Laterite and General Notes on the Soil

Laterite and General Notes on the Soil in Southwestern Western Australia

Robert Gibson

The southwest of Western Australia is world famous for its wealth of endemic plant species, including an abundance of carnivorous plants. When viewed by those used to seeing Drosera and Utricularia confined to wet peaty soil of the temperate Northern Hemisphere the abundance and widespread distribution of the native carnivores is at first puzzling. Whilst peat bogs do occur in this region, they are of very limited extent. The majority of carnivorous plant habitats occur in seasonally moist to saturated soil which is composed largely of laterite or sand. In the following article the basic soil components in this botanic wonderland are described. As will be revealed it could be argued that the soil composition may have played a large role in the evolution of such a floral paradise.

There are five basic components of the soils in southwestern Australia: duricrust, clay, sand, rock fragments and organic matter. The combination of these in any one area is determined by the geological history, drainage and climate of the area. Over a short distance the soil composition and profile can change rapidly. There may be scarcely perceptible changes in slope, surface drainage or climate over this distance but there will probably be a dramatic change in plant species composition. The geology of this area has also been an important factor in the resultant soils. The most abundant rocks are granites and gneisses, which are rich in the minerals quartz and feldspar. Quartz is physically durable and generally chemically inert, whereas feldspars are slightly softer and weather readily to form clays. Linear belts of “greenstone” also occur, which are composed of iron-rich minerals.

Duricrust is probably the most well known element of the soil in this part of the World. It is a conspicuous surfacial layer formed by laterite weathering, and is sometimes itself called “laterite”. Lateritic weathering is an intense chemical breakdown and produces a very distinctive layered profile, and which is tied with the activity of ground water movement (Figure 1). Initially oxygen-rich rainwater filters through the soil and decaying rock, and under warm, seasonally humid conditions is able to dissolve most minerals. In doing so the level of oxygen decreases, affecting the solubility of some dissolved compounds, particularly iron oxides.

In surficial environments iron is generally oxidised to the insoluble ferric (+3) state, however, in oxygen-poor conditions it changes to the water-soluble ferrous (+2) state during which it is readily transported in ground water. Where iron-rich rocks are present around the granite plutons of this region, and insoluble iron oxide and hydroxide chemical lag may form. In other cases all surficial and near-surface iron is dissolved, and eventually reprecipitated where the ground water reaches the surface and a higher oxygen concentration. The duricrust, or ferricrete, forms, and which is highly resistant to physical weathering. It may be massive or granular in texture, the later are known as “pisolites”, and often includes oxides and hydroxides of aluminium (White, 1994). In this chemical environment quartz and calcite may also be precipitated just below the oils surface and form “silcrete” and “calcrete” respectively.

Where the oxygen-poor ground water moves through the decaying rock it removes all but insoluble clay, which forms a layer known as “saprolite”. At depth, relict original rock textures are still retained forming a basal layer of “saprock”. This is adjacent to the top of the fresh rock, which may be tens of metres below the surface. In general lateritic weathering represents a significant volume reduction of rock. It appears to be a fossil. It appears that laterite weathering throughout southwestern Australia is a fossil feature which was produced when the region experienced a monsoonal climate; this changed relatively recently, within the last 4 million years to the present Mediterranean climate.

With a change in climate and a reduction, or cessation, of lateritic weathering this distinctive soil and rock profile has been progressively dissected (Figure 2). Once the hard duricrust cap has been breached physical erosion of the clay zone occurred rapidly, and eventually fresh rock was exposed. During lateritic weathering the majority of minerals in granites and gneisses were dissolved, including quartz. However, with a change in weathering regime the layers of the lateritic weathering profile were locally stripped away revealing fresh rock. This began to break down but, this time, it yielded rock fragments, clay and quartz sand. The various mineral components: duricrust, clay, rock fragments and quartz sand were variably transported, deposited and concentrated throughout the landscape producing complex soil profiles.

Plant matter is another important soil component which is produced in most abundance, and accumulates most quickly along drainage lines, in swamps, and in creek and river beds. The locally produced peat is composed primarily of the remains of sedges, small mosses and other wetland plant species. It is worth noting that whilst sphagnum moss does occur in Western Australia, it is only recorded from two small sites on the extreme south coast and is thus not a significant peat-forming species.

The nutrient value of most of the soil components within this region is very low. Lateritic duricrust has been intensely weathered and all soluble minerals have been removed. Quartz sand is intrinsically inert. Peat in the wetlands comprises a store of plant nutrients which are locked up until slow decay or fire releases them. Due to the mineralogy of the underlying granites and gneisses, even the more mineral-rich clays and rock fragments are still poor in many essential plant nutrients.

