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Soil organic matter fractions and microaggregation in a Ultisol under cultivation and secondary forest in south-eastern Nigeria.

Introduction

The importance of soil organic matter (SOM) in restoration of soil fertility and development of good soil structure has been widely studied (Palm et al. 1997, 2001; Koutika et al. 1997). Elliot and Coleman (1988) remarked that aggregates physically protect SOM by forming a barrier between microorganisms, microbial microbial

pertaining to or emanating from a microbe.


microbial digestion
the breakdown of organic material, especially feedstuffs, by microbial organisms.
 enzymes, and their substrates. Zhang and Horn (2001) observed that SOM in turn enhances soil aggregate stability or soil strength by increasing friction between particles and in binding effects due to increased menisci menisci

plural form of meniscus.
 forces. Goldberg et al. (1990) indicated that SOM can act as an aggregating or segregating material or have no noticeable influence on aggregate stability, depending on its composition in soil and or the relative contributions of other aggregating agents.

Despite the importance of SOM to soil fertility management and soil physical and rheological rhe·ol·o·gy  
n.
The study of the deformation and flow of matter.



rheo·log
 properties, changes in land use can have serious effects on the content and distribution of SOM in the soil. Parfitt et al. (2003) showed that there was a clear influence of land use on C and N mineralisation in the soils they studied in New Zealand New Zealand (zē`lənd), island country (2005 est. pop. 4,035,000), 104,454 sq mi (270,534 sq km), in the S Pacific Ocean, over 1,000 mi (1,600 km) SE of Australia. The capital is Wellington; the largest city and leading port is Auckland. . Land use changes have been observed to affect the SOM-associated particle size Particle size, also called grain size, refers to the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials.  fractions in the soil (Capriel et al. 1992; Quiroga et al. 1996). Whereas the total SOM under conservation tillage affects macroaggregate stability, the micoaggregate stability is mainly controlled by stable organic matter and therefore depends on soil parameters such as texture (Quiroga 1994; Quiroga et al. 1996; Mrabet et al. 2001).

Tisdall and Oades (1982) demonstrated a framework for aggregate hierarchy which described how mineral particles are bound together with soil microflora microflora /mi·cro·flo·ra/ (-flor´ah) the microscopic vegetable organisms of a special region.
Microflora
The bacterial population in the intestine.
 into microaggregates. Quiroga et al. (1996) indicated that physical fractionation fractionation /frac·tion·a·tion/ (frak?shun-a´shun)
1. in radiology, division of the total dose of radiation into small doses administered at intervals.

2.
 techniques enable fractions of total organic matter to be differentiated into coarse organic matter, young organic matter, and the biological, stable organic matter associated with fine fractions. However, Mrabet et al. (2001) rather referred to the coarse fraction associated organic matter as particulate organic matter.

In the degraded Ultisols of south-eastern Nigeria, land use changes due to deforestation deforestation

Process of clearing forests. Rates of deforestation are particularly high in the tropics, where the poor quality of the soil has led to the practice of routine clear-cutting to make new soil available for agricultural use.
 for farm lands, coupled with high SOM mineralisation rates due to high temperature, have led to a serious decrease in SOM. Previous studies in the area (Spaccini et al. 2001, 2002, 2004) all concentrated on the influence of addition of organic residue to the topsoils of tropical ecosystems. There are very few studies on the distribution of native SOM within the soil profiles of Ultisols. The soil physical properties have been affected by severe SOM depletion, leading to severe soil erosion, leaching, and excessive runoff. The objectives of this study were (i) to identify the soil properties and soil organic carbon (SOC) distribution within the soil profiles and microaggregate size fractions, (ii) to evaluate their roles in the microaggregate stability of the soils, and (iii) to determine the effect of cultivation on SOC distribution and microaggregation. The aim was to develop a framework that will guide the management of these soils and improve their stability so as to reduce soil erosion losses.

Materials and methods

Environment and soils

The study area is located between 6[degrees]44' and 6[degrees]55'N and 7[degrees]11' and 7[degrees]28''E in south-eastern Nigeria. The climate of the area is mainly humid tropical with mean annual precipitation of 1500-1600 mm. Average temperature is 30[degrees]C with the difference between winter and summer temperature always <5[degrees]C. The rainy season is between April and October. The vegetation is derived savanna savanna or savannah (both: səvăn`ə), tropical or subtropical grassland lying on the margin of the trade wind belts.  (Igbozurike 1975) and the underlying geology is mainly weathered sandstone (Ajali Formation).

The soils are classified as Typic Paleustult (Soil Survey Staff 1998) or Haplic Nitosol (FAO FAO,
n See Food and Agriculture Organization.
 1990). The soils are deep, acid, and low in plant-available nutrient including cation exchange capacity In soil science, cation exchange capacity (CEC) is the capacity of a soil for ion exchange of positively charged ions between the soil and the soil solution. A positively-charged ion, which has fewer electrons than protons, is known as a cation due to its attraction to cathodes.  (CEC (Central Electronic Complex) The set of hardware that defines a mainframe, which includes the CPU(s), memory, channels, controllers and power supplies included in the box. Some CECs, such as IBM's Multiprise 2000 and 3000, include data storage devices as well. ) (Table 1). Jungerius (1964) observed that the SOM content is very low, and leaching including soil erosion by water remains the major cause of degradation of these soils. The major silicate silicate, chemical compound containing silicon, oxygen, and one or more metals, e.g., aluminum, barium, beryllium, calcium, iron, magnesium, manganese, potassium, sodium, or zirconium. Silicates may be considered chemically as salts of the various silicic acids.  clay of the soil is kaolinite kaolinite (kā`əlĭnīt), clay mineral crystallizing in the monoclinic system and forming the chief constituent of china clay and kaolin. , quartz, and gibbsite Gibbs´ite

n. 1. (Min.) A hydrate of alumina.

Noun 1. gibbsite - white crystalline mineral consisting of aluminum hydroxide; a constituent of bauxite and a source of alumina
 (Table 1). The soils are mainly cultivated with annual root crop (Manihot spp.), while the secondary forest is rejuvenated secondary forest which previously was a deforested tropical forest. During the period of deforestation, these secondary forested soils were extensively cultivated.

