Soil testing sways decisions

By Dairy News

SOIL TESTING provides valuable information on important soil characteristics and can greatly assist in cost-effective decisions about fertilisers and other soil additions on your farm.

It is essential to collect soil samples which accurately represent the paddock or area being assessed.

In pastures, soil samples are generally collected at a standard depth of 10 cm. This information aims to help you interpret standard soil test information.

Soil type

Soil type is reported as the colour and the texture of the soil.

Colour and texture are indicators of properties of the soil and are taken into account when interpreting other soil chemical results.

Organic carbon

Organic carbon is a measure of the organic matter present in soil.

Organic matter results from partly decayed plant and animal residues in various degrees of decomposition.

Soil organic matter assists in maintaining soil structure and the supply and retention of nutrients, air and water.

If a soil is low in organic matter, the soil test will result in a low organic carbon level.

Soil pH

Soil pH is a measure of the alkalinity or acidity of the soil.

A pH value of 7 is neutral. Values below 7 are defined as acidic and those above are alkaline.

The soil pH can influence the availability of nutrients to plants and potential toxicity of aluminium and hydrogen.

In most Australian soil tests, the pH of the soil is measured in water (pH water) or calcium chloride (pH CaCl2).

Soil pH CaCl2 values are usually between 0.5 to 1.1 units lower than pH (water).

The pH(water) value readily reflects current soil conditions, but is subject to seasonal variations.

The CaCl2 test is useful for long-term monitoring of pH and is less subject to seasonal variations.

Aim to keep the pH level above 5.3 (water) or 4.5 (CaCl2).

Available phosphorus

Phosphorus is essential for plant growth and is vital for early root formation.

Soil minerals can react strongly with applied phosphorus and only a small proportion may be available for plant uptake.

In Victorian pasture soils, plant available phosphorus is usually tested using the Olsen P test and results are presented in milligrams per kilogram (mg/kg) or parts per million (ppm).

Available potassium

Potassium is needed for a wide range of important processes within the plant including cell wall development, flowering and seed set.

Available potassium is measured by the Colwell K or Exchangeable K soil tests.

Because the holding and supply capacity of potassium in soils can differ, the appropriate target for available potassium depends on soil type.

When potassium levels are high, potassium inputs can be reduced from the fertiliser regime until levels fall.

Available sulphur

Sulphur is essential for nitrogen fixation by legumes. It is usually measured by the potassium chloride (KCl 40) test, and is reported as mg/kg.

This test takes into account some of the sulphur that will become available during the growing season from the breakdown of organic forms of sulphur.

Sulphur is considered adequate when the levels are > 4 mg/kg using the CPC test.

Sulphur is considered adequate when the levels are > 8 mg/kg using the Blair (KCl 40) test.

Phosphorus buffering index

When phosphorus is applied to soils as fertiliser, it reacts with soil components and becomes less available for plant uptake.

This reactivity between applied phosphorus and the soil is called ‘phosphorus buffering capacity’ and is measured by the Phosphorus Buffering Index (PBI) soil test.

Consequently, a soil with a high PBI value will require more phosphorus fertiliser than a soil with a low PBI.

– Agriculture Victoria