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

There is a story going around that the ratios of available cations in soils affects crops and pastures.  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.

Amounts over ratios

The consensus now in soil science and plant nutrition is that amounts, not ratios, of nutrients, are important. 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.

However, people still talk about cation ratios. BCSR supporters argue that balancing soils leads to improved crop performance and soil health. Often the recommendation is to apply lime and/or gypsum as calcium is lower than the BCSR ‘ideal’.

Adding minerals to soil to move exchangeable cation ratios towards the ‘ideal soil’ is rarely harmful. But the current evidence suggests it’s not the most cost effective path to yield benefits.  

Critical levels predict response to fertiliser

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.

Improving soil health

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.

Liming

Choose a liming material firstly on cost, including freight and spreading, of neutralising value. A lime comparison calculator can assess the value of different lime sources. Plant nutrients in the lime may be important but the dominant effect is pH change.

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