Making Sense of Chemical Indicators

Most indicators of soil chemical quality measure dynamic soil properties i.e. properties that change over time and with management. These indicators are used to guide management decisions over the period of a rotation. It is important to monitor these indicators as they can act as constraints to yield, restricting crop growth and preventing the yield potential from being achieved.

Making Sense of Chemical Indicators - Qld

Most indicators of soil chemical quality measure dynamic soil properties i.e. properties that change over time and with management. These indicators are used to guide management decisions over the period of a rotation. It is important to monitor these indicators as they can act as constraints to yield, restricting crop growth and preventing the yield potential from being achieved.

Cations and Cation Exchange Capacity

Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants.

Cations and Cation Exchange Capacity - Tas

Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants.

Cations and Cation Exchange Capacity - Qld

Cation exchange capacity (CEC) is a measure of the soil’s ability to hold positively charged ions. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants.

Soil Acidity

Soil acidity is a major environmental and economic concern. Approximately 50 % of Australian agricultural land or 50 million ha have surface pH values less than or equal to 5.5 which is below the optimal level to prevent subsoil acidification. If untreated, acidity will become a problem in the subsurface soils, which are more difficult and expensive to ameliorate. Subsurface acidity is already a major problem for large areas of Western Australia and New South Wales. It is estimated that 12 to 24 million ha is extremely to highly acidic with pH values less than or equal to 4.8.

Soil Acidity - Tas

Soil acidity is a major environmental and economic concern. Approximately 50 % of Australian agricultural land or 50 million hectares have surface pH values less than the optimal level to prevent subsurface acidification. Research in Tasmania indicates that the trend for topsoil pH in cropping areas is for increasing pH due to the history of lime applications in these areas, in contrast to large areas of cropping country on mainland Australia. If untreated, acidity will become a problem in the subsurface soils, which are more difficult and expensive to ameliorate.

Soil Acidity - Qld

Queensland has more than 500 000 hectares of agricultural and pastoral land that has acidified or is at risk of acidification. Soils most at risk are lighter-textured sands and loams with low organic matter levels, and the naturally acidic red clay loam soils commonly found in areas such as the South Burnett and Atherton Tableland. Soils least at risk are the neutral to alkaline clay soils (e.g. brigalow soils and the black clay soils of the Darling Downs and Central Queensland).

Acidic soils cause significant losses in production, and where the choice of crops is restricted to acid tolerant species and varieties, profitable market opportunities may be reduced. In pastures grown on acidic soils, production will be reduced and some legume species may fail to persist.

Soil pH - SA

Acid soils occur in areas of South Australia with high rainfall where basic ions (sodium, potassium, magnesium and calcium) have been removed by leaching. Nitrate leaching also contributes to significant soil acidification under high rainfall. Very frequent legume cropping can reduce pH in non-calcareous soils. Soils high in sulphur may become very acidic due to the dominance of certain chemical (oxidation-reduction) reactions.
Soil alkalinity is mainly caused by bicarbonates and carbonates, although phosphates, borates and some organic molecules can contribute. In a soil with pH from 7 to 8.2, bicarbonates and carbonates of calcium and magnesium dominate. Problems are encountered in alkaline soils when sodium occurs or accumulates and forms salts such as sodium bicarbonate and sodium carbonate. These are highly soluble and increase the soil pH above 8.

Soil Acidity - NSW

In crop production, acidic soils cause significant yield penalties, and can prevent legumes capturing nitrogen. In pastures, production is reduced, deep-rooted species that increase water usage may not thrive, and some legumes may fail to persist.

pH should be maintained above 5.2 in the topsoil and 4.8 in the subsoil to maintain the soil resource and avoid production losses.

Managing Soil Acidity - WA

Soil acidification is an ongoing and unavoidable result of productive agriculture. The main practices that cause soil acidification include removing harvested products and leaching of nitrate from soil. Because soil acidification is an ongoing result of farming, management also needs to be ongoing.

Water Repellency

Water repellent soils occupy more than 5 million hectares of western and southern Australia. Decomposition of hydrophobic (or water repelling) waxy materials originating from plant residues can coat soil particles preventing the infiltration of water into the soil profile. Soils with a small surface area (e.g. sand) are more prone to water repellency as it takes less hydrophobic material to coat individual particles, compared to silt or clay.
The result of water repellency is generally uneven water distribution in the soil profile which leads to patchy and uneven plant emergence. Moving from an alternately wet and dry soil can make it difficult to control the depth of sowing causing further problems with establishment. Water can remain ponded on the soil surface to be evaporated or lost as runoff. Lack of plant cover and heavy autumn or summer rains can result in significant runoff and erosion on sloping sites.

