Base Saturation

Soil holds cations on negatively charged sites belonging to clay and organic matter, and the total capacity of those sites is the cation exchange capacity. Base saturation asks what fraction is occupied by the basic cations — calcium, magnesium, potassium and sodium — rather than by the acidic ones, aluminium and hydrogen.

The number is a different way of stating acidity from pH, and a more informative one for a lime decision. pH measures the acidity currently in solution; base saturation measures the reserve sitting on the exchange, which is what has to be neutralised. Two soils at pH 5.5 can need very different amounts of lime, and their CEC and base saturation are what tell them apart — a sandy soil with a CEC of 5 needs a fraction of the lime a clay with a CEC of 30 does.

Most crops are comfortable between 60 and 80%. Below about 50%, aluminium becomes soluble enough to damage roots, and aluminium toxicity — not any nutrient deficiency — becomes the limiting factor on acid soils. Above 95% the risk inverts: phosphorus, iron, manganese and zinc all become less available as pH climbs toward and past neutrality.

The one claim to treat sceptically is the "ideal ratio" school, which holds that calcium, magnesium and potassium should sit at fixed proportions of the exchange — commonly quoted as 65:10:5. Decades of trials have failed to show yield responses to correcting ratios within the normal range, and the mainstream position is that sufficiency of each cation matters and their ratio to one another does not. Sodium is the genuine exception: above about 15% of the exchange it disperses soil structure, which is a physical problem gypsum addresses and lime does not.