Field study and laboratory studies

Cultivated and rejuvenated secondary forest sites occurring within the same soil type were identified for the study. Four soil profile pits were cited, with 2 each from secondary forest and cultivated areas. The aim was to duplicate the profiles within each land use to enable comparison. The cultivated areas have been cleared for more than 10 years. All the soil profiles were described and soil samples collected from the pedogenetic horizons of each soil profile. All samples were collected in the dry season (January-February). These soil samples were air-dried, sieved through a 2-mm mesh, and analysed in triplicate as described below. The soil used for saturated hydraulic conductivity Hydraulic conductivity, symbolically represented as , is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures.  was sampled with cores in triplicate from each horizon. These cores keep the soils in an undisturbed form.

Separation of particle size fractions

The 2000-200, 200-63, 63-2, and <2 [micro]m aggregate size fractions were separated from the <2.00 mm particles without prior chemical treatment. Low sonication sonication /son·i·ca·tion/ (son?i-ka´shun) exposure to sound waves; disruption of bacteria by exposure to high-frequency sound waves.

son·i·ca·tion
n.
 energy was used to disperse the soils so as to simulate natural disruptive forces such as from raindrop impact and avoid organic matter redistribution among the separates. This procedure was adopted by Spaccini et al. (2001) and consist of suspending 40 g of the <2.00-mm, air-dried sample in 100 mL of deionised water in a beaker and dispersing with an ultrasonic probe (Ultrasonic Liquid Processor, XL-Series Sonicator) placed at about 1 cm depth in the soil-water suspension. The suspension was sonicated for 2 min yielding a total energy of 240 J or 2.40 J/mL. Excessive sonication energies may cause undesirable artefacts especially in the <2 [micro]m fraction. The dispersed soil was then sieved in a series to separate the 2000-200 [micro]m and 200-63 [micro]m aggregate sizes. The 63-2 [micro]m fraction was separated through centrifugation Centrifugation

A mechanical method of separating immiscible liquids or solids from liquids by the application of centrifugal force. This force can be very great, and separations which proceed slowly by gravity can be speeded up enormously in centrifugal
 at 90 G for 6 min to maximise yield; centrifugation was repeated 2 more times on the residue. The <2 [micro]m fraction was obtained after centrifugation at 3100 G for 30 min. All the separations were done in triplicate while fractions were air-dried and stored for the determination of SOC and total nitrogen. Water-stable aggregates (WSA) <0.25 mm were obtained by the method of Kemper and Rosenau (1986) using a <0.25 mm sieve to collect WSA below that diameter range.

Soil analysis

Particle size distribution The particle size distribution[1] ("PSD") of a powder, or granular material, or particles dispersed in fluid, is a list of values or a mathematical function that defines the relative amounts of particles present, sorted according to size.  of the <2 mm fine earth fractions was measured by the hydrometer hydrometer (hīdrŏm`ətər), device used to determine directly the specific gravity of a liquid. It usually consists of a thin glass tube closed at both ends, with one end enlarged into a bulb that contains fine lead shot or mercury to  method as described by Gee and Bauder (1986). The clay and silt obtained from particle size analysis with chemical dispersant dis·per·sant  
n. Chemistry
A liquid or gas added to a mixture to promote dispersion or to maintain dispersed particles in suspension.
 was regarded as total clay (TC) and total silt (TSilt), while clay and silt obtained after particle size analysis using deionised water only were the water-dispersible clay (WDC WDC Washington DC, USA
WDC Western Digital Corporation
WDC World Data Center
WDC Warwick District Council (UK)
WDC World Diamond Council
WDC Workforce Development Center
WDC Wisconsin Democracy Campaign
) and water-dispersible silt WDSi), respectively. Soil pH was measured in a 1:2.5 soil : 0.1 M KCl suspensions. The soil saturated hydraulic conductivity was measured using the method of Klute and Dirksen (1986). The SOC in both whole soil (<2.0mm) and soil fractions was determined by the Walkley-Black method described by Nelson and Sommers (1982). Total nitrogen in both soil and soil fractions was determined by the micro-Kjeldahl method of Bremner and Mulvaney (1982). Exchangeable cations were determined by the method of Thomas (1982). The major clay minerals were analysed and identified using the X-ray diffraction technique (XRD XRD X-Ray Diffraction
XRD Crossroad
XRD X-Ray Diode
) after pretreatments of the soil samples to remove impurities (Whittig and Allardice 1986). Exchangeable sodium percentage (ESP (1) (Enhanced Service Provider) An organization that adds value to basic telephone service by offering such features as call-forwarding, call-detailing and protocol conversion. ) and sodium adsorption adsorption, adhesion of the molecules of liquids, gases, and dissolved substances to the surfaces of solids, as opposed to absorption, in which the molecules actually enter the absorbing medium (see adhesion and cohesion).  ratio (SAR (Segmentation And Reassembly) The protocol that converts data to cells for transmission over an ATM network. It is the lower part of the ATM Adaption Layer (AAL), which is responsible for the entire operation. See AAL.

SAR - segmentation and reassembly
) were calculated using the following equations:

ESP = (Exchangeable [Na.sup.+]/CEC) x 100 (1)

SAR = Exchangeable [Na.sup.+] / ([square root of [Ca.sup.2+] + [Mg..sup.2+]/2) (2)

The soil microaggregate stability indices were calculated as shown below:

Dispersion ratio (DR) = [(WDSi + WDC)/(Tsilt + TC)] (3)

Clay dispersion ratio (CDR (1) See CD-R and extension.

(2) (Call Detail Reporting) See call accounting.

(3) (Common Data Rate) A standard sampling rate for digital video for 480i and 576i systems. The rate is 13.5 MHz. See ITU-R BT.
) = WDC/TC (4)

Clay-flocculation index (CFI CFI
abbr.
cost, freight, and insurance
) being an index of clay aggregation was calculated as:

CFI = [(TC - WDC)/TC] (5)

Aggregated silt + clay (ASC ASC Ambulatory surgery center, see there ) = [TC + Tsilt] - [WDC + WDSi] (6)

The higher the CDR, DR, and WSA <0.25 mm the greater is the ability of the soil to disperse, while the higher the CFI and the ASC the better aggregated is the soil.

