101 Questions answered
How Crop Type and Farming Practices Influence Soil Acidification Rates
Short answer: Yes, legumes acidify the soil more than grains, but other cropping practices such as applying nitrogen fertiliser, high yields and cutting for hay also speed up soil acidification.
Longer answer: Legume crops acidify the soil faster than cereals and grasses because they produce nitrate nitrogen (see Question 10 ‘Why does farming acidify the soil?’). When legumes fix nitrogen from the atmosphere which is then converted to nitrate, two hydrogen ions are released. If the nitrate isn’t taken up by the crop (whereupon the crop releases hydroxide that will bind with the hydrogen to form water) the hydrogen ions contribute to soil acidity.
As cereal crops don’t produce their own nitrogen, they don’t contribute to soil acidity in the same way as legumes. However, the acidity directly from ammonium based fertilisers and nitrate leaching is more significant than that from legumes, particularly in the Mallee environment.
Crops that remove more material from the paddock – such as lucerne cut for hay – cause faster soil acidification than harvesting just grain. This is because the crop removes more alkaline nutrients, e.g. calcium and magnesium, from the paddock. In the same vein, higher yielding crops will accelerate acidity more than lower yielding crops because they remove more alkaline nutrients from the soil.
A very rough rule of thumb to estimate the acidification rate is you will need 10 kilograms of lime per ton of grain produced and roughly 1 kilogram of lime required for each kilogram of DAP, MAP, SoA and urea. The Maintenance Lime Rate Calculator will do more detailed calculations.
