Investigating soils and understanding 
the regolith

Introduction

In early April 2008  the NUFG began excavating pits within each of the five farms participating in the Climate Change Project. The objective was to gain a better appreciation of the physical and chemical properties of the substrate that trees and other agricultural species grow within. The effort was not restricted to sampling surface and subsoils. Investigations were extended to include exploration of the parent materials that the soils had formed upon.    

The objective was to explore the uppermost part of the 'regolith'. This term is used by geologists and refers to the unconsolidated sediments and weathered rock materials, including soil, that rests over the deeper rocks that form the base of landscapes.  NUFG is interested in the biophysical and chemical nature of the regolith below the immediate soil zone. The roots of trees and other perennial vegetation often extend to great depths within this material.

Construction of the first pit begins at Gunbower

Investigative techniques

With the assistance of Ian Rankin Excavating NUFG established pits 2.5 metres deep and 1.0 metres wide.  Samples were collected at 0.5 metre intervals and within horizons of specific interest. The samples were packaged ready for laboratory determination of salinity and pH. 

At each site a log of the materials encountered was documented. This included recording the texture and colour of each horizon, the presence or absence of any root material and any minerals including limestone nodules, gypsum,  manganese or iron.  

The Gunbower Property

This property lies south of the township of Gunbower and west of the immediate floodplain of the Murray River in the northern extremities of the Riverine Plains in Victoria. The low relief terrain distant from the upland valleys of the Dividing Ranges is known as the Patho Plains. 

The Gunbower property is located about 5 kilometres east of Kow Swamp and encompasses a meandering depression within a small part of a much larger complex anabranch system that supports the Torrumbarry irrigation region. 

Parts of the meandering depression is subject to salinity and waterlogging that results from a shallow watertable that is a consequence of a groundwater mound associated with the irrigated land.    

 Over the past decade the landholders that own the Gunbower property have re-established large areas of native tree plantations along the depression and within some adjacent lands. Large stands of native forest now appear throughout the lower lying lands, amidst slightly more elevated terrain used for cereal cropping and irrigated grazing. 

The area that chosen for investigation of the soil and regolith lies within a subtle depression that is a tributary of the large anabranch. This area lay within well established native forest plantations that had been established in the latter part of the 1990s. The principle species in the plantation were Sugar Gum (Eucalyptus cladocalyx) and Flat Top Yate (Eucalyptus occidentalis).
 

Soil and regolith Gunbower site 1

Gunbower Site 1

Depth (cm)


Description

0 - 20

Dark brown clay loam
Abundant tree roots

20 - 120 

Yellow brown silty clay
Finer tree roots

120 - 250 

Yellow brown sandy clay
Mixture of fine sand 
and clay
Fine tree roots present as dark organic material

Comments:  Difficult to know the status of tree roots, some appeared to be alive and others dead.  Apparent, however, that tree roots are at times extending to 2.5 metres in depth.

  

Soil and regolith Gunbower site 2

Gunbower Site 2

Depth (cm)


Description

0 - 10

Grey brown silty clay loam

10 - 40 

Dark brown clay

40 - 75 


Dark brown clay
Gypsum abundant both as 
powder and small crystals
75 - 120



 
Brown silty clay
Strong plate-like fabric
Blocks 5 mm across  

Comments:  Gypsum layer is quite striking. Carbonate nodules found throughout he profile.  

The Drummartin Properties

The Drummartin properties are located on the immediate flood plain of the Bendigo Creek east of Kamarooka. This is dryland terrain that supports cereal cropping and grazing. Both properties also have well established native forestry plantations, and both support extensive areas of lucerne.

Drummartin Site 1

Investigations at the first property focused on a an area of the floodplain supporting a large stand of dryland lucerne. As per the description below the excavation found the soil type to be a classic red sodasol. 

