Swan Coastal Plain - Reading

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Cathleen Gregory
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1 Swan Coastal Plain Almost all the soils of the Swan Coastal Plain are formed by material deposited by rivers and wind. The Yilgarn Block, east and south of the scarp bordering the Swan Coastal Plain, rose about 40 to 50 million years ago. Erosion of the block by rivers and streams, and the eroded material was either deposited onto the Swan Coastal Plain or washed into the sea. The youngest dunes are the Quindalup dunes nearest the coast Next come the Spearwood dunes, and farthest from the coast are the oldest Bassendean dunes Limestone occurs as pipes (after roots) in profiles of the Quindalup dunes and the western margin of the Spearwood dunes, and as limestone rock at the base of both the Quindalup and Spearwood dune sequences (Tamala Limestone). There is no limestone under the Bassendean dunes. Between the dune systems, inlets, lakes and swamps occur. Inlets (e.g. the Leschenault Inlet) occur between the Quindalup and Spearwood dunes. Lakes and swamps occur between all three dune systems The soils under the swamps and lakes are rich in organic matter, so they form peat swamps, and have often been drained and used for vegetable production. Activity: Construct a cross section profile of the major parts of the SCP. Use the Gozzard (2007) Guidebook to assist.

2 Quindalup Dunes (Calcareous Sand) composed of unconsolidated sand (quartz grains) and shell fragments shell fragments are mostly calcium carbonate, so the sands are alkaline shell fragments dissolve, calcium moves down the profile and is deposited as lime cement, initially around plant roots, to form pipes, which are exposed by wind erosion of sand from the surface of dunes at or near the coast (i.e. the Quindalup dunes), or on the western margin of the Spearwood dunes The Pinnacles calcite is deposited to form limestone at the bottom of the profile of the dunes limestone at the base of the dunes is dissolved by water to form caves e.g. Yanchep Caves good quality water suitable for irrigation is stored in the caves, and is used to water gardens and parks on the Quindalup and Spearwood dunes, or to water vegetable crops grown on the Spearwood dunes Spearwood Dunes higher than the Quindalup and Bassendean dunes formed about 40,000 years ago, and comprise red/brown, yellow and pale yellow/grey sands sand grains can be coated with both iron and aluminium oxides, with the amount of iron oxide coating the sands largely responsible for the colour of the sands The greater the amount of iron coating the sand, the darker (red/brown) the colour iron and aluminium coating the sand grains increases the capacity of the sands to retain phosphorus (P) The darkest coloured red/brown sands occur on the far west of the Spearwood dune system, nearest the Quindalup dunes darker coloured sands are closely associated with limestone pipes and rock, and are exposed by wind erosion of the yellow sand that overlayed the darker sand and limestone eroded sand was blown inland, to expose the darker coloured sand and the limestone. The red/brown sands have been called Cottesloe Sands, or terra rosa soils yellow sands in this sequence are referred to as the Karrakatta Sands they become less distinctly coloured as they age because the iron (and aluminium) coating the sands is leached paler coloured Spearwood sands are farther inland red/brown sands are only found near the Quindalup dunes, and are typically deep in the profile (unless exposed by wind erosion) and are coated with the most amount of iron (and aluminium) the yellow phase of the Karrakatta sands, are to the west of the Spearwood dune system the yellow/grey phase of the Karrakatta sands, occur to the east

3 Bassendean Dunes these are the oldest of the three dune systems, thought to be about 800,000 years old, and so are the most leached, infertile and acidic sands contain little silt or clay, and very low levels of nutrient elements, with nutrient element content being associated with the presence of organic matter dunes typically form low-lying hills with poorly-drained areas between the hills. The depth to ground-water varies from close to the surface up to 10 m below the surface water is suitable for irrigation Three types of Bassendean Sands have been identified: Jandakot Sands: are well drained sands on the crests and upper slopes of the low hills or ridges: in their B horizon they have an iron podzol (contains iron oxide, which appears as a bright orange layer, that can include hardened concretions) the water table is about 10 m below the surface of the Jandakot sands Gavin Sands: Joel Sands: occur further down slope from the crest, where the water table is about 2 m below the surface. In the B horizon, an iron-humus podzol (a bright orange-brown layer which contains iron oxide and organic matter) occurs, which can be partly cemented (the constituents are compacted and bound together) often a sandy loam (loam texture due to organic matter), occurs in the lowest part of the dune system, where the water table is very close to the surface, and seasonal waterlogging occurs at the wettest time of the year (typically June to mid-august) The B horizon is a humus (soil organic matter that has been changed physically and chemically by soil animals, fungi, bacteria, algae and protozoa to humus) podzol, comprising a strongly cemented dark brown organic B horizon

