Susan Gosling, Jeff Garnham,.
Geoderma, 37 (1986) 207--220 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
207
AGGREGATION OF CLAY IN SIX NEW ZEALAND SOIL TYPES AS MEASURED BY DISAGGREGATION PROCEDURES
G.J. CHURCHMAN and K.R. TATE N.Z. Soil Bureau, Department of Scientific and Industrial Research, Lower Hutt, (New Zealand) (Received July 8, 1985; accepted after revision January 20, 1986)
ABSTRACT Churchman, G.J. and Tate, K.R., 1986. Aggregation of clay in six New Zealand soil types as measured by disaggregation procedures. Geoderma, 37: 207--220. Measurements were made of the yields and carbon concentrations of clay fractions obtained by a number of disaggregation procedures from samples of a range of New Zealand soils. Disaggregation was carried out in water by: minimal shaking by hand; prolonged mechanical shaking; shaking with sodium resin; ultrasonication; acetylacetone treatment; periodate--borate treatment; peroxidation; and peroxidation followed by citrate--dithionite--bicarbonate treatment. There was virtually no clay yielded by any of the six samples when gently suspended in water. Either a combined hydrogen peroxide and sodium citrate--dithionite--bicarbonate treatment or an exhaustive (10x repeated) hydrogen peroxide treatment produced the highest yield of clay except where ultrasonication of the field moist soil sample brought about strong disaggregation. Considerable physical disruption of primary particles could occur on ultrasonication. Airdrying had a variety of effects. In one notable case with a soil having a highly smectitic clay mineralogy, air-drying led to considerable resistance of the sample against disaggregation into clay. Exhaustive peroxidation was usually highly dispersive but it suppressed clay dispersion in samples of two soils from volcanic ash with allophanic clay mineralogies. There was a wide range between samples of the different soils in the concentrations and amounts of carbon in their clay fractions. A high concentration of carbon was present in clay fractions of all samples after even the most severe chemical treatment, with those of the allophanic soils having the highest concentrations of carbon after these treatments.
INTRODUCTION M a n y t h e o r i e s h a v e b e e n p r o p o s e d t o e x p l a i n t h e c a u s e s o f a g g r e g a t i o n in s o i l s , e.g. r e v i e w s b y H a r r i s e t al. ( 1 9 6 6 ) a n d T i s d a l l a n d O a d e s ( 1 9 8 2 ) . E m e r s o n ( 1 9 5 9 ) c o n s i d e r e d t h a t c l a y p a r t i c l e s a d h e r e t o o n e a n o t h e r , in the first instance, to form domains and that these domains were then bound to one another and to larger quartz particles by organic matter. Edwards and Bremner (1967) suggested that clays and organic matter are linked to one
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© 1986 Elsevier Science Publishers B.V.