Soil and regolith Drummartin Site 1

 Drummartin Site 1

Depth (cm)

Description

0 - 20

Grey brown fine sandy loam

20 - 65





Dark red medium clay
Strong texture contrast with the topsoil
Evidence of sodicity in the upper uppermost horizon
Abundant carbonate nodules

5 - 250




 

Red/yellow clay
Active lucerne roots penetrating to at least 2.4 metres
Large carbonate nodules abundant  

Comments:  Soil profile has been de-watered by lucerne and, accordingly, it is very dry and very dense 

 

Lucerne root extracted from a depth of 2.4 metres

Drummartin Site 2

Site 2 is located on a property north of the Drummartin school. The focus for investigation was the soil and regolith  beneath and 8 year old native farm forestry plantation located on the near floodplain of the Bendigo Creek. 

Soil and regolith Drummartin site 2

Drummartin Site 2

Depth (cm)

Description

0 - 30

Grey brown fine sandy loam

30 - 50

Dark red-brown silty clay
Abundant carbonate nodules
Abundant tree roots 

50 - 250 

Brown silty clay
Abundant carbonate nodules
Abundant tree roots

Comments:  Appears to be a very similar profile to that recorded for site 1 but at a greater moisture content and more reduced indicating periods of waterlogging

Kamarooka 

The Kamarooka property comprises land subject to a shallow watertable, saline groundwater,  and extensive areas of dryland salinity. The sampling site is approximately 50 metres inside the southern boundary of a Northern United Forestry Group plantation established during 2004.

 Soil and regolith Kamarooka 

Kamarooka

Depth (cm)

Description

0 - 15

Brown fine sandy loam

15 - 45

Brown silty clay
Abundant tree roots 

45 - 160

Brown silty clay
Large carbonate nodules  Concretions (up to 3 cm diameter) 
Tree roots present


160 - 265

Dark red silty clay
Possible palaeosol (fossil soil). Absence of carbonate nodules 

Comments:  Appears to be a very similar profile to that recorded for site 1 but at a greater moisture content and more reduced indicating periods of waterlogging

Leichardt

The Leichardt property comprises low relief gently undulating lands that are common along the weathered bedrock foothills of the Great Dividing Ranges of northern Victoria. The groundwater beneath the farm is quite deep and there is little risk of salinity developing from saline groundwater discharge.  

The soils on this farm are the classic red sodosols found throughout the foothills and plains of northern Victoria. At Leichardt the sodosols are formed on  weathered bedrock.  

Red sodasol formed on weathered meta-sedimentary rocks at Leichardt

Kamarooka

Depth (cm)

Description

0 - 30


Brown fine sandy loam
Abundant tree roots

30 - 110

Medium red clay
Large carbonate nodules and concretions (3 cm diameter)
Some tree roots 

110 240

Red clay matrix & rock fragments 
Quartz fragments and iron/manganese concretions 
Vertical tree root 1 cm diameter penetrating to 240 cm and beyond

Comments:  Large carbonate concretions formed throughout the B horizon clays. Very distinct texture contrast between A and B horizons.  Obvious upper B horizon sodicity issues common to this soil type. Manganese and/or iron concretions in the weathered bedrock are suggestive of alternating seasonal wetting and drying.    

Vertical roots of Sugar Gum (E. cladocalyx) penetrating the subsoil 
and deep into weathered bedrock at Leichardt

 

Results of Chemical Analyses 

Site

Depth
(m)

Salinity
(uS/cm) 
pH

 

Leichardt 0-0.1 130 6.2
0.5 110 8.8
0.9 320 9.4
1.5 150 9.1
2.0 320 9.4
Kamarooka 0-0.1 1000 6.0
0.5 1400 8.3
1.0 1900 8.6
1.5 1700 8.8
2.3

 

1500 8.9
Drummartin 1 0-0.2 320 5.6
0.5 36 7.7
1.0 150 9.0
1.5 160 9.1
2.3

 

140 9.3
Drummartin 2 0-0.1 87 6.0
0.5 190 8.5
1.5 920 9.0
2.0 1000 9.0
2.4 850 9.1
Gunbower
1
0-0.1 1000 7.0
0.25 2900 7.4
1.0 2400 8.8
1.5 2600 7.9
2.0 3000 7.6

 

Gunbower
2
0-0.1 610 6.3
200? 740 8.2
300? 850 7.4
400?
1.0

 

3800
940

 

7.4
9.2