Understanding soil tests for pastures

Soil testing provides valuable information on important soil characteristics. The results will greatly assist in cost effective decisions about fertilisers and other soil additions on your farm. It is essential to collect soil samples that accurately represent the paddock or area being assessed. In pastures, soil samples are generally collected at a standard depth of 10cm.

Soil type

Soil type is reported as the colour and the texture of the soil. Both 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. It assists in maintaining:

• soil structure
• the supply and retention of nutrients
• air
• water

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

Table 1. Organic carbon percentages (%) over a range of conditions

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).

Table 2. Ranges of Olsen P and their availability to plants

Available potassium

Potassium is needed for a wide range of important processes within the plant including:

• cell wall development
• flowering
• seed set.

Available potassium is measured by the Colwell K or Exchangeable K soil tests. The appropriate target for available potassium depends on soil type, because the holding and supply capacity of potassium in soils can differ.

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

Table 3. Soil type and target levels of potassium (mg/kg)

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.

Table 4. Phosphorus Buffering Index (PBI) classes

Cation exchange capacity

The cation exchange capacity (CEC) of a soil is the measure of the soil's capacity to hold important cations (positively charged ions) such as:

• calcium
• magnesium
• sodium
• potassium.

Some laboratories also include aluminium. The CEC measure provides an indication of the types and amount of calcium, potassium and magnesium available and associated ratios. Exchangeable sodium is useful for determining potential soil structural problems.

The CEC of the soil is largely dependent on the amount and type of clay and organic matter that is present. The exchangeable cations are usually reported in meq/100g soil and also as a percentage of total cations.

Exchangeable calcium

Calcium is a necessary plant nutrient. It plays a key role in maintaining soil structure and is generally present in high concentrations in the soil solution — even at low pH.

Exchangeable calcium should be 5 to 10 meq/100g and in the range of 65 to 80% of the total cations present.

Exchangeable magnesium

Magnesium is also a necessary plant nutrient. It is usually present in sufficient quantities to satisfy plant requirements.

• Exchangeable magnesium should be 1 to 3 meq/100g and in the range of 10 to 20% of the total cations present.
• If exchangeable magnesium is more than 20% of the sum of cations present, it may result in potassium deficiency in plants and animals.

Exchangeable potassium

Potassium is an essential plant nutrient and is required in larger amounts.

• Exchangeable potassium should be 0.3 to 0.7 meq/100g and in the range of 3 to 8% of the total cations present.
• If the exchangeable potassium level is more than 10% of the sum of cations, it may cause magnesium deficiency in plants and animals.

Exchangeable sodium

Ideally exchangeable sodium should be <0.7 meq/100g and less than 1% of the total cations present.

• If sodium makes up 6% or more of the total cations present, then the soil may be sodic and susceptible to dispersion — where a soil may lose structural integrity, compact and form surface crusts.
• The application of gypsum (CaSO4) can help alleviate excess sodium in the short term.

Exchangeable aluminium

High exchangeable aluminium concentrations can be common in very low pH soils, and may be toxic to plants. High aluminium levels can be reduced by applying lime.

• Aluminium levels generally fall to harmless levels once the pH(water) exceeds 5.6 to 5.8.
• The exchangeable aluminium level should be less than 1% of the CEC.

If the exchangeable aluminium level is 10% or higher, lime should be applied to ensure good pasture growing conditions

Calcium to magnesium ratio

Well structured soils generally have twice the amount of exchangeable calcium to exchangeable magnesium.

• If the calcium to magnesium ratio is less than 2:1, then this may indicate reduced soil stability.
• In contrast, a calcium to magnesium ratio of more than 10:1 indicates a potential magnesium deficiency in plants and animals.

Magnesium to potassium ratio

The amount of magnesium should be one and a half times greater than the amount of potassium.

If the ratio of magnesium to potassium is less than 1.5:1 — this indicates an increased likelihood of magnesium deficiency in plants and potential grass tetany in classes of grazing animals.

Salinity

Soil salinity is a measure of the total soluble salts present. High levels of soluble salts in the root zone may affect water and nutrient uptake and adversely affect plant growth. Plants are more susceptible to salinity in their germination and seedling stage than in later stages of growth.

Soil salinity is generally determined by measuring the electrical conductivity (EC) of the soil sample, with results deciSiemens per metre (dS/m). Ideal levels are less than 0.2 dS/m.

Different pasture species have varying tolerance to soil salinity. Salt tolerance of plants is usually based on a different test — the electrical conductivity of a saturated extract method, ECe, which is also measured in dS/m. Salinity levels are satisfactory for all pasture species if the ECe is under 1 dS/m.

Table 5. Plant tolerance levels to salinity

Some soil test reports do not include a value for plant tolerance levels. They only report the electrical conductivity value.

To convert an electrical conductivity (EC) result to a plant tolerance level (ECe), multiply the EC value by a factor that dependent on soil texture using the values in Table 6.

Table 6. Conversion factor of various soil types for EC to ECe

Contacts and services available

The department provides courses on the nutrient requirements of farms to beef, sheep and dairy producers. These courses provide information on managing soil factors:

• that can limit plant growth
• determine nutrient requirements of pastures
• nutrient budgeting for your own farm
• calculating rates and cost of fertilisers
• using fertilisers

For information on a course near you, contact your local beef, sheep or dairy extension officer, or our customer service centre, phone: 136 186