Originally Posted by Kiril
From your link.
The procedure for measuring exchangeable Al+3 is not well suited for routine soil testing procedure. For this reason a buffer solution pH 6.6 is used to measure extractable acidity (Ac) in North Carolina soils. The buffered solution extracts both exchangeable Al3+ and the pH dependent acidity (H+) which becomes ionized up to pH 6.6. The lime rate to apply is calculated with the following equation.
tons CaCO3 / acre = Ac [(desired pH - soil pH) / (6.6 - soil pH)]
The desired pH for a soil is the pH at which the activity of Al+3 is neutralized. The effect of soil organic matter in decreasing the activity of Al+3 has been taken into account by establishing desired pH for each of three classes of soils (mineral, mineral-organic and organic) based on their organic matter content. The desired pH at which exchangeable Al+3 is essentially neutralized is 6 for mineral soils, 5.5 for mineral-organic soils, and 5 for organic soils.
Furthermore, without a value for Ac
the formula is worthless.
Ok, thanks Kiril. For all values of desired pH less than 6.6 this formula makes sense.