Data analysis

The soil properties and SOC fractions measured were subjected to linear correlation analyses with microaggregate stability indices. Many soil properties were shown to intercorrelate in a manner that does not show any pattern. To remove the disturbances caused by these intercorrelations and so determine the underlying structure of interrelations among the parameters, principal component analysis using the SPSS A statistical package from SPSS, Inc., Chicago (www.spss.com) that runs on PCs, most mainframes and minis and is used extensively in marketing research. It provides over 50 statistical processes, including regression analysis, correlation and analysis of variance.  10 computer package was also used to decompose de·com·pose  
v. de·com·posed, de·com·pos·ing, de·com·pos·es

v.tr.
1. To separate into components or basic elements.

2. To cause to rot.

v.intr.
1.
 the SOC fractions and soil properties affecting soil microaggregate stability indices in the soils to fewer principal components. The varimax rotation explained the interpretation of eigenvector (mathematics) eigenvector - A vector which, when acted on by a particular linear transformation, produces a scalar multiple of the original vector. The scalar in question is called the eigenvalue corresponding to this eigenvector.  (loading) and was used to determine the major soil factors affecting microaggregation in the soils.

Results

Soil characteristics

The soils are coarse to medium textured and often with very little silt content (Table 1). This concurs with findings for soils similar to those studied (Akamigbo 1984; Igwe et al. 1999). Generally, soils formed on these sedimentary sandstones have very little silt content. They are acid with pH values in KCl ranging from 3.7 to 4.2. Table 1 also presents the values of exchangeable cations and CEC for the soils. These properties are very low, reflecting the low inherent fertility of these soils. Enwezor et al. (1981) referred to these soils as acid sands because of their low pH values and low retention of essential plant nutrients often caused by leaching due to high rainfall. The ESP was 0.6-5.1% and the SAR ranged from 0.04 on profile P3 to 0.22 on P2. There was no definite trend in the occurrence of these soil properties within the soil profile.

The major silicate mineral silicate mineral

Any of a large group of silicon-oxygen compounds that are widely distributed throughout much of the solar system. The silicates make up about 95% of the Earth's crust and upper mantle, occurring as the major constituents of most igneous rocks and in
 of the clay fraction was kaolinite which contributes >50% of the total minerals followed by quartz and the oxides and hydroxides of Fe and Al. Soil saturated hydraulic conductivity (Ks) was 0.6-32.6 cm/h in cultivated profiles (P1 and P2), and it ranged from 0.5 to 101.8 cm/h in forest soils (P3 and P4). In most cases highest Ks values were obtained on the topsoil or the horizon below the topsoil. These values tend to decrease with depth (Table 2). This is a reflection of the high permeability rates recorded for these soils.

Soil microaggregate stability

The values for WDC are presented (Table 2). The soil profiles on cultivated soils have higher WDSi than those of forest soils. The DR for the cultivated soils and those of the forest soils are shown (Table 2), along with CDR and CFI. The CDR for the soils was 0.19-0.72, while CFI was 0.28-0.81. The ASC values were 6.76-18.76% on the same profile. The WSA <0.25 mm are also shown on Table 2.

Distribution of soil organic carbon and total nitrogen within the soil profiles and soil fractions from cultivated and forest soils

The SOC for the <2.00mm (whole soil) in most cases decreased with depth of the soil profiles. In most cases the highest SOC content was obtained for the topsoil. There were, however, some fluctuations in the profiles for SOC values with depth. The distribution, the mean values, and the percent coefficient of variation Coefficient of Variation

A measure of investment risk that defines risk as the standard deviation per unit of expected return.
 (CV %) for each profile are shown (Table 3). The overall values of the entire SOC were greater in forest soils than the cultivated soils. In most soil profiles of cultivated and secondary forest land uses, the 2000-200 [micro]m associated SOC was the least, followed by 200-63 [micro]m associated SOC. The greatest amount of SOC was associated with the <2 [micro]m soil fractions in most of the soil profiles. The SOC associated with aggregate size fractions 2000-63 [micro]m was therefore the least, although most of the 2000-200 [micro]m associated SOC seems to have been generated from recently deposited materials.

The values of total nitrogen mean and CV in both whole soils and soil particle fractions in soil profiles followed the same trend as SOC. As in the SOC the least amount of associated total nitrogen occurred in the 2000-200 [micro]m size fraction, followed by 200-63 [micro]m then 63-2 p.m then <2 [micro]m fractions (Table 4). There was not wide variation among the total nitrogen concentration of soils within the forest and cultivated land use and the aggregate sizes.

Relationships between microaggregation indices, soil organic carbon fractions, and soil properties

The correlation coefficient Correlation Coefficient

A measure that determines the degree to which two variable's movements are associated.

The correlation coefficient is calculated as:
 matrix presenting the relationships between the microaggregate stability indices is presented in Table 5. WDC correlated significantly positively with CDR but negatively with WDSi, clay content, and CFI. WDSi also correlated significantly with CDR (negative) and CFI (positive). Dispersion ratio correlated significantly negatively with ASC (r= -[0.64.sup.*]), and CFI correlated significantly negatively with CDR and positively with ASC. CDR correlated negatively with ASC (r = -[1.67.sup.*]).

Clay content correlated significantly negatively with <2 [micro]m microaggregate size associated SOC, 63-2 [micro]m associated SOC, <63 [micro]m associated SOC, and CEC. A significant positive correlation Noun 1. positive correlation - a correlation in which large values of one variable are associated with large values of the other and small with small; the correlation coefficient is between 0 and +1
direct correlation
 existed between clay and ESP. The WDC correlated significantly positively with <2, 63-2, 2000-200, <63, and 2000-63 [micro]m microaggregate size associated SOC, and exchangeable [K.sup.+] (Table 6). WDSi negatively correlated with exchangeable [K.sup.+], while DR positively correlated with CEC and [Ca.sup.2+]. The CFI correlated significantly with the <63 [micro]m aggregate size associated SOC fraction, CEC, and ESP. Also, CDR correlated positively with <63 [micro]m size associated SOC and CEC but negatively with ESP. The ASC correlated significantly positively with ESP but negatively with CEC. Similarly, WSA <0.25 mm correlated negatively with total SOC, and all the SOC associated sized fractions and exchangeable [K.sup.+]. Exchangeable [Na.sup.+] and ESP correlated positively with WSA <0.25 mm (Table 6).