Salinity - NSW

A saline soil is one that contains sufficient soluble salts (most commonly sodium chloride) to adversely affect the growth of most plants. Salinity reduces a plant’s ability to extract water from the soil and can cause toxicities from specific ions. The point at which the growth of different plants is adversely affected varies. Some species are very sensitive to salt while others, such as saltbush, are very salt-tolerant.

Soils develop salinity via interactions with groundwater. If groundwater rises to within 2 metres of the soil surface, water can rise by capillary action to the surface. When this happens, the water can bring salt into the root zone and when the water evaporates at the soil surface, concentrated salts are left behind.

Optimising Soil Nutrition

Most soils contain reserves of nutrients that would otherwise have to be applied in fertilisers. Knowledge of your soil characteristics can aid in accessing these nutrients.

Growers can employ a range of management strategies to ameliorate and manage soil constraints to minimise negative yield impacts. Some of these include liming for soil acidity, deep ripping and using rotations to minimise soil borne pathogens.

Optimising Soil Nutrition - Qld

• Most soils contain reserves of nutrients that would otherwise have to be applied in fertilisers.
• Growers can minimise fertiliser inputs by optimising the use of these nutrient reserves by plants.
• Maximising root abundance and rooting depth means roots can take up nutrients and water in the subsoil.
• Use of soil reserves of nitrogen can be optimised by testing for the different forms of nitrogen and using the results when deciding on fertiliser application rates.
• Good yield estimates are critical when estimating nitrogen requirements.

Nitrogen - WA

Over 98 % of all nitrogen present in the soil is in an organic form (i.e. contains carbon in its molecular structure). Organic nitrogen cannot be taken up by the plant until it is mineralized by microbes into inorganic forms (principally nitrate (NO₃) and ammonia (NH₄)). Mineralisation occurs throughout the growing season providing a steady, continuous supply of nitrogen to the crop. This process can be stimulated significantly by cultivation.

Nitrogen - Qld

Over 98 % of all nitrogen (N) present in the soil is in an organic form. This organic N is converted (mineralised), by microbes into inorganic forms such as nitrate and ammonia, which are then available for crop use. Mineralisation can occur throughout the growing season providing a steady, continuous supply of nitrogen to the crop.

The positively charged ammonium ion (NH₄⁺)is immobile in soil, does not leach, and is an intermediary in the conversion of organic-N to nitrate (NO₃^-). Nitrate on the other hand has a negative charge and is highly mobile in the soil. This mobility provides a nitrogen source that moves readily towards plant roots, but can also be leached out of reach of the plant root system. Nitrate is used by the plant to make precursors to plant proteins.

Nitrogen - NSW

Nitrogen is used by the plant to make precursors to plant proteins. As proteins make up much of the content of cells, nitrogen is needed in greater quantity that any other mineral nutrient. Nitrogen is highly mobile within the growing plant allowing it to move the nitrogen to tissues that can use it more effectively. Older leaves tend to exhibit nitrogen deficiency symptoms first. Nitrogen plays an essential role in the production of chlorophyll; deficiency is displayed as yellowing leaves and reduced tillering , and leads to reduced crop yields.

Phosphorus - WA

Many soils in Australia and particularly Western Australia are ancient and highly weathered with very low levels of natural phosphorus. As a result, these soils were amongst the most acutely phosphorus deficient found anywhere in the world when cleared for agriculture. Profitable crop production on most wheat-belt soils of Western Australia has only been possible by applying significant quantities of phosphorous fertilisers. More recently it has also been a requirement especially on sandy soils in high rainfall areas that detrimental impacts of fertiliser both on-site and off-site must be minimised if utilisation of the soil resource is to be sustainable. This will only be achieved if fertiliser input is matched to crop and pasture demands.