208 another, via polyvalent metals, to give clay size (< 2 p m ) complexes which themselves are bound together to form microaggregates (< 250 pm) and then macroaggregates. A model of aggregate organisation wherein the causes of aggregation were different at different size levels was developed by Tisdall and Oades (1982). They considered that inorganic agents bound the finest (submicrometre) particles together, persistent organic matter was associated with polyvalent metal cations in binding material of several micrometres in diameter into aggregates, while transient polysaccharides and temporary agents such as roots and fungal hyphae bound together entities with diameters of several tenths of a millimetre. Several authors (e.g. Deshpande et al., 1968; Krishna Murti et al., 1977; Shanmuganathan and Oades, 1982; Emerson, 1983; Oades, 1984) have paid particular attention to the components involved at the fundamental level of binding of fine particles. Organic matter can serve as either an aggregating agent or a dispersing (i.e. disaggregating) agent at this level (Emerson, 1983; Oades, 1984). The nature of the clays themselves can affect the size of aggregates that result (Krishna Murti et al., 1977), though the nature of the exchangeable cations may have an over-riding effect (Emerson, 1983). The particular role that iron and aluminium oxides play in aggregation has been controversial. Thus Deshpande et al. (1968) considered that aluminium oxides were more important aggregating agents than iron oxides, while Krishna Murti et al. (1977) found that clay-iron oxide interactions were basic to aggregation in many soils. The addition of Fe (III) polycations to softs can effect flocculation and improve physical properties (Shanmuganathan and Oades, 1982). In this present study the causes of the aggregation of clays were investigated in samples of a wide range of New Zealand soils by selectively removing or disturbing aggregating agents. The extent of clay produced by these treatments was then related to the particular aggregating agents removed or disturbed by each treatment, revealing the different relative roles of these aggregating agents in the various softs. Many investigations of the causes of aggregation have previously used this approach (e.g. Clapp and Emerson, 1965; Hamblin and Greenland, 1977). All soils in our study were sampled under either native tussock or introduced pasture, and only surface layers were used. MATERIALS AND METHODS Soils Some of the characteristics of the softs and sites are shown in Table I. All soil samples were collected in autumn and were stored field-moist in plastic bags and kept at 4°C prior to their treatment and analyses.
0--5
0--5
0--5
0--16
0--10
Harihari loamy sand
Waimakariri fine sandy loam
Puniu silty clay loam
Egmont black loam
Taupo sandy silt
6.6
7.9
7.8" s
4.6* 3
4.9 * 1
7.5 * '
Organic carbon (%)
Molloy and Blakemore, 1974. Churchman, 1980. Ross et al., 1982a. T.H. Webb, pers. comm. Ross et al., 1982b.
0--6
McKerrow fine sandy loam
,1 ,2 ,3 ,4 ,s
Sample depths (cm)
Soil
Some characteristics of soils and sites
TABLE I
Typic Vitrandept .9
Entic Dystrandept .8
Aquic Dystrochept .6
Fluventic Ustochrept .4
Typic Fluvaquent .2
Typic Dystrochrept .2
Soil Taxonomy
"6 Ross et al., 1981. ,7 New Zealand Soil Bureau, 1968. , s Henmi and Parfitt, 1980. ,9 R.L. Parfitt, pers. comm.
Central yellow-brown loam .7 Yellow-brown pumice soil .6
Allophane .7
Allophane .6
Central gley soil *s
Recent soil
Podzolised high-country yellow-brown earth* 1 Gley recent soil .1
New Zealand
Soil classification
Halloysite
Mica, chlorite + kaolinite
Mica .2
Micabeidellite .2
Dominant clay minerals
Haast River floodplain, Westland ($87/164014) Islington Freezing Works Christchurch ($83/886552) Te Kopua, Waikato (N74/725132) Mokoia, Taranaki (N129/931238) Wharepaina, nr. Waiotapu (NS5/793738)
nr. Haast Pass, Westland ($98/168869)
Locality (Map Ref. ) NZMS1
Rhyolitic pumice
Andesitic ash
Pumice alluvium
Greywacke alluvium
Schist alluvium
Weathered schist
Parent material
Pasture
Pasture
Pasture
Ryegrassclover pasture
Pasture + tussock
Tussock grasses
Vegetation
CD
b~
Prolonged shaking (S)
Prolonged shaking with sodium resin (Na)
Short ultrasonic of wet soil (Uwet)
Short ultrasonic of dry soil (Udry) Acetylacetone soak (AcAc)
Periodate/borate treatments (P-B)
Exhaustive peroxidation (H2 02 )
Hydrogen peroxide/ citrate--dithionite-bicarbonate treatment (H2 O2 -CDB)
2.
3.
4.
5.
7.
8.
9.
6.
Sample in c. 500 ml water in cylinder; soaked for ~ 30 min. Cylinder slowly inverted and re-inverted ; repeated 4 times. Sample in 100 ml water in 250 ml plastic bottle. Bottle shaken for 16 h in end-over-end shaker at 50 rev./min. Sample + 1.0 g of Na-exchanged Dowex 50W-X8 resin in 100 ml water and then treated as in 2 above.
