Rates of Calcium to Magnesium in Soil

Rob Norton, ANZ IPNI, Nigel Wilhelm, SARDI - GRDC

Type: Webpage
Knowledge level: Advanced

Farm Table says:

Properly managed fertilizers support farming systems that provide economic, social and environmental benefits. On the other hand, poorly managed nutrient applications can decrease profitability and increase nutrient losses, potentially degrading water and air.

Do I need to worry about the rate of calcium to magnesium in the soil?

What is the problem?

There is a story going around that the ratios of available cations in soils affects crops and pastures.  The consensus now in soil science and plant nutrition is that amounts, not ratios, of nutrients, are important.

What did the research involve?

The idea of a ‘balanced’ soil gained favour in the USA from work in the 1940s.

The ‘ideal soil’ had approximately 65% Ca, 10% Mg, and 5% K as exchangeable cations. This concept became known as the Base Cation Saturation Rate (BCSR). Correcting a soil’s exchangeable cation ratios would improve crop performance.

By today’s standards, the experiments underpinning the BSCR ‘ideal soil’ were not well designed or interpreted.

  • the pH effects of treatments were not considered
  • significant difference among treatments was not assessed

Two Australian soil scientists completed a detailed review of preferred cation ratios in 2006. They looked at the original data and more recent studies. When calcium (Ca), magnesium (Mg), and potassium (K) were within ranges commonly found in soils, their ratios did not affect crop yields or soil properties.

What were the key findings?

Strong evidence supports the ‘sufficiency’ level of available nutrients framework. Sufficiency defines ‘critical’ levels of individual nutrients in soils. Below the critical level crops will respond to added fertiliser. Above the critical level crops will probably not respond to fertiliser. Critical levels are based on field trials that measure yield outcomes.

The best available guide for N, P, K and S decisions in Australian conditions is the Better Fertiliser Decisions for Cropping (BFDC) Interrogator. Critical ranges are specific to soils and crops. Nutrients that test below their critical range should be supplied to crops.


Final comment

Effective actions to improve soil health are:

  • managing pH
  • adding essential nutrients which are in deficient supply
  • maintaining groundcover
  • having living plants present whenever possible


2016 - Australia - Rob Norton, ANZ IPNI, Nigel Wilhelm, SARDI - GRDC
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