A Vertical Segment Of A Soil Biology Essay

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A soil profile is a vertical segment of a soil which encompasses all the distinct layers of physical, chemical and biological composition (Botkin, Porter & Skinner, 1999, p. 129) from the soil surface to the underlying parent material, other substrate material or to a defined depth (Charman & Murphy, 2000). A particular soil profile, with its different horizons is an expression of the various additions to and removals from the soil as well as transformations and translocation within it (Brown, Isbell, Jacqueir & Mckenzie, 2004, p.30). The above processes are in turn influenced by the type of parent rock and/or surrounding landform as well as climatic conditions, biota and hydrology (Moore, 1998, p.31). Therefore a soil profiles are a useful tool to gauge the various geological, pedological and anthropogenic processes that have effected the formation of different soils. For instance the relative ages of different soils can be compared through the number and differentiation of horizons in their respective profiles (Froend, 2008). The mineral composition of those horizons provide an insight into the types and sources of parent material as well as the types of weathering they underwent whilst the organic matter illustrates the effects of climatic conditions and biota (Brown et al., 2004, pp.30-31).

The aim of this study was to compare the properties of two contrasting soils in the Bullsbrook region and analyse the processes that influence their formation. The first site (hereafter Site 1) was block of natural bushland located on a gently sloping plain near Jess Road, Bullsbrook. The vegetation of the site suggested it was a regrowth marri woodland whilst 2 types of Xanthorrhoea spp. were also present. Site 1 had sandy soil surface with heavy leaf litter. The site showed evidence of previous fire disturbance and was known to be frequented by kangaroos and emus. The second site (hereafter site 2) was an existing vertical exposure located midway through on a steep slope near Crestmoor pass. The vegetation suggested it was a cleared wandoo forest whilst one type Xanthorrhoea spp. The soil surface of site 2 was more coarse grained. The two sites were located within 2-3 km of each other.


Them methods followed for this study were those specified in Froend (2008, pp.11-18). The field data was recorded according to the format outlined in Isbell & Mcdonald (1998, pp.103-152). However for the particle size analysis 0.2 g/l was added to the hydrometer reading for every degree above 20C temperature instead of adding 0.2 g/l for every degree above 19.5C temperature as outlined by Froend (2008, p.18).


Organization of soil horizons

Whilst site 1 had only five distinct horizons site 2 had six distinct horizons (Table 1). Though both profiles possessed A1 and B2 horizons the A1 and B2 horizons of site 2 had a redder hue and a higher chroma in munsell colour (Table 1). Only site 2 had the transitional horizons A3 and B3. Whilst the profile in site 2 extended to the specified depth of 120cm, the profile in site 1 extended only extended down to 55cm due to an R horizon (Table 1). Mottling was present only in the B2 and B3 horizons of site 2and the A1 horizon in site 1. Though the mottles in the three horizons were similar in colour and, there were differences in abundance, size, contrast and distinctiveness of boundaries (Table 2).

Table 1

Depth, colour and notation of the soil horizons in soil profiles of site 1 and 2


Site 1 Site 2

Depth (cm) Horizon Munsell colour Depth (cm) Horizon Munsell colour

3-2 O1 - 3-2 O1 -

2-0 O2 -

2-0 O2 - 0-10 A1 10yr 3/2 very

dark greyish brown

0-20 A1 5y 2.5/1 Black 11-18 A3 7.5yr 5/8

Strong brown

21-55 B2 10y 5/2 19-78 B2 5yr 4/6

Greyish brown Yellowish red

55 + R 78-120+ B3 10yr 5/5

Brownish yellow

Table 2

Properties of mottles found in horizons B2 and B3 in site 2

A1(site1) B2 (site2) B3

Abundance Few Few Common

Size Fine Fine Medium

Contrast Distinct Distinct Prominent

Colour Pale Pale Pale

Distinctness of Sharp Diffuse Diffuse


Physical properties (structure, consistency and field texture)

Site 1 was devoid of peds. However the A horizons of site 2 had crumb shaped peds whilst the B horizons contained polyhedral shaped peds (Table 3). Though both the A1 and B2 horizons of site 1 were of loose consistency the A3 horizon of site 2 was friable and both B horizons had firm soil. The field texture results for site 1 showed the soil was of a sandy nature whilst site 2 had more clay with the clay content increasing from A1 to B3 (Table 3).