Inversion (I)
1.
Sample in 100 ml of 0.1 M aqueous acetylacetone solution. Soaked for 8 days with occasional gentle shaking. Acetylacetone then removed by filtering suspension, then washing with water until filtrate colour disappears (treatment after Martin and Reeve, 1957, and Hamblin and Greenland, 1977). Sample in 100 ml of 0.02 M sodium periodate. Soaked for 16 h. Periodate then removed (by decanting and filtering) and 100 ml of 0.1 M sodium borate adjusted pH 9.6 with dil./NaOH added. Soaked for 2 h. Borate then removed and soil washed with water until pH was 8.0 ( t r e a t m e n t a f t e r Greenland et al., 1962). Sample in 10 ml of 100 vol. H2 O2. Reacted overnight then boiled in water bath until effervescence ceased. Repeated 9 times. Sample then centrifuged and washed with water. Sample given two H 2 02 treatments as in 8 above. After supernatant removed, 3 0 m l of 0.3 M sodium citrate (C) + 5 ml of 1 M sodium bicarbonate (B) added, sample in centrifuge tube heated in boiling water bath and c. 1 g sodium dithionite (D) added, with 5 min stirring. Sample centrifuged, CDB treatment repeated and sample washed with water (treatment after Mehra and Jackson, 1960).
Sample in 100 ml water in beaker enclosed in ice jacket. Treated for 3 rain. with an MSE 150 Watt Ultrasonic Disintegrator Mark 2 with 19 mm diam. probe. Temperature always < 30 ° C. Total heat input measured in insulated beaker at 7620 J. Sample air dried (20°C) and then treated as in 4 above.
Details of method
Maximum chemical disaggregation
Maximum organic matter removal
Comparison with results of 4 shows effect of airdrying Removes cations associated with organic matter (o.m.) but destroys little o.m. (Hamblin and Greenland, 1977) Polysaccharides removed
Comparison with results of 2 shows effects of replacing surface cations by Na ÷ Ultrasonic disruption and dispersion
Physical disaggregation
Minimal disaggregation
Effect of method
O
Methods used for selective disturbance of aggregating agents
Number and name of method (abbreviation)
b~
TABLE II
211
Methods All treatments for the selective disturbance of aggregating agents were carried out with a sample of field-moist soil that was equivalent to 10 g oven-dried weight. They are set out in Table II, in the anticipated order of increasing energy of disaggregation within, firstly, the physical treatments (Nos. 1--5), and, secondly the chemical treatments (Nos. 6--9). A 'long ultrasonic' treatment of 30 min was also carried out on samples of two of the softs only (McKerrow and Harihari). The temperature was kept below 45°C by enclosing the beaker containing the sample within an ice jacket for this particular treatment. Following each of these treatments, size fractions were measured and/or collected by either of two procedures. After the inversion treatment (No. 1, Table II), a pipette m e t h o d was used without a dispersing agent to measure both clay ( < 0.002 mm equivalent spherical diameter or e.s.d.) and 'clay + silt' ( < 0 . 0 2 m m e.s.d.) size fractions. A centrifuge and gravity sedimentation procedure (Jackson, 1956) was used after the other treatments to both measure and collect clay size fractions. Each step of the procedure was repeated six times. Carbon contents of the clay size fractions following each of the soil treatments were obtained using a high-frequency induction furnace (Blakemore et al., 1977). Clay fractions from samples of two of the soils (McKerrow and Harihari) were also examined by transmission electron microscopy. For this purpose, representative clay fractions were collected from suspensions prior to drying and a small a m o u n t of sodium ethyl mercuric thiosalicylate (Fluka A.G. Switzerland) was added to the suspension to prevent microbial contamination during storage at 4°C. Particles with indistinct edges were probably indicative of organic matter and other non-crystalline soil constituents, both separate and associated with minerals, whereas particles with sharp edges typified clay minerals. Evidence from this technique is necessarily partial and selective but can be supportive of evidence from other techniques. RESULTS AND INTERPRETATIONS Effects o f treatments on particle size distributions The distribution of each soil sample into sand ( > 20 pm), silt (2--20 pm) and clay ( < 2 ttm) fractions following the minimum disaggregation inversion treatment showed that all samples were very largely composed of entities larger than 20 pm prior to the stronger disaggregation treatments. Furthermore, none of the surface layers, upon sampling, had more than 0.7% of clay-size material. The percentage of soil material which was disaggregated into clay-size particles by the various treatments described in Table II is given in Fig. 1.