The physical properties of the soil components plays an important role in producing a variable soil terrain. The lateritic duricrust, where coherent, generally forms a barrier to water movement, which is then concentrated in cracks and clefts. In contrast a layer of pisolites is highly porous and acts as a mineral mulch. Quartz sand is also highly porous and, where it occurs above impervious fresh rock, clay or coherent lateritic duricrust, it may hold significant volumes of water. Similarly peat accumulations act like a sponge in storing water. Fresh granite and gneiss are generally impervious, forming large rounded outcrops, however, water may be stored with fractures and along geological contacts, and is often temporarily stored with the surface layer of mosses and herbs where they are established. Clay rich soils often have good water storage ability, and may become seasonally waterlogged where hard pans are developed. During the seasonal summer drought the soil is exposed to intense solar radiation. Due to the low thermal conductivity of quartz sand, lateritic duricrust, and dry peat the soil surface may become extremely hot (over 50°C) whilst the a few centimetres down soil conditions are cool and humid.

The scene is now set to look at a few factors in the carnivorous plant diversity in southwestern Western Australia. In general soils are nutrient poor, with variable water holding ability, but winter rainfall and the air temperature during the coolest 6 months of the year are conducive to plant growth. As a general rule it is now thought that where nutrient-poor soils are developed no single species is able to dominate the landscape, instead co-dominance is the rule, with many species growing together in a small area. This is also well developed in such places as the Cape Province of South Africa and the sandstone plateaux of southeastern Brazil. Even with the advent of the carnivorous syndrome, this small, but most interesting group of plants has not been able take up the role as the dominant plant group. At least not yet.

Throughout southwestern Western Australia the carnivorous plant flora has evolved and spread into the most environments; an achievement helped greatly by the development of survival mechanisms for the seasonal summer drought. The two main groups of Drosera survive the summer by retreating to tubers safely below the scorching heat, or in the heat-resistant stipule buds of pygmy sundews. Most Utricularia and sundews also survive as seeds which lodge in cracks in the soil. Byblis gigantea survives as swollen roots deep within the soil. In general, the native carnivorous plants occur in great diversity and abundance in a great range of different environments.

Each of the main soil types is utilised by a range of carnivorous plants; a few examples are now given. The coherent lateritic duricrust is home to such species as D. barbigera, D. miniata, D. stolonifera ssp. porrecta. Pisolitic laterite soil supports D. hyperostigma, D. erythrorhiza ssp magna, D. erythrorhiza ssp squamosa, D. erythrorhiza ssp “Roleystone Red”, D. stolonifera ssp porrecta. Thin quartz sand over coherent laterite supports the northern population of Byblis gigantea. : Drosera macrophylla is a great coloniser of exposed saprolite. Clefts in fresh exposed granite and gneiss are home to D. macrantha ssp macrantha, D. menziesii ssp menziesii, Utricularia menziesii and D. browniana. Accumulations of rock fragments supports D. microphila (around Esperance), D. scorpiodes and D. macrantha ssp macrantha. Deep quartz sand is the preferred soil of D. zonaria, D. erythrorhiza ssp erythrorhiza, D. stolonifera ssp humilis D. enneaba and D. paleacea ssp paleacea. Seasonally wet peaty sand is the preferred habitat for D. rosulata, D. helodes, D. nitidula ssp nitidula and the southern population of Byblis gigantea. Permanently to seasonally wet peat is the preferred medium for Cephalotus follicularis, Utricularia volubilis, D. hamiltonii, D. binata, D. pulchella, D. gigantea, D. stolonifera ssp monticola and D. neesii ssp neesii. This list is incomplete and many of the native carnivorous plant species grow in several different soil combinations.

The five soil components: lateritic duricrust, clay, rock fragments, quartz sand and organic matter within southwestern Western Australia have formed together under differing conditions, some of which are no longer active. They now occur as a complex mosaic, with highly variable physical conditions, but similar low nutrient status, which supports an incredible diversity of plants which exist in co-dominance. This also applies to the native carnivorous plant flora. It is hoped that this outline will assist in knowledge of the position of these plants in the wild and in their successful cultivation.

Reference:

White, M. E. 1994. After the Greening: the Browning of Australia. Kangaroo Press, Sydney.

Figure Captions:

Figure 1: Active Tertiary laterite weathering profile over granite. Strong chemical leaching, from oxygenated ground water dissolves the majority of rock-forming minerals as it passes through a layer of decaying rock. Clay-rich saprolite is left behind. Insoluble iron oxide-rich duricrust forms an insoluble chemical lag (shown by diagonal lines and circles), and may also be precipitated in accumulated sediment when the ground water reach the sediment. Arrows indicate the general movement of ground water. At depth, fresh rock occurs (indicated by crosses).

Figure 2: After a change in climate, physical weathering dominated over nominal chemical changes. Laterite weathering ceases, and its profile is dissected. this leads to highly diverse soil profile changes where fresh rock, duricrust, saprolite and saprock and variable exposed and covered by a mixture of their own fragmented residue. In addition, wind and water-sorted quartz sand (stippled) and peat accumulations (shown in black) may locally occur. Carnivorous plants have evolved to fill niches in almost all of these soil types.