Reading soil samples can be complicated, especially with the extra numbers that come from extensive testing packages. Wouldn’t it be nice to understand what all the numbers tell you about your soil?
In this article we provide an overview of a sample soil report commonly seen in our region. Results are typically broken into four sections — chemical and physical, cation exchange capacity, macronutrients, and micronutrients.
Physical properties
Soil texture: Texture is based on mineral composition — broken down to parts of clay, silt, and sand.
Soil colour: A visual assessment of the mineral material colour.
Chemical properties
pH: Nutrient availability is optimised in the majority of pasture grasses and legumes at pH 5.5-6.5. pH influences nutrient availability and cation exchange.
Electrical conductivity: This is a measure of salts in the soil, which are extracted in a solution. A good level is less than 4 dS/m.
Organic matter: Soils in our region are known to be naturally low. Organic matter does not have specific critical levels for extensive grazing.
Cation Exchange Capacity (CEC)
CEC is a measure of cations present in the soil. Cations are positively charged ions. A CEC greater than 4 meq/100g, with sodium <1% and aluminum <5% of cations, in extensive grazing country is a good result. CEC can determine calcium, magnesium, and potassium deficiencies.
Macronutrients
Nitrogen is critical for plant energy production. In extensive grazing practices, there are no defined critical limits.
Phosphorus is known to be a limiting nutrient in Australia, and our region is no exception. Phosphorus levels in a Colwell P test below 10 mg/kg will indicate poor plant nutrition and productivity. A common result in our region is between 10 and 20 mg/kg, which may pose issues for legumes establishing root nodes.
Potassium is generally not a major limiting factor on pasture growth in northern Queensland. Critical limits for growth depend on soil type, sandy soils require higher levels to support growth, as potassium is easily leached from soil.
Sulphur levels required for pasture production are low when nitrogen and potassium are limiting. Legumes have a greater requirement for sulphur as this supports nitrogen fixation in root nodes.
Micronutrients
Micronutrients are at low levels in much of northern Queensland. Soil used for extensive grazing is often tested for iron, manganese, zinc, boron, copper, and silicon. Low levels will indicate a deficiency, but symptoms are often not pronounced in plants as micronutrients may not be the most limiting nutrient.
Read the full article, including how to read a sample soil test, in the Gulf Croaker:
Funded by the Queensland Government’s Natural Resources Recovery Program.