There were some intercorrelations within the parameters, and some strong correlations occurred randomly among the parameters. To remove the disturbances caused by these interrelationships and determine the underlying structure of interrelations among the properties tested, they were subjected to principal component analysis (PCA). PCA reduced the 14 soil properties tested to 4 orthogonal components having eigenvalues greater than unity and together accounting for 82.07% of the total variance within the variables. The loadings on the 4 components are shown (Table 7). Component 1 explained 44.36% of the total variance, has significant loadings (i.e. greater than [+ or -] 0.74) on 2000-63 [micro]m fraction associated SOC, <63 [micro]m associated SOC, 200-63 [micro]m associated SOC, <2 [micro]m associated SOC, 63-2 [micro]m SOC, 2000-200 [micro]m SOC, and <2 [micro]m SOC. This component described the organic matter fractions. Component 2 explained 17.97% of the total variance and loaded significantly on exchangeable [Na.sup.+]. Component 3, which explained 12.08% of the total variance, loaded significantly on [Mg.sup.2+] and SAR. In component 4, 7.66% of the total variance was explained and loaded highly on CEC and ESE ESE
abbr.
east-southeast

Noun 1. ESE - the compass point midway between east and southeast
east southeast
 In all of components 2, 3, and 4 the underlying dimension appears to be exchangeable bases. The component defining variables (CDVs) are those variables which loaded highest in each component. In this study the CDVs are 2000-63 [micro]m microaggregate size associated SOC, [Na.sup.+], [Mg.sup.2+], and CEC.

Discussion

The roles of SOM in a fragile soil such as the one studied have been shown to include nutrient supply and prevention of soil degradation due to erosion and soil structural degradation (Six et al. 2000; Buschiazzo et al. 2001). The SOC contents of the soil fractions played a very significant role in the microaggregation mechanism in these soils. From the correlation coefficient matrix in Table 6 and PCA (Table 7), the role of SOC was clearly manifested especially on the soil dispersion indices or the unstable aggregates such as WSA <0.25 mm, CDR, and WDC as shown in their relationship with stability indices. As already mentioned, their role in ASC was masked but the PCA was able to expose the role of SOC in microaggregate stability. Many authors (Elliot 1986; Cambardella and Elliot 1993; Six et al. 2002) have shown that increased microbial activity depletes SOM, which eventually leads to lower production of microbially derived binding agents and loss of aggregation. Also, Kolbl and Kogel-Knabner (2004) demonstrated that it was possible to have fine aggregate bounded SOC manifesting in microaggregation if the fine fraction contents were high. According to according to
prep.
1. As stated or indicated by; on the authority of: according to historians.

2. In keeping with: according to instructions.

3.
 them, in soils with low fine fraction contents, the SOC contents are highly degraded. In our study the fine fractions of <2 [micro]m size fractions were low and often <25%. The high WSA <0.25 mm and the moderate to low CDR and DR values were the consequence of the low SOC in the soil irrespective of irrespective of
prep.
Without consideration of; regardless of.

irrespective of
preposition despite 
 the land use except on the topsoil of Profile 1. This is a peculiar situation as more SOM was expected for the forest land use than the cultivated soils. However, the studied forest is a secondary type being rejuvenated from deforestation and often combed and cultivated underneath the canopy.

Boix-Fayos et al. (2001) indicated that microaggregation depended on clay while macroaggregation depends on SOM. In our study, the 2000-63 [micro]m associated SOC loaded significantly on the PCA and therefore related best with ASC (Table 7). This was followed by <63 [micro]m associated SOC, then 200-63 [micro]m associated SOC, <2 [micro]m associated SOC, 63-2 [micro]m associated SOC, 2000-200 [micro]m associated SOC, and finally SOC of the whole soil. The 2000-63 [micro]m associated SOC may have provided such important parameters as carbohydrates (polysaccharides), humic hu·mic  
adj.
Of, relating to, or derived from humus.

Adj. 1. humic - of or relating to or derived from humus; "humic acid"
 substances, and exudates that enhance microbial growth and fungi hyphae hy·pha  
n. pl. hy·phae
Any of the threadlike filaments forming the mycelium of a fungus.



[New Latin, from Greek huph
. Ball et al. (1996) emphasised that the fraction of SOC occurring within this range is actively involved in stability of soil aggregates. Caravaca et al. (2004) found that macroaggregates were not controlled by the stable fraction of organic matter in the finer fractions (<20 [micro]m) but by those in >20 [micro]m fractions. Webber (1965) also reported that there is the tendency for microaggregates of soils to be preferentially enriched in total carbohydrates. This is because carbohydrates are easily absorbed to microaggregates in the soil. Spaccini et al. (2001) explained that carbohydrates in microaggregates are more easily absorbed as polymers of various forms and dimensions to either clay or humic fractions which abound in the microaggregates. In this way, according to Piccolo piccolo, small transverse flute pitched an octave higher than the standard flute. Its tone is bright and shrill, and it can produce the highest notes in the orchestral range. The piccolo is used in orchestras and especially in military bands. See fife.  and Mbagwu (1990) the carbohydrates are better protected from microbial degradation than those in the larger aggregates, hence the higher ASC with the 2000-63 [micro]m sized SOC. Chenu et al. (1994) observed that soil aggregates, especially the microaggregates, were stabilised by polysaccharides adsorbed on clays and soil minerals. The effect of carbohydrates on soil aggregation is transient and depends on regular decomposition of residues. Tisdall and Oades (1982) indicated that microaggregates 20-250 [micro]m are not destroyed by agricultural practices and are held together by a number of temporary agents such as hyphae, fungi, and bacteria. They observed that the binding agents of these small aggregates are aromatic humic materials associated with the aluminosilicates through complexes with polyvalent polyvalent /poly·va·lent/ (-va´lent) multivalent.

pol·y·va·lent
adj.
1. Acting against or interacting with more than one kind of antigen, antibody, toxin, or microorganism.