212 )i, C~ay
s~
-
~
+'
Na~-
c~ q
~-
r .
t Uwet
*~
Udry I .
H~Od~_ CDB/ 0
t
×
N
N
M
P*O ,/~. 0'~
,f
-,
Na
N
..
Uwe t
N
X
.'
Udry
~'
AcAc
,.
P-B
H202 / CDB
I×
"
S
H202
[ ×
"
AcAc ~-
M
[
N
.. "
,_~
Na Uw~ Udry AcAc P
B
H20~ H202;
CDB
Fig. 1. P e r c e n t a g e o f each soil s a m p l e in t h e clay ( < 2/~m) f r a c t i o n f o l l o w i n g disaggreg a t i o n t r e a t m e n t s : I = i n v e r s i o n ; S --- p r o l o n g e d s h a k i n g ; Na = s h a k i n g w i t h Na-resin; U w e t = 3 m i n u l t r a s o n i c s o f field-moist soil; U d r y = 3 m i n . u l t r a s o n i c s o f air-dried soil; AcAc = a c e t y l a c e t o n e soak; P-B = p e r i o d a t e - - b o r a t e ; H 2 0 2 = e x h a u s t i v e ( 1 0 x ) pero x i d a t i o n ; H 2 0 2 / C D B = h y d r o g e n p e r o x i d e + c i t r a t e - - d i t h i o n i t e - - b i c a r b o n a t e (twice). Crosses i n d i c a t e m e a n values a n d e r r o r bars s t a n d a r d errors in m e a n s , w h e r e errors e x c e e d 0.1 p e r c e n t . Plots are for t h e various soil samples as follows ( a ) M c K e r r o w ; ( b ) H a r i h a r i ; (c) Waimakariri; (d) E g m o n t ; (e) T a u p o ; (f) P u n i u .
213
The results were replicated between two and ten times by one or more of five different operators; average values are plotted, together with their standard errors. The results, when plotted in this way, provide a t y p e of "profile of disaggregation" of the various soils. They also show that the results of the various disaggregating treatments described in Table II are not operator-dependent. A comparison of the different 'profiles of disaggregation' given for the various soils in Fig. 1 indicates that only one soil (Waimakariri) (Fig. l c ) shows a progressive increase in the proportion of clay within the sets of, firstly, physical treatments (from I-~ Udry) and, secondly, chemical treatments (from AcAc ~ H: 02 and H 2 02/CDB). Each of these sets of treatments are arranged, as in Table II, in anticipated increasing energy of disaggregation. Both treatments involving peroxidation, however, had a similar disaggregating effect on the Waimakariri soil sample. The 'profiles of disaggregation' for the other softs deviate from the reasonably smooth pattern set by the Waimakariri soil in different ways. These are characterised as follows. (1) For McKerrow soil (Fig. la), ultrasonic treatment of the wet sample produced as much clay as the two strongest chemical treatments (exhaustive peroxidation and hydrogen peroxide/citrate--dithionite--bicarbonate). Furthermore, air-drying causes aggregation of clay in this soil as shown by the results of ultrasonic treatment of the air-dried soil sample. This contrasts with the results for both the Waimakariri and Puniu soil samples (Fig. l c and f), where ultrasonic treatment of soil samples that had been dried led to larger amounts of clay than for the samples that had been kept moist. (2) There were no significant differences between the various disaggregating treatments, apart from the inversion treatment, for the Harihari soil sample (Fig. lb). All treatments of this soil sample yielded only small percentages ( ~ 5%) of clay, but the pattern in Fig. l b is similar to that for McKerrow soil in Fig. la. Furthermore, a longer (30 min) ultrasonic treatment of samples of both the Harihari and McKerrow soils was very effective in the production of clay (11% clay was produced from the Harihari soil sample; 26% clay from the McKerrow soil sample). (3) The Egmont soil sample was disaggregated to a similar extent by all treatments except the inversion treatment, exhaustive peroxidation and the strongest chemical treatment (peroxidation plus citrate--dithionite-bicarbonate; Fig. ld). The inversion treatment, as expected, produced the minimum a m o u n t of clay, while the strongest chemical treatment led to the maximum a m o u n t of clay. In comparison with those of other soils, however, the peroxidation treatment of the Egmont soil sample produced only a small a m o u n t of clay. (4) Apart from the inversion treatment, the amounts of clay released from the Taupo soil sample by the various treatments were hardly significantly different from one another (Fig. le). Ultrasonic treatment of soil samples that had been kept moist appeared to produce the highest yield of clay
