What is the connection between soil pH, acidity, nutrients and amount of lime required to raise the soil pH?
When we think of soil acidity most people think of pH. But pH is a measure of active acidity which can be measured with a meter, test strips or indicator solution or powder. In simple terms they measure hydrogen ions in water that’s in the soil.
But there is a ‘pool’ of acidity that is held in the soil. This is called exchangeable acidity and it creates a balance with pH in the soil solution.
An important property of soils is their ability to hold nutrients such as calcium, magnesium and potassium and make them available to plants. This is called the ‘exchange’ capacity and it is generally larger for soils with more clays and organic matter. But this capacity can be partly taken up by exchangeable acidity.
For agricultural soils generally, as pH increases (less hydrogen ions), exchangeable acidity decreases. But also with increasing pH the total exchange capacity of the soil increases and this capacity is taken up with a larger proportion of desirable nutrients. In soils with pH around 5.5 to 7 exchangeable acidity should taper off as pH rises with more of the available exchange capacity occupied by nutrients.
Soil samples were taken on a West Gippsland dairy farm at the same 3 sites described in previous entries. Exchangeable acidity was extracted with KCl salt solution. Exchangeable calcium and magnesium were extracted using Double Acid (Mehlich 1).
Typical figures for exchangeable acidity reported for other soils range from 0.5 to 1 meq% so Sites 2 and 3 have low exchangeable acidity.
Typical values for exchangeable calcium can range from 0.23 to 12.5 meq%. Typical values for exchangeable magnesium range from 0.25 to 4.2 meq%. Calcium levels are moderate at Site 1 to high at Sites 2 and 3. Magnesium levels are low / moderate at Site 1, moderate at Site 2 and high at Site 3.
The unit meq% used to express acidity and nutrients is designed to allow a direct comparison between the amounts of each held on exchange sites in the soil. It also provides the mechanism for working out how much lime to apply to soil.
As lime is applied to soil it slowly reacts with the soil acidity. The active (pH) acidity is constantly replenished from the exchangeable acidity but in the process some of the calcium (and magnesium for Dolomite type lime) becomes attached to the exchange sites. The lime will displace some of the exchangeable acidity. This raises the proportion of desirable nutrients to acidity and in doing so, raises the pH.
One approach for working out how much lime to apply to raise the pH is to calculate how much would be required to neutralize the exchangeable acidity. At least this takes the guess work out of liming. Tests like the ones shown here can be used to monitor progress.
Another related approach is to estimate or measure the occupied exchange capacity then by using a diagram of pH vs exchange capacity decide the percentage change required to raise the pH a particular amount. See the reference below for more details.
So far, tests for organic matter, pH and some major nutrients have shown significant differences in fertility between paddocks on a dairy farm.
Further reading: Soil test interpretations by Apps Labs.