Table 3

Structure, consistency and field texture of the soil horizons of soil profiles in site 1 and 2

Site 1 Site 2

Horizon Structure Consistency Field Horizon Structure Consistency Field

Texture Texture

A1 Apedal Loose Sandy A1 Crumbs loose Light sandy

loam clay loam

B2 Apedal Loose Sandy A3 Crumbs friable Sandy clay

B1 Polyhedral firm Light clay

B3 Polyhedral firm Medium clay

Chemical properties (moisture, electrical conductivity and pH)

The soil moisture percentages of the O2 and A (A1) horizons of site 1 were higher than those of the O2 and A (Average of A1 and A3) horizons in site 2 (Fig. 1) However the soil moisture percentage of the site 2 B horizon (Average of B1 and B3) was higher than that of the B (B2) horizon in site 1(Fig 1.). All the soil horizons in both sites contained less than 5% moisture. The electrical conductivity of the O2 horizon of site 1 was higher than that of the O2 horizon in site 2, though the A and B horizons of site 2 had higher conductivity compared to the A and B horizons of site 1 respectively (Fig 2.). The electrical conductivities of all the horizons in both sites were far lower than 600 micro-siemens/cm.

Figure 1. Comparison of soil moisture between O2, A & B horizons in sites 1 and 2

Figure 2. Comparison of electrical conductivity between O2, A & B horizons in site 1

and 2. Each value being the mean of two replicates.

For both the 1:5 water extract (Fig 3.1) and the 1:5 CaCl2 extract (Fig 3.2) the pH values for O2 horizons in both profiles were equally acidic. The A horizon of site 1 had a lower pH than the A horizon of site 2 in both extracts. However whilst the 1:5 water extract pH for the B horizon of site 1 was higher than the site 2 B horizon (Fig 3.1), it's 1:5 CaCl2 extract pH was comparatively lower (Fig 3.2). All the soil horizons across both sites had ph values lower than 6.5 and higher than 3.8 .

Figure 3.1 Comparison of soil pH (1:5 water extract) between soil horizons O2, A &

between sites 1 and 2

Figure 3.2 Comparison of soil pH (1:5 CaCl2 extract) between soil horizons O2, A &

in sites 1 and 2

Organic matter, roots and coarse fragments

The percentage of organic matter is slightly higher in the O2 horizon is slightly higher than the organic matter percentage in the O2 horizon of site 2. However the A1 horizon of site 2 has more than double the organic matter percentage than the site 1 A1 horizon (Fig. 4). Roots were present in the O2 and A1 horizons in both profiles, though coarse material was only present in the A1 horizon of site 1 as charcoal.

Figure 4. Comparison of the 'loss on ignition' (organic matter) percentages between

soil horizons O2 and A1 in sites 1 and 2

Soil particle analysis and water repellency

The soil horizons in site 1 had a high percentage of sand whereas the B1and B3 horizons of in site 2 had a very high percentage of clay (Table 4). Horizon A1 in site 1 had nearly equal percentages of sand and clay. The A3 horizon of site 2 though having a high percentage of sand, had 3 times the percentage of silt than any other horizon in either site (Table 4). The O2 horizon of site 1 was water repellent whilst the O2 horizon of site 2 was non-water repellent.