214 w h i l e e x h a u s t i v e p e r o x i d a t i o n a p p a r e n t l y s u p p r e s s e d c l a y y i e l d r e l a t i v e t o all except the inversion treatment. (5) Both exhaustive peroxidation and the strongest chemical treatment h a d a h i g h l y d i s a g g r e g a t i n g e f f e c t u p o n t h e P u n i u soil s a m p l e s ( F i g . l f ) . O t h e r w i s e all t r e a t m e n t s e x c e p t t h e i n v e r s i o n t r e a t m e n t h a d s i m i l a r s l i g h t disaggregating effects.
Effects o f treatments on organic matter distribution T a b l e I I I gives t h e c a r b o n (C) c o n c e n t r a t i o n s o f c l a y f r a c t i o n s r e s u l t i n g f r o m e a c h o f t h e d i s a g g r e g a t i o n t r e a t m e n t s o f t h e soil s a m p l e s . T a b l e I V , derived from Table III and Fig. 1 by multiplying carbon concentrations by t h e a v e r a g e c l a y c o n t e n t s , gives t h e a m o u n t s o f c a r b o n in t h e c l a y f r a c t i o n s o f t h e v a r i o u s soil s a m p l e s . T h e a m o u n t s o f c a r b o n in t h e c l a y - s i z e f r a c t i o n s a r e e x p r e s s e d as p e r c e n t a g e s o f t h e m a s s o f t h e w h o l e soil s a m p l e s . TABLE III Carbon concentrations (%) of clay fractions of soil samples after disaggregation treatments Treatment
McKerrow
Harihari
Waimakariri Egmont
Taupo
Puniu
2. Prolonged shaking 3. Shaking with Na resin 4. Short ultrasonic, wet soil 5. Short ultrasonic, dry soil 6. Acetylacetone soak 7. Periodate--borate 8. Exhaustive H202 9. H2 O2-CDB
16.7 28.8 24.6
28.6 18.6 24.5
9.2 10.0 11.0
20.3 16.6 17.1
21.5 23.3 23.6
9.6 8.3 8.7
16.3
18.5
9.3
17.0
22.4
7.9
23.7 26.8 4.0 8.3
30.9 14.5 5.0 9.2
8.9 7.4 1.3 5.6
18.4 13.8 15.5 15.2
21.7 22.8 8.4 13.7
7.3 5.6 1.9 7.8
TABLE IV Amount of carbon in the clay fractions (as a percentage of whole soil samples) Treatment
McKerrow
Harihari
Waimakariri
Egmont
Taupo
Puniu
2. Prolonged shaking 3. Shaking with Na resin 4. Short ultrasonic, wet soil 5. Short ultrasonic, dry soil 6. Acetylacetone soak 7. Periodate--borate 8. Exhaustive H 2 02 9. H2 O2-CDB
1.1 2.1 3.5
0.6 0.4 1.1
0.6 1.0 1.4
1.3 1.2 1.5
0.8 1.2 1.7
1.2 1.2 0.5
0.4
0.4
1.2
1.1
1.0
1.2
1.2 2.6 0.6 1.0
0.4 0.3 0.2 0.2
0.7 0.9 0.2 0.9
1.1 1.0 0.5 4.4
1.1 1.2 0.2 0.6
0.5 0.8 0.7 3.4
215 Table III shows that a wide range of C concentrations resulted from the various treatments of both the McKerrow and Harihari soil samples, but, with the exceptions of the two most severe treatments (peroxidation, with and without citrate--dithionite--bicarbonate), C concentrations in the clay fractions were much more even for the samples of the other four soils. The variations in the C concentrations of clays from the McKerrow soil sample partly reflect the heterogeneous nature of the soil (McQueen, 1982) so that replicate C concentration values could differ by as much as 9%. Variations in these values for samples of the other soils were much lower. For the Egmont soil sample, the C concentration of the clay fraction was very similar regardless of treatment, including the most severe chemical treatments. Table IV shows that the most effective treatment for bringing carbon into the clay fractions was usually the ultrasonic treatment of the undried soil sample. Na resin treatment of the McKerrow soil sample also brought a relatively high amount of C into the clay fraction. Table III shows that the carbon was highly concentrated in the clay fraction of the McKerrow soil sample as a result of Na resin treatment even