2.
 cation cation (kăt'ī`ən), atom or group of atoms carrying a positive charge. The charge results because there are more protons than electrons in the cation.  bridges (C-P-OM). However, this assertion does not downplay the role of SOC associated with <63 [micro]m in WDC, CDR, CFI, and WSA <0.25 mm. SOC in the finer fractions also contributed significantly to microaggregate formation (Table 6).

The role of the exchangeable cations and CEC are also of note (Table 7). They are also involved in the microaggregate stability of these soils by acting as polymer linkages. Polymer bridges must have occurred between the organic materials such that organo-mineral complexes are formed. This goes to support the earlier remark made by Edwards and Bremner (1967) that the nature of aggregation was such that the polyvalent bonding with clay and SOC occurs on the soil surfaces. According to them, the soil organic matter, clay, and polyvalent cations are linked in the following manner: clay--cation--organic matter--clay. These cations behave as junctions of a net composed of polymeric chains of organic matter. The results of this study, however, do not assume to preclude the aggregate stability functions of oxides and hydroxides of Fe, Al, and Mn, which are very common in highly weathered tropical soils.

Conclusions

The soils were loose, deep, coarse-textured, and low in basic cations and SOC content both in the whole soil and the particle size fractions. Although most of fine aggregate associated SOC was in the <2 [micro]m content, they were unable to control the microaggregate stability of the soil. However, the SOC associated with 2000-63 [micro]m size fractions was assumed to have controlling effect on the aggregated silt + clay (ASC), which was our measure of microaggregate stability of the soil. This does not downplay the significant roles of SOC associated with <63 [micro]m aggregate fractions in microaggregate stability. The assumed role of coarse fraction associated SOC in this study does not preclude the role of polyvalent metals such as Fe, Al, and Mn oxides in aggregate stability of tropical soils.

In this study there was no major variation in SOC contents of soils within the cultivated and secondary forest except in the topsoil and the relative quantity in the soil profile. The reason could be that the secondary forest is just being rejuvenated and the forest is combed and cultivated beneath. High mineralisation rate and low clay contents of the soils which could have acted as a pool for SOC may have been responsible for the low SOC values.

Acknowledgments

We are grateful to the Swedish International Development Cooperation Agency Sida (sometimes SIDA but not officially spelled with capital letters) is a Swedish governmental agency that answers to the Swedish Ministry for Foreign Affairs. Sida is an acronym for the Swedish International Development Cooperation Agency (or in Swedish  (SIDA) for providing the funding under the framework of Regular Associate Programme of Abdus Salam International Centre for Theoretical Physics The Abdus Salam International Centre for Theoretical Physics operates under a tripartite agreement among the Italian Government, UNESCO, and the International Atomic Energy Agency (IAEA) (both agencies of the United Nations) to foster advanced studies and research, especially in  (ICTP ICTP International Centre for Theoretical Physics (Trieste, Italy)
ICTP International Council of Tourism Partners
ICTP Individual and Collective Training Plan
ICTP Intensified Combat Training Program
) to one of the authors (CAI (1) (Computer-Assisted Instruction) Same as CBT.

(2) See CA.

CAI - Computer-Aided Instruction
). We also thank the ICTP Trieste, Italy, for their hospitality. This manuscript was completed while one of the authors (CAI) was on a visit to ICTP. The contribution of Alexander yon Humboldt-Foundation, Bonn, Germany through 'The Equipment Donation Programme' is acknowledged.

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 indicators. CATENA ca·te·na  
n. pl. ca·te·nae or ca·te·nas
A closely linked series, especially of excerpted writings or commentaries.



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A plural of symposium.
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n. 1. Tendency to become diffused; tendency, as of heat, to become equalized by spreading through a conducting medium.
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A city of eastern Argentina on the Bahía Blanca, an inlet of the Atlantic Ocean southwest of Buenos Aires. It is a major shipping and commercial center. Population: 314,000.

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Of, relating to, or being the geographic areas adjacent to the Tropics.


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of the region lying between the tropics and temperate lands

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Manuscript received 15 June 2005, accepted 30 May 2006

C. A. Igwe (A,B) and D. Nwokocha (A)

(A) Department of Soil Science, University of Nigeria The University of Nigeria is in the Enugu State town of Nsukka. It was founded by Dr Benjamin Nnamdi Azikiwe, the first president of Nigeria. It is the first indigenous university in Nigeria. , Nsukka, Nigeria.

(B) Corresponding author. Email: charigwe1@hotmail.com
Table 1. Major characteristics of the representative soil profile
TC, Textural class; SCL, sandy clay loam; SL, sandy loam; LS, loamy
sand; ESP, exchangeable sodium percentage; SAR, sodium adsorption
ratio; [Ka.sup.++], kaolinite in abundance (>50%); [Q.sup.+], quartz
in moderate quantity next to kaolinite; [Gi.sup.+], gibbsite

                  Particle size distribution (%)
Soil depth
(m)             Clay          Silt          Sand

Profile 1. Typic Paleustult cultivated soil

0-0.20           22             6            72
0.20-0.65        22             2            76
0.65-1.00        20             4            76
1.00-1.35        24             4            72
1.35-1.80        26             4            70

Profile 2. Typic Paleustult cultivated soil

0-0.23           22             4            74
0.23-0.45        24             2            72
0.45-0.80        20             4            76
0.80-1.20        24             2            74
1.20-1.75        10             6            84

Profile 3. Typic Paleustult secondary forest soil

0-0.18           14             4            82
0.18-0.40        14             4            82
0.40-0.70        20             4            76
0.70-1.20        24             2            74
1.20-1.80        24             2            74

Profile 4. Typic Paleustult secondary forest soil

0-0.21           18             2            80
0.21-0.45        20             2            78
0.45-0.86        24             2            74
0.86-1.20        22             6            72
1.20-1.90        24             4            72

Soil depth                     pH
(m)              TC            KCl

Profile 1. Typic Paleustult cultivated soil

0-0.20           SCL           3.9
0.20-0.65        SCL           4.0
0.65-1.00        SL            4.0
1.00-1.35        SCL           4.1
1.35-1.80        SCL           4.2