4 Pinjarra Plain Soils soils are complex, and comprise a successive layering of soils formed from erosion of material from the scarp and east of the scarp. sivers and streams have mostly carried the eroded material, which is deposited from the water as fans of alluvium. she plain is made up of layers of soils of different ages and occupies about one third of the Swan Coastal Plain, and most of it has been cleared and sown to pasture for the grazing industries, mostly dairy, with some beef. Only small remnants of the indigenous (native) vegetation remain. the oldest soils on the Pinjarra Plain are the Coolup Soils. The profile of the Coolup soils is similar to that of the laterite formed east of the scarp, because similar material that gave rise to the laterite soils east of the scarp were eroded and deposited on the plain and therefore produced a similar profile. the profile is a sand over lateritic ironstone gravel over predominantly kaolinite clay, with a mottled zone in the clay layer just below the laterite mottles are formed from iron leached from the ironstone gravel. The iron is reprecipitated and may result in the formation of spherical iron nodules or concretions Coolup soils are highly leached and infertile, with low cation exchange capacities (CEC), with almost all nutrients coming from organic matter at the surface Coolup soil, in the top 30 cm, contains about 90% sand, 6% silt and 4% clay. It is classified as sand. below this, the soil comprises about 60% to 80% sand, 5% silt and from 15% to 31% clay, with the percentage clay content increasing with depth, so the subsoil is a sandy loam to a sandy clay loam. ACTIVITY: Construct soil profile columns for all sub-groups in this major category in conjunction with the cross-section Wellesley Soils are next oldest after the Coolup series they are: fine textured (high levels of silt and clay) alluvium dark brown/grey soils contain 45% to 55% clay, predominantly kaolinite surface of the soil is self-mulching, and large cracks appear in the surface when the soil is dry kaolinite does not expand and contract as the soil moisture content changes so the reason why the soils develop cracks when dry is not known, and cracks rapidly disappear as the soil gets wetter soils occur in the lowest parts of the western margin of the Plain soils are commonly wet and salty owing to water draining onto them from the area due east of the Wellesley soils where the newer soils described below are found Bassendean sands are usually found to the west of the Wellesley soils

5 these soils had a low ph near the surface, typically 4.5 increasing to near neutral ph with depth soils contain from 45% to 55% clay, and are classified as medium clay silt varies from about 25% near the surface (top 30 to 40 cm), to 10% at depth, sand varies from about 25% to 40% with depth Younger soils formed on the Plain are derived from alluvium deposited on top of the Wellesley and Coolup soils younger soils usually occur to the east of the Wellesley soils and west of the Coolup soils ACTIVITY: Define regolith, organic matter, kaolinite clay and other components of a soil matrix. The next soils formed are Boyanup Soils soils are loams that are bright yellow-brown, so they are distinctive and easily identified this is the major soil in the Harvey area they have a high silt content, which decreases from about 20% to 10% with depth, while the clay content increases from about 25% to 50% with depth Boyanup soils often have gilgai areas, which are scattered mounds and hollows which have been attributed to the shrinkage and swelling of the Wellesley soil present in the subsoil as the water content of the subsoil changes Blythwood Soils are the next soils formed these soils occur alongside present-day rivers and streams, and were formed from alluvium deposited when streams cut through the old Coolup system they are red, and contain a lot of sand (from about 80 per cent near the surface, to about 60 per cent at depth) throughout the soil profile soils are duplex soils, containing much less clay at the surface (top 30 cm) than at depth about 80% sand, 13% silt and 7% clay in the top 30 cm (loamy sand) compared with 10% silt and 20% to 35% clay below about 30 cm (loam to a clay loam) Belhus Soils are the next soils formed these soils are confined to terraces of the Swan River in the Swan Valley they are similar to the Blythwood soils, but they contain more sand and therefore are better drained, and are ideal for viticulture Dardanup Soils are next deposits, and occur as a broad alluvial fan over the older systems of soils these soils are found in the Dardanup, Harvey and Waroona areas they are dark reddish brown soils at the surface, becoming dark brown below about 30 cm

6 in the top 30 cm, the soils contain about 80% sand, 15% silt and 5% clay ( loamy sands) Below this they contain about 70% sand, 15% silt and 15% clay ( sandy loams) Pyrton Soils are similar to the Dardanup soils, but occur as low river terraces rather than as alluvium fans at depth, the Pyrton soils change abruptly from a dark brown sandy loam to a layer of fine white to pale yellow sand The final system of soils is the Vasse Soils soils differ from all the above soils because they are formed from estuarine alluvia, and are at about sea level found near the Vasse and Wonnerup Inlets, east of Busselton and north of Capel they are very variable soils, comprising layers of clay, shells, marine algae and coarse calcareous sand contain much organic matter, these were the first soils cleared in the area and were used to grow vegetables they are now mainly pastured and used for grazing Ridge Hill Shelf very narrow strip of soil, at most 3 km wide, adjacent to the scarp composed of coarse deeply weathered ironstone gravel, that makes up more than 50% of the soil by weight, and coarse sand the soil is very old, has a very poor water-holding capacity, and is extremely infertile this soil is typically present at the highest elevation of the Swan coastal plain the soils has poor water-holding capacity, so water is freely drained from the soil and it rapidly dries out after rain this soil is frequently dry, even in the growing season there is good quality water suitable for irrigation in these soils so they can be used for vegetable production

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