though this treatment was hardly more productive of clay than shaking alone (Fig. la). Periodate-borate oxidation also led to a considerable amount of carbon in the clay fraction of the McKerrow soil sample b u t had no comparable effect in samples of the other soils. In the Egmont soil sample, peroxidation produced a small a m o u n t of clay (Fig. l d ) with a high carbon concentration (Table III), leading to a small a m o u n t of carbon in the clay fraction, as indicated in Table IV. The large amounts of carbon brought into the clay fraction of the Egmont and also Puniu soil samples by the combined peroxidation and citrate--dithionite-bicarbonate treatment (Table IV) bear out the very strong disaggregation of these two soils resulting from this treatment (see Fig. 1). The similarity between the concentrations of carbon in the clay fractions of the Egmont soil sample that were obtained by b o t h peroxidation alone, which brings a b o u t little dispersion of clay, and peroxidation followed by citrate-dithionite--bicarbonate, which is much more dispersive, suggests that carbon was uniformly distributed between the clay and the coarser fractions prior to these treatments. In the Puniu soil sample, there is a very large difference (c. 6%) in carbon concentrations of the clay fraction between peroxidation alone (10x) and peroxidation (2x) with CDB (Table III). The clay percentage produced by the latter treatment, however, only exceeds that from peroxidation alone by approx. 8% (Fig. lf). This small increment in the clay fraction is apparently greatly enriched in organic matter (Table IV). More extensive removal of carbon by exhaustive peroxidation alone than by the combined H202(2x)-CDB treatment may have contributed to this difference in the clay carbon concentrations. The CDB-susceptible aggregation observed in the Puniu soil sample may be due to the presence of some allophane (Cotching, 1978}, as in the Egmont soil sample. Comparison of the amounts of carbon in the clay fraction (Table IV)
216 following ultrasonic treatment of McKerrow and Harihari soil samples, before and after drying, shows that the clays extracted from the samples after drying contained less carbon. This result supports indications from Fig. I (a and b) that the binding of clay-size material into larger size fractions via organic matter in these softs is substantially enhanced by drying and thereby is resistant to disaggregation and subsequent ultrasonication. Transmission electron microscopy All physical treatments {shaking, short ultrasonic, and also a 30-min, long ultrasonic treatment), as well as the acetylacetone treatment, of the McKerrow and Harihari soil samples resulted in particles in the clay fractions largely showing indistinct edges by electron microscopy. Close associations between minerals and organic matter are indicated by this observation. Both exhaustive peroxidation and H 2 O2-CDB treatments left clay-sized particles with sharp edges, indicating mineral particles unassociated with organic matter. The 30-min ultrasonic treatment revealed a greater concentration of very small particles than were present in both samples following any other treatment. DISCUSSION Virtually all of the clay-size material in each of the soils studied was bound into aggregates of larger than clay size in the undisturbed soil. Organic matter is an important aggregating agent binding clay-size material into larger aggregates in several of the soils, especially McKerrow, Waimakariri and Puniu soils (Fig. 1). Organic matter appears to be a dispersing agent rather than an aggregating agent in the two allophanic soils, especially the Egmont soil. Peroxidation enhanced clay aggregation (Fig. 1). Besides removing organic matter from soils, peroxidation also lowers the pH of the suspensions (Douglas and Fiessinger, 1971). Allophanic soils can disperse at low pH (Maeda et al., 1977). The effect of peroxidation of the Egmont soil