Profile 2. Typic Paleustult cultivated soil

0-0.23           SCL           3.9
0.23-0.45        SCL           4.0
0.45-0.80        SL            4.1
0.80-1.20        SCL           4.2
1.20-1.75        LS            4.2

Profile 3. Typic Paleustult secondary forest soil

0-0.18           SL            4.0
0.18-0.40        SL            3.8
0.40-0.70        SL            3.8
0.70-1.20        SCL           3.9
1.20-1.80        SCL           4.0

Profile 4. Typic Paleustult secondary forest soil

0-0.21           SL            3.7
0.21-0.45        SL            3.8
0.45-0.86        SCL           3.7
0.86-1.20        SCL           3.9
1.20-1.90        SCL           4.0

                         Exchangeable cations (cmol/kg)
Soil depth
(m)          [Na.sup.+]     [K.sup.+]    [Ca.sup.2+]   [Mg.sup.2+]

Profile 1. Typic Paleustult cultivated soil

0-0.20          0.09          0.07          0.40          0.40
0.20-0.65       0.08          0.02          0.40          0.60
0.65-1.00       0.07          0.02          0.20          0.60
1.00-1.35       0.07          0.01          0.10          0.50
1.35-1.80       0.06          0.03          0.10          0.50

Profile 2. Typic Paleustult cultivated soil

0-0.23          0.07          0.04          0.10          0.20
0.23-0.45       0.08          0.02          0.10          0.36
0.45-0.80       0.07          0.02          0.10          0.10
0.80-1.20       0.08          0.06          0.10          0.30
1.20-1.75       0.07          0.02          0.10          0.50

Profile 3. Typic Paleustult secondary forest soil

0-0.18          0.07          0.04          0.20          0.60
0.18-0.40       0.02          0.08          0.20          0.40
0.40-0.70       0.06          0.04          0.10          0.10
0.70-1.20       0.07          0.02          0.10          0.50
1.20-1.80       0.07          0.04          0.20          0.20

Profile 4. Typic Paleustult secondary forest soil

0-0.21          0.08          0.09          0.30          0.50
0.21-0.45       0.08          0.07          0.10          0.30
0.45-0.86       0.07          0.06          0.20          0.40
0.86-1.20       0.06          0.08          0.20          0.50
1.20-1.90       0.06          0.04          0.10          0.80

Soil depth       CEC
(m)           (cmol/kg)        ESP           SAR

Profile 1. Typic Paleustult cultivated soil

0-0.20          1.76           5.1          0.14
0.20-0.65       2.92           2.7          0.11
0.65-1.00       2.68           2.6          0.11
1.00-1.35       2.40           2.9          0.13
1.35-1.80       2.50           2.4          0.11

Profile 2. Typic Paleustult cultivated soil

0-0.23          3.50           2.0          0.18
0.23-0.45       2.70           3.0          0.16
0.45-0.80       2.60           2.7          0.22
0.80-1.20       2.80           2.9          0.18
1.20-1.75       4.50           1.6          0.13

Profile 3. Typic Paleustult secondary forest soil

0-0.18          3.10           2.3          0.11
0.18-0.40       3.20           0.6          0.04
0.40-0.70       2.80           2.5          0.19
0.70-1.20       2.40           2.9          0.13
1.20-1.80       2.10           3.3          0.16

Profile 4. Typic Paleustult secondary forest soil

0-0.21          3.20           2.5          0.13
0.21-0.45       2.60           3.1          0.18
0.45-0.86       3.00           2.3          0.13
0.86-1.20       1.80           3.3          0.10
1.20-1.90       2.60           2.3          0.09

Soil depth
(m)          Mineralogy

Profile 1. Typic Paleustult cultivated soil

0-0.20       [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]
0.20-0.65    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.65-1.00    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
1.00-1.35    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]
1.35-1.80    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]

Profile 2. Typic Paleustult cultivated soil

0-0.23       [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.23-0.45    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.45-0.80    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.80-1.20    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]
1.20-1.75    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]

Profile 3. Typic Paleustult secondary forest soil

0-0.18       [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.18-0.40    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.40-0.70    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.70-1.20    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]
1.20-1.80    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]

Profile 4. Typic Paleustult secondary forest soil

0-0.21       [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.21-0.45    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.45-0.86    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]
0.86-1.20    [Ka.sup.++]; [Q.sup.+]. [Gi.sup.+]
1.20-1.90    [Ka.sup.++]; [Q.sup.+]; [Gi.sup.+]

Table 2. Some microaggregate stability indices of the soils

WDC, Water-dispersible clay; WDSi, water-dispersible silt; Ksat,
saturated hydraulic conductivity; DR, dispersion ratio; CDR, clay
dispersion ratio; CFI, clay flocculation index; ASC, aggregated
silt+clay; WSA, water-stable aggregates <0.25 mm

Soil depth     WDC        WDSi                   DR
(m)                                  Ksat
               (%)                  (Cm/h)

Profile 1. Typic Paleustult cultivated soil

0-0.20          7           6        9.2        0.47
0.20-0.65       5          10        9.0        0.63
0.65-1.00       5          10        9.0        0.63
1.00-1.35       5           6        4.2        0.40
1.35-1.80       5           8        4.8        0.44

Profile 2. Typic Paleustult cultivated soil

0-0.23          5           6        32.6       0.43
0.23-0.45       7           6         4.8       0.42
0.45-0.80       5           4         4.1       0.38
0.80-1.20       5           6         5.1       0.43
1.20-1.75       7           2         0.6       0.57

Profile 3. Typic Paleustult secondary forest soil

0-0.18          7           2        84.4       0.51
0.18-0.40       7           2        25.5       0.51
0.40-0.70       7           4         3.2       0.46
0.70-1.20       5           8         4.3       0.50
1.20-1.80       7           4        58.0       0.43

Profile 4. Typic Paleustult secondary forest soil

0-0.21          7           2        24.1       0.46
0.21-0.45       5           4       101.8       0.41
0.45-0.86       7           6         4.9       0.50
0.86-1.20       7           4         1.4       0.40
1.20-1.90       5           4         0.5       0.32

Soil depth     CDR        CFI        ASC        WSA
(m)
                                           (%)

