Soil Process Impacts
Soil acidity (or alkalinity) refers to the concentration of hydrogen ions (H+) in the soil. Measured on the pH scale, soil acidity is an invisible condition that directly affects soil fertility and toxicity by determining which elements in the soil are available for absorption by plants. Soil acidity increases (soil acidification) by: removal of agricultural product from the paddock, leaching of nitrogen as nitrate below the root zone; inappropriate use of nitrogenous fertilizers and build up of organic matter (Slattery & Hollier, 2002). Many of the soils in the Australian state of Victoria are naturally acidic, however about 30,000 square kilometres or 23% of Victoria’s agricultural soils suffer reduced productivity due to increased acidity (Slattery & Hollier 2002).
Soil acidification has an impact on of soil biodiversity. It reduces the numbers of most macrofauna including, for example, earthworm numbers (important in maintaining structural quality of the topsoil for plant growth). Also affected is rhizobium survival and persistence. Decomposition and nitrogen fixation may be reduced which affects the survival of native vegetation; biodiversity may further decline as certain weeds proliferate under declining native vegetation (Slattery & Hollier 2002; Hollier & Reid 2005).
In strongly acid soils the associated toxicity may lead to decreased plant cover, leaving the soil susceptible to erosion by water and wind. Extremely low pH soils may suffer from structural decline as a result of reduced microrganisms and organic matter – this brings a susceptibility to erosion under high rainfall events, drought and agricultural disturbance (Slattery & Hollier, 2002)
Soil structure decline
Well-developed, healthy soils are complex systems in which physical soil structure is as important as chemical content. Soil pores, which are maximised in a well-structured soil, allow oxygen and moisture to infiltrate to depths and plant roots to penetrate to obtain moisture and nutrients (Aplin 1998).
Biological activity helps in the maintenance of relatively open soil structure as well as facilitating decomposition and the transportation and transformation of soil nutrients. Changes in soil structure can lead to reduced accessibility by plants to necessary substances.
Traditional agricultural practices have generally caused declining soil structure: cultivation, for example, causes the mechanical mixing of the soil, compacting and sheering of aggregates and filling of pore spaces - organic matter is also exposed to a greater rate of decay and oxidation (Young & Young, 2001). Soil structure is essential to soil health and fertility – soil structure decline has a direct impact on soil and surface food chain and biodiversity as a consequence.
Soil sodicity refers the content of sodium in the soil compared to other cations, for example calcium. Sodium, compared to other cations, tends to cause soil particles to repel rather than attract each other – soils of high sodium content are said to be sodic to be of high sodicity. Sodicity may increase if additional sodium is introduced, such as under irrigation – increasing sodicity leads to soil structure decline, hard-setting of soils in dry conditions, erosion; all conditions that are detrimental to soil fertility, health and biodiversity.
Soil salinisation is the concentration of salt within the soil profile or on the soil surface. Excessive salt directly affects the composition of plants and animals due to varying salt tolerance – along with various physical and chemical changes to the soil including structural decline and in the extreme, denudation, exposure to soil erosion and export of salts to waterways. Soil salinity has localised and regional effects on biodiversity, ranging for example, from changes in plant composition and survival at a local discharge site through to regional changes in water quality and aquatic life.
Soil erosion leads to a loss of topsoil, organic matter and nutrients; it breaks down soil structure and decreases water storage capacity, in turn reducing fertility and the availability of water to plant roots. Soil erosion is therefore a major threat to biodiversity (NSW Government, 2006)
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