sample on particle size distribution (Fig. 1) is contrary to dispersion, however, so it can be concluded that the removal of organic matter brought a b o u t clay aggregation and suppressed dispersion. Substantial dispersion of clays in this sample only occurred upon citrate--dithionite removal of iron and aluminium oxides. Organic matter removal by peroxidation, however, was far from complete (Table III). Samples of all soils retained some carbon in their clay fractions following peroxidation but those of Egmont and Taupo retained very high percentages. This carbon is probably associated with allophane. The fraction of organic matter removed by peroxidation had been responsible for dispersion of the clays and its removal led to the allophane previously associated with it becoming aggregated into silt- and sand-sized entities. The fraction of organic matter that was retained appears to play no significant part in aggregation.
217 The soil samples differed in the stability of their aggregates of clays to physical treatments (Fig. 1). Aggregates of clays in the Harihari, Waimakariri and Taupo soil samples were generally quite unstable, whereas those in the sample of McKerrow soil were particularly unstable when the field moist sample was ultrasonicated. Aggregates of clays in the Egmont and Puniu soil samples were generally stable to physical treatments. Drying stabilised clays in aggregates in the McKerrow soil sample, particularly, and the Harihari soil sample to a lesser extent, against ultrasonic treatment. Aggregates of clays in the Puniu soil sample were less stable after drying than before while the samples of the other three soils showed little effect of drying. Both the amount and particle size (as revealed by electron microscopy) of clay produced by extended ultrasonic treatment of the Harihari and McKerrow soil samples suggests that ultrasound brings about comminution of particles in these soils. The fact that drying had an aggregating effect that was largely resistant to ultrasonic treatment, especially in the McKerrow soil sample, indicates the effect of drying is very strong. Drying appeared to fix organic matter in aggregates against its movement into the clay fraction of the McKerrow soil sample (Table III). Both the strongly aggregating effect of drying the McKerrow soil sample and the strong attachment of organic matter within this sample after drying indicate the effect of smectite, which dominates the clay mineralogy of this sample in the form of mica-beidellite. Further studies of this and a related soil (Theng et al., 1986) confirm the strong attachment of organic matter to the mica-beidellites probably through the formation of interlayer organic complexes with the smectite. Generally, periodate--borate was more effective than acetylacetone in dispersing clays (Fig. 1). It appears that polysaccharides played some role in aggregating clay, especially in the McKerrow soil sample, where the periodate--borate treatment had the largest relative effect. A previous investigation (Barron et al., 1980) of the organic matter in a clay fraction of this soil by 13C-n.m.r. has indicated a predominance of O-alkyl carbon. Periodate--borate oxidation of polysaccharides increased the carbon content of the clay fraction of only the McKerrow soil sample (Table III). In this sample, the significant redistribution of clay-size material from larger aggregates into the clay fraction upon polysaccharide oxidation probably brought accompanying organic matter into this fraction. Na resin treatment had a similar effect on organic matter but not on clay dispersion. Here, the replacement of polyvalent cations on the clay surface by Na÷ probably displaced organic matter associated with these cations that had allowed clays to aggregate into coarser size fractions. It is significant that the concentration of organic matter in the clay fraction following such treatments as those with Na resin and periodate--borate (for the McKerrow soil sample) and acetylacetone (for the Harihari soil sample) should be greater than those obtained by shaking or even ultrasonic treatment (Table III). This strong physical treatment might be expected to
