101 Questions answered
Why Sandy Soils Acidify Quickly and Clay Soils Resist Change
Sandy soils have a higher risk of developing acidity because they have a low pH buffering capacity, meaning they can’t resist pH change very well.
Soils that can buffer against pH change have a higher cation exchange capacity (CEC), which comes from clay and organic matter. Generally, more clay and organic matter means a higher CEC and buffering capacity.
pH measures the concentration of hydrogen ions in the soil solution. Hydrogen in the soil is adsorbed to soil particles and in the soil solution. Soils with higher concentrations of hydrogen are more acidic. The more exchange sites a soil has to hold hydrogen ions, i.e. the higher the CEC, the harder it is to change the soil pH.
Applying lime neutralises hydrogen ions in the soil solution (see Question 18 ‘Should I use lime or gypsum on my acidic soils?’). Because sandy soils have fewer exchange sites and therefore fewer hydrogen ions adsorbed to soil particles, it takes less lime to neutralise the hydrogen ions.
Note that clay soils can still become acidic. Over time, ongoing nitrogen fertiliser applications and harvesting will gradually increase hydrogen ion concentrations in the soil. Clays are slower to acidify but when they do, they need more lime to raise soil pH. It takes less lime to raise pH again in a sandy soil than in clayey soils.
Acidity is something to watch, particularly in variable paddocks with different soil types. Paddocks with a mix of soil types can develop acidity in susceptible soils, even when there are adjacent zones containing calcareous loams or limestone reefs.
As a rough rule, the amount of lime needed to increase pH by about 1 pH unit (e.g. from pHCa 4.5 to 5.5) in the 0–10 cm layer is 2 t/ha for a sand, 3 t/ha for a sandy loam and 4 t/ha for a loam. If organic matter is low, these rates may be significantly less.
