16.1  Soil formation

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Soils form in response to interactions between soil-forming factors such as climate, parent material, biotic inputs, top-og-raphy and time. Outcomes from this ‘pedogenesis’ fall into four broad groups, entisols, oxisols, vertisols and alfisols (Table 16.1, Figure 16.1). Evolution of natural ecosystems and agri-cultural options vary accordingly, with more productive communities based on sites that are richer in plant-available nutrients.

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Table 16.1

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Figure 16.1 Soils in Australia comprise four major groups: (a) entisols (deep sands), (b) vertisols (cracking clays), c) oxisols (massive sequioxidic soils) and (d) alfisols (texture contrast soils, and in this case with differential development of the A horizon). (Based on Hubble et al. 1983, a source with a much more comprehensive account of soil types in Australia)

Nutrients for plant growth are derived from minerals in parent material by weathering, and four main processes are involved: oxidation/reduction, hydrolysis, solution and hydration. Minerals such as kaolinite and iron oxides tend to accumulate in soils because of their resistance to weathering but they are low in plant nutrients.

Elements released from parent material migrate during pedogenesis according to their mobility and solubility which in turn are functions of the pH and oxidation/reduction (redox) potential of the soil system. Basic cations such as Ca2+ and Mg2+ and elements such as P, S and Cl do not form insoluble hydroxides and are readily leached, whereas Al, Fe and Mn form insoluble hydroxides and accumulate.

Compared to New Zealand and much of the northern hemisphere, parent materials of Australian soils are old and weathered. Where the northern hemisphere was swept clean by glaciation during the Pleistocene, parent materials have been weathering for less than 10 000 years. By contrast, soils of inland Australia have formed on parent materials that have experienced many cycles of weathering, erosion and deposition. Some of these weathered materials have been either deposited or exposed (by denudation of the landscape) only recently. Soils formed on parent materials such as these unconsolidated sediments or sedimentary rocks are young in terms of pedogenesis but nevertheless low in nutrients, a feature usually associated with soils that have been developing for tens of thousands of years. In other areas such as central Queensland, red and yellow earths (oxisols) have been developing in excess of one million years. Texture-contrast soils (alfisols and ultisols), so common around the eastern seaboard and used for dryland grain cropping, have taken in excess of 25 000 years to form. By contrast, soils of New Zealand are young inasmuch as they have formed on recently deposited or recently exposed materials, but are nutrient rich compared to Australia. For example, a large area of the North Island is mantled by pumice with resulting soils <4000 years old. Volcanic loamy clay soils in the same region can be >50 000 years old.

Because so many soils have been developing for such a long time, soil characteristics cannot be attributed to recent climate, and as a corollary present-day distribution of soils cannot be explained by climatic variables alone (largely rainfall and temperature). Nevertheless, abrupt boundaries between a number of major soil zones in southeastern Australia are attributable to major changes in chemical and/or physical properties of parent materials.

Soil nutrient status is often closely correlated with parent material. For example, in eastern New South Wales and southern Queensland, chocolate soils (mollisols) and black earths (vertisols) occur only on basic parent rocks such as basalts whereas the red-brown earths (alfisols) occur only on granites and acidic sediments. These contrasting parent materials produce sharp differences in nutrient resources of resulting soils. Average P content (surface horizon) of a black earth is 870 ppm compared with a red-brown earth where P is usually <200 ppm (Table 16.1).

Parent material exerts a strong influence on pedogenesis in drier climates and especially at the beginning of soil devel-opment. In particular, chemical characteristics of parent rock or sediment are largely responsible for chemical fertility of resulting soils. Soils developed over rocks and sediments high in ferromagnesian minerals such as olivine and augite have high levels of basic cations and trace elements necessary for plant growth. They are also the least resistant to chemical and physical breakdown. Quartz, muscovite, zircon, tourmaline and iron oxides such as goethite and hematite are the most stable and also contain only minor amounts of the elements needed for plant growth.

Physical and chemical characteristics of parent rocks can also have a marked influence on physical fertility of soils. Resistance to erosive agents (wind and water) is largely a function of aggregate stability, a property which is dependent on organic matter, divalent cations and clay type. Formation of water-stable aggregates is also greatly enhanced by pro-liferation of plant roots and their associated filamentous micro-organisms. Regularly cropped soils thus benefit greatly from a pasture (especially grass) phase.

Development of agriculture and horticulture in Australia has been markedly influenced by two soil characteristics related to the nature of the parent material, namely salinity and nutrient status. Large areas of mainland Australia are mantled by saline and/or sodic soils which have developed from sedimentary materials in which salts have accumulated over millions of years. In many areas where surface soils were not
initially saline or sodic, secondary salinisation has followed land clearing. Their potential for salinisation and sodification is high because of a saline water table close to the surface (Section 17.1).

By world standards, large areas of Australia are deficient in major nutrients (P, N, K and S) as well as trace elements (Mo, Cu, Zn and Mn). To underline this contrast, the overall P average of Australian soils is around 300 ppm whereas average P in the USA is >500 ppm and in England about 650 ppm. Low nutrient status, especially P and trace elements, is largely a consequence of time and parent material. Weathering of primary minerals and leaching of products in Late Tertiary periods have left many profiles low in basic cations, trace elements and P. These deficiencies have exercised a strong selection pressure in nature, and call for major nutrient additions in agriculture. An appreciation of soil–plant interactions ensures that such additions are used to best effect.

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