218 comminute plant fragments into smaller particle sizes. Any such effect must be slight, compared with that brought about by the above chemical treatments of the McKerrow and Harihari soil samples. In these cases, the organic: matter that was concentrated in the clay fraction is of a different type from that of plant material; prior humification by biological and/or chemical agents is suggested. Generally, Na resin treatments had no significant effect on clay dispersion (by comparison with shaking alone), though a small increase in dispersion is apparent for the Waimakariri soil sample. The criterion for increased dispersion in this work, however, is an increase in the proportion of ~ 2 p m fraction only. Earlier work has shown us (Churchman and Tate, 1981) that the distribution of material within the clay fraction of some allophanic soils is shifted towards finer sizes as a result of Na-resin treatment. In addition, we have found that the amount of clay produced by Na-resin treatment can vary with the t y p e of resin used.
Comparisons with other studies Some theories of aggregation (e.g. Emerson, 1959) do not specify the particular agents binding clay particles together. In their microaggregation model, however, Edwards and Bremner {1967) suggested that clays are linked together through polyvalent metals and organic matter into primary units which themselves coalesce into fine sand- and silt-size microaggregates. The present results suggest that clay-organic matter bonding with little or no polyvalent metal involvement is clearly very important in some soils (e.g. Waimakariri, Puniu, McKerrow and Harihari soils). In others however (e.g. Egmont and Taupo soils), organic matter appears to separate primary units rather than to bind them together. Emerson (1983) and Oades (1984) both noted that organic matter can play the dual role of either aggregating or dispersing clays. A general understanding of the organisation of microaggregates into larger units (macroaggregates) was not among the aims of this present study. Nevertheless, the results of this study can be compared with those of McQueen (1982) who investigated the macroaggregation of the McKerrow and Harihari softs using material from the same samplings as in this present work. McQueen found that the influence of organic matter governed the macroaggregate stability of both softs and that the smectitic mineralogy of the McKerrow soil stabilised that soil on drying over that of the micaceous Harihari soils, even at the macroaggregate size level. There appeared to be some relationship between microaggregation (< 300 t~m) and macroaggregation in these two soils, although McQueen (1982) also found that aggregates in the intermediate 300--53 t~m size range were more stable in the Harihari soil than in the McKerrow soil.
219 CONCLUSIONS
Treatment of samples of surface layers from six different types of soils from New Zealand with nine procedures designed to either remove, disturb, or enhance particular aggregating agents has shown that the causes of aggregation of clays vary with the type of soft. The variations appeared to be closely related to the differences in clay mineralogy between the various soils. Compared with those containing predominantly non-expanding clay minerals (mica, chlorite, kaolinite or halloysite), clays from surface layers of a soil with a highly smectitic clay mineralogy were aggregated by airdrying whereas allophanic clays were aggregated, rather than dispersed, by peroxidation. Ultrasonication produced clay by disrupting coarser particles from soils containing considerable mica or mica-derived minerals. ACKNOWLEDGEMENTS
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