Toxicities and Deficiencies

Essential Nutrients | Soil pH | Soil Sampling | Nitrogen Cycle

Common deficiencies in agricultural soils

Most Victorian soils have limiting supplies of nitrogen and phosphorus for regular crop production. Further, soils in high rainfall areas often have limiting supplies of potassium and sulphur. Trace element deficiencies include copper, zinc, boron and molybdenum. Molybdenum deficiencies commonly occur in acidic soils, while zinc deficiencies commonly occur in alkaline soils. Deficiencies of copper, selenium and cobalt important for animal health have also been recorded in areas where plant growth has not been affected.

Common toxicities in agricultural soils

Some nutrients when present in ‘surplus’ quantities can restrict and damage plant growth. In Victorian soils, acidic soils have had and still have aluminium and manganese toxicities. Saline soils, alkaline soils, clayey soils, poorly drained soils and semi-arid soils can have specific toxicities of sodium, chloride and boron.

Your paddock and common nutrients

‘Common’ deficiencies and toxicities are provided by field research conducted by farmers, fertiliser companies and the Department of Primary Industries and its predecessors, state-wide statistics and district average. However, unique combinations of soil type, topography and climate at the paddock scale mean that different areas of a paddock have to be treated on their own merits. Such local peculiarities can produce deficiencies or toxicities that may be atypical for the district, the farm and the paddock.

Why soil pH is important to plant nutrition?

The solubility of some elements important to plant and animal nutrition are controlled by soil pH. In particularly, the availability of phosphorus, molybdenum, zinc, aluminium, manganese and iron are affected by soil pH. Application of lime has and can be used to improve the availability of phosphorous and molybdenum, and to reduce the phytotoxicity of aluminium and manganese. Acidification of soil can improve the availability of iron and zinc.

Testing for deficiencies and toxicities

Where soil testing is undertaken it is important that a representative soil sample is collected. Instructions are available at soil sampling and how to take a soil sample.
Some considerations when interpreting soil tests:

• The availability of some elements, particularly phosphorus, iron and molybdenum and the phytotoxicity of aluminium and manganese, can be inferred from measuring soil pH
• Soil testing of phosphorous, potassium and sulfur measures the amount of these nutrients available to the plant, it does not quantify the amount of nutrient that a plant will uptake in a season
• Soil nitrate tests are used to assess the amount of inorganic nitrogen available at the start of the season. Analysis of total soil nitrogen can indicate soil nitrogen reserves to manage ley rotations