Base-cation saturation ratio

Base-cation saturation ratio (BCSR) is a method of interpreting soil test results that is widely used in sustainable agriculture, supported by the National Sustainable Agriculture Information Service (ATTRA) and claimed to be successfully in use on over a million acres (4,000 km²) of farmland worldwide. The traditional method, as used by most university laboratories, is known variously as the 'sufficiency level', sufficiency level of available nutrients (SLAN), or Index(UK) system. The sufficiency level system is concerned only with keeping plant-available nutrient levels within a well studied range, making sure there is neither a deficiency nor an excess. In the BCSR system, soil cations are balanced according to varying ratios often stated as giving 'ideal' or 'balanced' soil. These ratios can be between individual cations, such as the calcium to magnesium ratio, or they may be expressed as a percentage saturation of the cation exchange capacity (CEC) of the soil. Most 'ideal soil' theories stress both approaches. (See also – Cation exchange capacity)

Strictly speaking, the 'base' cations are limited to calcium, magnesium, potassium, and sodium, and these are the primary nutrients that BCSR methods are most concerned with balancing. However, many proponents of 'ideal soil' theories also stress the importance of balancing the anions phosphorus, sulphur and chlorine as well as numerous minor and trace elements. The conventional SLAN system does not generally test for minor and trace elements unless there is sufficient cause to suspect a deficiency or toxicity.

BCSR supporters argue that a soil balanced using their methods leads to greater crop yield and nutritional quality, as well as increasing the soil biological activity and the physical properties of tilth, aeration, and moisture retention. There is currently no publicly available research or trial data to support these claims, but BCSR systems are fairly widely used in organic farms and many positive testimonials from farmers and gardeners can be found on the internet and in alternative agriculture literature. Under most circumstances following BCSR systems will not lead to negative effects. The main concern for farmers is simply the unnecessary expense of applying soil amendments beyond what the crop can actually utilise.

The cation exchange principle was discovered by Thomas Way and John Bennet Lawes at Rothamsted Experimental Station in the 19th century. In 1892 Oscar Loew observed that both calcium and magnesium can be toxic to plants when there is an excess of one and a deficiency of the other, thus suggesting there may be an optimal Ca:Mg ratio. In 1901 Oscar Loew and D.W. May did further testing and suggested an ideal Ca:Mg ratio of 5 to 4, though for several species maximum growth was obtained across a wide range of ratios. In 1916 Lipman reviewed the literature up to that point and concluded that while some researchers seemed to have identified 'optimal' Ca:Mg ratios for certain species, there was no evidence that the Ca:Mg ratio influenced growth.

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