Table 4

Percentages of sand, silt and clay of soil horizons in site 1 and site 2, obtained from soil particle analysis

Site 1 Site 2

Horizon % sand % clay %silt Horizon % sand % clay % silt

O2 63 34 3 O2 65 34 1

A1 82 14 4 A1 46 49 5

B2 71 29 0 A3 56 29 15

B1 16 79 5

B3 26 69 5


Assigning of soil horizons

The first mineral horizons in both site 1 and 2 were identified as A1 horizons due to them having a darker colour than the underlying horizons and the evidence of biological activity (Isbell & Mcdonald,1998, p.105) shown by the presence of roots. The horizon below A1 in site 1 was classified as a B2 horizon due to it's possession of a munsell colour of higher chroma and a higher percentage of clay (Isbell & Mcdonald, 1998, pp.105-106). The B2 horizon of site 2 was identified by it's redder hue, firm consistency and it having the highest percentage of clay in the profile (Isbell & Mcdonald,1998, p.106). The transitional horizon between A1 and B2 in site 2 was classified as A3 since it's properties were dominated by those of A1 (Isbell & Mcdonald,1998, p.105) such as crumb shaped peds, friable consistency and the lack of mottles. Though it contained similar amounts of sand, clay and silt the horizon underlying B2 in site 2 was classified as B3 based on it's yellowish munsell colour.

Classification into soil groups

The apedal structure, high organic carbon level (Moore,1998, p.319) and the high sand percentage with 10-20% clay percentage shown in the particle size analysis suggests that the soil in site 1 is of sandy loam texture (Moore, 1998, p.322). Since a cemented layer was found at a depth of less than 80cm the soil profile in site 1 could be considered shallow (Shoknecht, 2002). The diagnostic pH of the profile (Charman & Murphy, 2000) was acidic (Shoknecht, 2002) whilst the salinity was low (Moore, 1998, p.318). The above properties combined with the presence of mottles led to the postulation of site 1 as brown loam (Shoknecht, 2002).

The presence of pedal organization, the relatively high moisture percentage in the B horizon and the high clay percentage evident in the soil particle analysis pointed towards site 2 being a clay soil (Moore, 1998, p.322). As the B horizons in site 2 contained over twice the clay content of the overlying A3 horizon the soil profile possessed clear textural differentiation (Brown, Isbell, Jacqueir & Mckenzie, 2004, p.36). Since the textural differentiation occurred within a depth of 80cm the site 2 soil profile could be classified as a duplex soil (Shoknecht, 2002). The diagnostic pH of the profile (Charman & Murphy, 2000) was acidic (Shoknecht, 2002) whilst the salinity was low (Moore, 1998, p.318). The above properties combined with the high percentage of organic matter (Moore,1998, p.319) gave rise to the postulation that the soil profile in site 2 maybe a Brown deep loamy duplex soil (Shoknecht, 2002).

Soil formation processes

Shoknecht (2002) states that brown loamy soils such as that of site 1 are formed from alluvial processes. The fact that soil in site 1 contains a high sand percentage supports the possibility of alluvial transport (Botkin, Porter & Skinner, 1999, p. 195). Moreover the relatively less differentiated soil profile in site 1 also points towards alluvial deposition (Froend, 2008). Therefore it could be hypothesised that the parent material of the soil in site 1 was derived from the weathering of sedimentary rocks located in the Gingin area (Moore, 1998, p.31). The soil profile of site 2 however contains clearly differentiated horizons suggesting a comparatively older origin (Froend, 2008). This compliments Moore (1998, p.38)'s statement that brown loamy duplex soils are formed by the in-situ weathering of granite.

The water repellence shown in the surface soil of site 1 is caused by the recondensation of vaporised non-polar waxes during forest fires (Froend, 2008). Evidence of fire in the vegetation and the presence of charcoal in the A1 horizon confirm the water repellence property of the site 1 soil.

Limitations of the study

Moore (1998, p.316) suggests that soil horizons thicker than 30 cm should be sub sampled. However the above procedure was not included in the methods for this study. Subsampling of the B2 horizons in site 1 and 2 as well as the B3 horizon in site 2 may have led to the identification of subhorizons in the respective soil profiles.


This study set out to compare and contrast the properties of two different soils in the Bullsbrook area and thereby interpret the processes that effected the development of those soils. Based on the findings of field and laboratory data combined with literature related to subject it was postulated that site 1 was made of

brown loam soil formed by alluvial deposition whilst site 2 contained a brown deep loamy duplex soil formed in situ by the weathering of granite.