Soil Chemical Attributes
Soil Salinity
In this module, aspects of naturally occurring salinity or 'primary salinity' and induced or 'secondary salinity' are discussed. Salts accumulate through evaporation of water from soil and transpiration by plants. High rainfall environments are often able to leach these salts from the root zone, but in dryland agricultural systems this is often not possible, particularly on soils high in clay. Salinity refers to the presence of soluble salts in soil and/or water, and can result in reduced plant growth or even death due to low water availability and ion toxicity.
Soil pH
Soil pH describes the soil's acidity or alkalinity. Extremely high or low pH (on a scale from 0 to 14, and in which 7 is neutral) can have a negative effect on the health of plants and soil biota. In soils with pH below 5.5, acid-sensitive agricultural plants are adversely affected and the risk of subsoil acidification increases. Soil pH is an important indicator of chemical processes that occur in soil, affecting soil processes governing nutrient availability. Although soil acidification is a natural process, it can be accelerated under agriculture. Soil pH represents a signficant constraint to production in many areas of Australia, with the economic loss from soil acidification across Australia, estimated to be up to six times higher than from dryland salinity.
Soil and Water Pollutants
By definition, pollutants are organic or inorganic substances present in harmful amounts (above background concentrations) within air, water and soil. Land contamination is defined as land that has a pollutant (or pollutants) at above-background concentrations causing, or with the potential to cause, adverse impacts to human health, the environment or any environmental value. The main causes of land and water contamination are largely a result of poor or inadequate practices linked with human habitation and development. Industry, mining, forestry, intensive agriculture (e.g. feedlots, horticulture, broadacre farming), roadworks and urban development all have the potential to pollute soils and waterways, if poorly managed .
Soil Fertility
Soil fertility can be used to describe the availability of nutrients for plant uptake and in the broader context can be derived from soil type, soil organic matter status, soil properties including pH, as well as the concentration of available nutrients and should be reflective of land use. The fertility of a soil reflects not only the turnover of organic and inorganic inputs, but also the management of soil condition.
The conversion of natural vegetation to agricultural land uses, changes soil processes and function to support a much less diversified but integral ecosystem. Soil fertility has over time become increasingly depleted in soil organic matter and nutrients, and inappropriate soil and plant management practices have often contributed to the development of constraints to continued production.
Nitrogen
Australian agricultural soils have become depleted in nitrogen (N) and organic matter over time, and in many situations are unable to supply sufficient N to meet crop requirements. Nitrogen is required for healthy plant growth and physiological development, and less than optimal amounts will constrain potential yield and protein quality. Oversupply can also adversely affect plant growth and yield. Farmers should aim to apply N to meet crop demand in order to reduce environmental losses and maximise fertiliser efficiency.
Phosphorus
Profitable production of grain and pasture crops in many regions has relied on the application of phosphorus (P), particularly in high rainfall (> 800 mm) areas and on highly weathered soils with very low levels of natural P. Native pastures are adapted to these low levels, but introduced crops and pastures are not. Phosphorus often reacts rapidly with other elements such as calcium and iron in soil, and can result in less than 5% of the total P applied being taken up by plants in the year of application. This adsorption effect increases with declining soil pH.
Potassium
A greater proportion of potassium (K) accumulates in the vegetative parts of a mature plants. Potassium is highly mobile in the soil and is readily leached from stubble and during the breakdown of organic matter - unless crops are cut for hay, or the stubble is burnt or harvested for feed prior to K leaching. This is particularly critical within the first month after harvest.
Sulfur
Regular use of superphosphate fertilisers containing S, has meant that S deficiencies were often not observed. However, because of the use of low S fertilisers (e.g. triple super, DAP and MAP) and the growing of canola which has a high sulfur requirement, S deficiency has now become more evident in many agricultural soils. Expression of S deficiency is more likely in soils with high N availability.
Recycled Nutrients
Current farming systems largely rely on energy-intensive inorganic fertilisers to supply nutrients, due to ease of application and ability to time applications in response to plant demand. However, increasingly organic wastes and manures are being considered as a lower cost, more environmentally friendly source of nutrients.