Profile 1. Typic Paleustult cultivated soil

0-0.20         0.32       0.68      14.76      52.40
0.20-0.65      0.23       0.77       8.76      41.00
0.65-1.00      0.25       0.75       8.76      53.83
1.00-1.35      0.21       0.80      16.76      53.83
1.35-1.80      0.19       0.81      16.76      53.55

Profile 2. Typic Paleustult cultivated soil

0-0.23         0.23       0.77      14.76      42.05
0.23-0.45      0.29       0.71      12.76      47.55
0.45-0.80      0.25       0.75      14.76      45.85
0.80-1.20      0.21       0.80      14.76      52.53
1.20-1.75      0.72       0.28       6.76      53.15

Profile 3. Typic Paleustult secondary forest soil

0-0.18         0.51       0.49       8.75      52.30
0.18-0.40      0.51       0.49       8.76      28.48
0.40-0.70      0.35       0.65      12.80      48.75
0.70-1.20      0.21       0.79      12.76      54.68
1.20-1.80      0.29       0.71      14.76      53.65

Profile 4. Typic Paleustult secondary forest soil

0-0.21         0.39       0.61      10.76      31.78
0.21-0.45      0.25       0.75      12.76      52.35
0.45-0.86      0.29       0.71      12.76      47.80
0.86-1.20      0.32       0.68      16.76      48.93
1.20-1.90      0.21       0.79      18.76      51.30

Table 3. Distribution of soil organic carbon (g/kg) within whole
soil (<2.00 mm sizes) and microaggregate sizes

CV %, Coefficient of variation

Soil depth    <2.00        <2        63-2
(m)             mm      [micro]m   [micro]m

Cultivated soils

Profile 1

0-0.20         8.4        25.1       32.3
0.20-0.65      4.4        18.3       14.4
0.65-1.00      3.5         9.6        9.2
1.00-1.35      3.4        11.9        8.0
1.35-1.80      1.9         3.6        5.6
Mean           4.3        13.7       13.9
CV %            56         60         78

Profile 2

0-0.23         6.0        14.0       11.9
0.23-0.45      1.8        16.8       13.6
0.45-0.80      2.2        13.9        6.6
0.80-1.20      2.4         8.0        6.8
1.20-1.75      0.8         6.4        6.0
Mean           2.6        11.8        9.0
CV %            75         37         39

Secondary forest soils

Profile 3

0-0.18         6.7        24.4       28.7
0.18-0.40      6.2        58.2       35.5
0.40-0.70      5.6        16.8       16.8
0.70-1.20      2.4         7.9        7.2
1.20-1.80      1.2         9.2        8.4
Mean           4.4        23.3       19.3
CV %            56         88         65

Profile 4

0-0.21         8.0        16.8        9.2
0.21-0.45      5.5        12.4       11.6
0.45-0.86      4.0        12.4       10.4
0.86-1.20      1.9        11.5       10.0
1.20-1.90      3.6        10.6        7.6
Mean           4.6        12.7        9.8
CV %            50         19         15

Soil depth    200-63    2000-200
(m)          [micro]m   [micro]m

Cultivated soils

Profile 1

0-0.20         10.4       5.6
0.20-0.65      10.0       5.6
0.65-1.00       6.0       2.4
1.00-1.35       6.8       2.8
1.35-1.80       5.2       3.2
Mean            7.7       3.9
CV %            31         40

Profile 2

0-0.23         7.6         2.8
0.23-0.45      7.2         2.8
0.45-0.80      8.4        15.3
0.80-1.20      3.6         1.6
1.20-1.75      4.0         3.2
Mean           6.2         2.4
CV %            36         33

Secondary forest soils

Profile 3

0-0.18        11.5         2.0
0.18-0.40     11.0         6.4
0.40-0.70      6.4         4.0
0.70-1.20      1.5         2.4
1.20-1.80      3.6         4.4
Mean           6.8         3.8
CV %            65          46

Profile 4

0-0.21        11.2         6.8
0.21-0.45      8.0         4.0
0.45-0.86      8.0         6.0
0.86-1.20      6.4         4.4
1.20-1.90      9.8         4.1
Mean           8.6         5.1
CV %            21          25

Table 4. Distribution of total nitrogen (g/kg) within whole soil
(<2.00 mm sizes) and microaggregate sizes

CV %, Coefficient of variation

Soil depth      <2         <2        63-2
(m)             mm      [micro]m   [micro]m

Cultivated soils

Profile 1

0-0.20         1.0        2.1        1.1
0.20-0.65      0.4        1.0        0.4
0.65-1.00      0.3        0.6        0.9
1.00-1.35      0.3        0.6        0.3
1.35-1.80      0.4        0.6        0.3
Mean           0.5        1.0        0.6
CV %            61         66         62

Profile 2

0-0.23         0.6        2.8        0.4
0.23-0.45      0.7        1.4        0.6
0.45-0.80      1.0        0.8        0.4
0.80-1.20      0.4        0.6        0.1
1.20-1.75      0.3        0.2        0.4
Mean           0.6        1.2        0.4
CV %            46         87         47

Secondary forest soils

Profile 3

0-0.18         0.8        2.0        1.3
0.18-0.40      1.0        2.0        1.5
0.40-0.70      0.8        3.4        0.7
0.70-1.20      0.4        0.8        0.3
1.20-1.80      0.6        0.3        0.4
Mean           0.7        1.7        0.8
CV %            32         71         64

Profile 4

0-0.21         0.3        1.8        0.8
0.21-0.45      0.6        1.3        0.6
0.45-0.86      0.6        0.6        0.4
0.86-1.20      0.7        0.8        0.4
1.20-1.90      0.3        0.6        0.4
Mean           0.5        1.0        0.5
CV %            37         51         34

Soil depth    200-63    2000-200
(m)          [micro]m   [micro]m

Cultivated soils

Profile 1

0-0.20         0.7        0.3
0.20-0.65      0.6        0.1
0.65-1.00      0.3        0.1
1.00-1.35      0.3        0.3
1.35-1.80      0.3        0.1
Mean           0.4        0.2
CV %            44         61

Profile 2

0-0.23         0.6        0.1
0.23-0.45      0.7        0.1
0.45-0.80      0.4        0.3
0.80-1.20      0.3        0.1
1.20-1.75      0.7        0.3
Mean           0.5        0.2
CV %            34         61