Phosphorus - Tas

Ancient and highly weathered soils with very low levels of natural phosphorus (P) dominate much of Australia. Many of our agricultural soils are therefore among the most acutely phosphorus deficient in the world, and profitable crop production has only been possible through significant applications of P-fertilisers. Phosphorus is an essential element for plants and animals for cell division and growth. Complex soil process influence the availability of phosphorus applied to the soil, with many soils able to ‘tie up’ phosphorus, making it unavailable to plants. Your soil’s ability to do this must be accounted for when determining requirements for crops and pastures.

Phosphorus - Qld

Ancient and highly weathered soils with very low levels of natural phosphorus dominate much of Australia, particularly Western Australia. Many of our agricultural soils are among the most acutely phosphorus deficient in the world, and profitable crop production has only been possible through significant applications of phosphorus-fertilisers.

Phosphorus is an essential element for plant and animal growth and important during cell division and growth. Complex soil process influence the availability of phosphorus applied to the soil, with many soils able to ‘tie up’ phosphorus, making it unavailable to plants. Your soil’s ability to do this must be measured when determining requirements for crops and pastures.

Phosphorus - NSW

Phosphorus is essential for plant growth, but few Australian soils have enough phosphorus for sustained crop and pasture production. Complex soil processes influence the availability of phosphorus applied to the soil, with many soils able to adsorb or ‘fix’ phosphorus, making it less available to plants. A soil’s ability to fix phosphorus must be measured when determining requirements for crops and pastures.

Soil Phosphorus Availability by DGT

Phosphorus (P) is a particularly complex nutrient in the soil because large amounts can be ‘locked’ up making it unavailable to plants. Measurement of P in soil, and determining fertiliser requirements, are therefore difficult as not all P measured will be available for plant uptake.

Diffusive Gradient in Thin-Films (DGT) is a new method of measuring soil phosphorus and other micronutrient levels, with tests showing it to be a more accurate measure of the growing crops nutrient requirement than more conventional methods. DGT-P is different to standard measures of soil phosphorus as it reflects the capacity of the soil to supply P in the soil rather than to the concentration of phosphorus in an extract. The DGT-P method aims to ‘mimic’ the plant root by only measuring the phosphorus in the soil that is accessible to the plant.

Potassium

Potassium (K) is one of the essential nutrients in plants. In recent years it has been identified as one of three nutrients in sufficiently short supply to limit crop yields on many soil types in Western Australia. Potassium is found in plants at levels above all other macro nutrients except carbon, oxygen, hydrogen and occasionally nitrogen.

Potassium has many functions including regulating opening and closing of stomata which are the breathing holes found on plant leaves and regulate moisture loss from the plant.

Potassium - Tas

Potassium (K) is one of the essential nutrients in plants and is one of three nutrients (including nitrogen and phosphorus) that are commonly in sufficiently short supply in the soil to limit crop yields on many soil types in Tasmania. Compared to other nutrients plants need relatively large amounts of K and over the years many soils have been depleted of K because of K removal in farm produce. Potassium has many functions including the regulation of the opening and closing of stomata which are the breathing holes in plant leaves that regulate moisture loss from the plant. For this reason K is known as the poor man’s irrigation because it can help crops tolerate dry spells.

Potassium - Qld

• Potassium deficiency is an emerging issue in Queensland cropping soils.
• Soil and plant testing is the most effective means of determining potassium requirements.
• It is important to maintain adequate potassium in soil as once deficiency symptoms emerge, costly fertiliser applications will be required.

Potassium - NSW

Potassium (K) is an essential plant nutrient. Potassium has many functions including the regulation of the opening and closing of stomata, the breathing holes on plant leaves that control moisture loss from the plant. Adequate potassium increases vigour and disease resistance of plants, helps to form and move starches, sugars and oils. Available potassium exists as an exchangeable cation associated with clay particles and humus.

Boron - WA

Boron (B) is essential for crop growth and development but in very small quantities. While the precise role of boron in plants is not fully known there is evidence to shown that boron is important for cell division, the production of nucleic acids (DNA, RNA), the movement of sugars across membranes and the development of reproductive structures (i.e. pollen tubes, fruit).

Molybdenum - NSW

Molybdenum is one of the ‘minor’ nutrients plants require for normal growth. Minor nutrients are only required in very small amounts, as little as 50 g/ha of molybdenum will satisfy the needs of most crops. Molybdenum is present in farmyard manure, in composts and mulches, in some chemical fertilisers and in seeds and other planting material, such as tubers and corms.