How are ppm values converted to meq per 100 g?

Divide the ppm value by the cation's equivalent weight and then by 10 to scale ppm to a per-100-gram basis. The equivalent weights are 200 for calcium, 120 for magnesium, 390 for potassium, and 230 for sodium, which already build in the 10x scaling, so meq/100g equals ppm divided by these factors.

What is base saturation?

Base saturation is the percentage of the cation exchange capacity occupied by the base cations calcium, magnesium, potassium, and sodium rather than by acidity (hydrogen and aluminium). Higher base saturation generally means a less acidic, more fertile soil.

What are ideal base saturation ratios?

A commonly cited balanced target is roughly 65 to 75 percent calcium, 10 to 15 percent magnesium, 2 to 5 percent potassium, and under 3 percent sodium. These ratios are guidelines; sufficiency levels of each nutrient matter more than hitting exact percentages.

How is the lime requirement estimated?

When a buffer pH is provided, the tool estimates lime need from the gap between buffer pH and a reference, since a lower buffer pH means more reserve acidity to neutralize. This is an approximation; use your lab's specific buffer method and recommendation for field rates.

What is a typical CEC value?

Sandy soils often have a CEC of 1 to 5 meq per 100 g, loams 5 to 15, and clay or high-organic-matter soils 15 to 30 or more. A higher CEC means the soil can hold more nutrients against leaching but also needs more lime to shift pH.

Soil Cation Exchange Capacity & Base Saturation Tool

A soil test reports cations in parts per million, but agronomic interpretation works in milliequivalents per 100 grams so that calcium, magnesium, potassium, and sodium can be added together and compared. This tool does that conversion, sums the cation exchange capacity, computes base saturation, and gives a first-pass lime requirement from buffer pH.

How it works

Each cation is converted from ppm to meq/100 g by dividing by its equivalent weight factor (which already includes the per-100-g scaling):

Ca meq/100g = Ca ppm / 200
Mg meq/100g = Mg ppm / 120
K  meq/100g = K  ppm / 390
Na meq/100g = Na ppm / 230
CEC         = Ca + Mg + K + Na (meq/100g) + exchangeable acidity
base sat %  = (sum of base cations / CEC) × 100

Base saturation for any single cation is its meq/100 g divided by the total CEC. When a buffer pH is entered, lime requirement is estimated from how far the buffer pH sits below a neutral reference, scaled to the target pH gap.

Tips and example

A soil testing 1600 ppm Ca, 240 ppm Mg, 195 ppm K, and 46 ppm Na converts to 8.0, 2.0, 0.5, and 0.2 meq/100 g. With 1.3 meq/100 g of acidity the CEC is 12.0 and base saturation is about 89 percent — calcium occupies 67 percent, a healthy balance. Treat the lime estimate as a starting point: always defer to your lab’s buffer-pH method and local extension recommendation for the actual application rate, and remember that the cation ratios are guidelines, not hard rules.