Secondary forest soils

Profile 3

0-0.18         0.8        0.3
0.18-0.40      1.0        0.4
0.40-0.70      0.8        0.3
0.70-1.20      0.3        0.1
1.20-1.80      0.4        0.1
Mean           0.7        0.2
CV %            45         56

Profile 4

0-0.21         1.3        0.3
0.21-0.45      0.6        0.1
0.45-0.86      0.6        0.3
0.86-1.20      0.4        0.3
1.20-1.90      0.6        0.1
Mean           0.7        0.2
CV %            49         50

Table 5. Correlation coefficients for the linear relationships
between microaggregate stability indices

WDC, Water-dispersible clay; WDSi, water-dispersible silt; DR,
dispersion ratio; CFI, clay flocculation index; CDR, clay
dispersion ratio; ASC, aggregated silt-F clay; WSA,
water-stable aggregates <0.25 mm

         WDC        WDSi       Clay        DR

WDC    --
WDSi   -0.58 **   --
Clay   -0.42 *     0.61 **   --
DR      0.10       0.37      -0.42 *    --
CFI    -0.68 **    0.67 **    0.91 **   -0.37
CDR     0.68 **   -0.67 **   -0.91 **    0.37
ASC    -0.31       0.10       0.76 **   -0.84 **
WSA    -0.25       0.30       0.32      -0.11

         CFI        CDR        ASC

WDC
WDSi
Clay
DR
CFI    --
CDR    -0.99 **   --
ASC     0.68 **   -0.67 **      --
WSA     0.24      -0.24        0.32

* P < 0.05; ** P < 0.01.

Table 6. Correlation coefficients for the linear relationships
between microaggregate stability indices, soil organic matter
fractions and soil properties

SOC, Soil organic carbon in whole soil; OC<2 [micro]m, SOC in <2
[micro]m fraction, etc. CEC, cation exchange capacity; ESP,
exchangeable sodium percentage; SAR, sodium adsorption ratio; WDC,
water-dispersible clay; WDSi, water-dispersible silt; DR,
dispersion ratio; CFI, clay flocculation index; CDR, clay
dispersion ratio; ASC, aggregated silt+clay; WSA, water-stable
aggregates <0.25 mm

                       Clay         WDC        WDSi         DR

SOC                  -0.31        0.11       -0.31       -0.02
OC<2 [micro]m        -0.45 *      0.39 *     -0.36        0.17
OC63-2 [micro]m      -0.42 *      0.45 *     -0.29        0.18
OC200-63 [micro]m    -0.31        0.23       -0.31       -0.01
OC2000-200pm         -0.13        0.39 *     -0.26        0.03
OC<63 [micro]m       -0.40 *      0.40 *     -0.32        0.15
OC2000-63 [micro]m   -0.36        0.45 *     -0.27        0.23
CEC                  -0.67 **     0.10       -0.31        0.39 *
[Ca.sup.2+]          -0.09        0.31        0.16        0.47 *
[Mg.sup.2+]          -0.06       -0.13        0.18        0.25
[K.sup.+]            -0.13        0.45 *     -0.46 *     -0.18
[Ng.sup.+]            0.29       -0.15        0.31        0.02
ESP                   0.50 *      0.01        0.30       -0.21
SAR                   0.22       -0.19        0.01       -0.32

                        CFI         CDR         ASC         WSA

SOC                  -0.14        0.14       -0.13       -0.44 *
OC<2 [micro]m        -0.39 *      0.39 *     -0.37       -0.69 **
OC63-2 [micro]m      -0.38 *      0.38 *     -0.32       -0.39 *
OC200-63 [micro]m    -0.19        0.19       -0.17       -0.59 **
OC2000-200pm         -0.16        0.15       -0.09       -0.67 **
OC<63 [micro]m       -0.34        0.34       -0.33       -0.69 **
OC2000-63 [micro]m   -0.33        0.33       -0.34       -0.81 **
CEC                  -0.63 **     0.63 **    -0.64 **    -0.32
[Ca.sup.2+]          -0.10        0.10       -0.31       -0.35
[Mg.sup.2+]          -0.07        0.07       -0.11        0.09
[K.sup.+]            -0.16        0.16        0.03       -0.50 *
[Ng.sup.+]            0.28       -0.28        0.07        0.42 *
ESP                   0.42 *     -0.42 *      0.42 *      0.49 *
SAR                   0.28       -0.28        0.23        0.28

* P < 0.05; ** P < 0.01.

Table 7. Principal component analysis of micoaggregate stability
factors after varimax rotation using ASC component

SOC, Soil organic carbon in whole soil; OC<2 [micro]m,
SOC in <2 [micro] m fraction, etc. CEC, cation exchange capacity;
ESP, exchangeable sodium percentage; SAR, sodium adsorption ratio

                                  Component

Variables               1        2        3        4

OC2000-63 [micro]m     0.929   -0.118    0.141   -0.149
OC<63 [micro]m         0.891   -0.376    0.064    0.018
OC200-63 [micro]m      0.819    0.105    0.209   -0.139
OC<2 [micro]m          0.812   -0.524    0.051   -0.070
OC63-2 [micro]m        0.794   -0.329    0.001    0.162
OC2000-200 [micro]m    0.778    0.178    0.016   -0.091
SOC                    0.757   -0.180   -0.028    0.085
[Ca.sup.2+]            0.680    0.462    0.357    0.207
[K.sup.+]              0.617   -0.030   -0.033    0.123
[Na.sup.+]            -0.196    0.881   -0.157    0.220
[Mg.sup.2+]           -0.069    0.072    0.967   -0.038
SAR                   -0.263    0.387   -0.863    0.051
CEC                   -0.033   -0.022   -0.011   -0.951
ESP                   -0.011    0.549   -0.138    0.805
Eigenvalue             6.21     2.52     1.69     1.07
% Variance            44.36    17.97    12.08     7.66
% Cum. variance       44.36    62.32    74.41    82.07
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Author:Igwe, C.A.; Nwokocha, D.
Publication:Australian Journal of Soil Research
Date:Sep 1, 2006
Words:8142
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