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Understanding the Link Between Farming and Soil Acidification
Soil acidification is a natural process, especially in high rainfall areas, but farming activities can accelerate it. The more productive the farm, the more acidifying it is. This is because productive farms tend to use more nitrogen fertilisers and export more grain or other products.
The type of nitrogen fertiliser used and the way it is converted into a plant-usable form influence how much the soil’s acidity increases.
When nitrogen fertilisers break down, they can release hydrogen ions, which contribute to the acidification of the soil (pH is a measure of hydrogen). For example, fertilisers like urea and ammonium nitrate releas eone hydrogen ion. When the plant uses the nitrate ion in the fertiliser, it releases one hydroxide ion to maintain electrical balance in the soil. The hydroxide combines with the hydrogen to make water and there is no acidifying effect. But, if the nitrate is not used, say it leaches, the plant does not excrete hydroxide and the hydrogen now contributes to soil acidification.
Some fertilisers are acidifying even if all the nitrate is taken up. For example, one molecule of diammonium phosphate (DAP) releases three hydrogen and two nitrate ions during nitrification, so there is always aspare hydrogen around contributing to acidification.
Ammonium based fertilisers such as sulphate of ammonia (SOA) and monoammonium phosphate (MAP)are the most acidifying because they generate two hydrogen ions for every ammonium molecule converted to nitrate.
Legumes, which fix nitrogen from the atmosphere, act in a similar way to nitrogen fertilisers. They can acidify the soil if the ammonium and nitrate produced by N-fixing bacteria aren’t all used by the crop or pasture.
Harvesting. Cutting hay, harvesting grain, and removing animal products such as wool and milk exports alkalinity from the paddock. As plants take up cations (e.g. nutrients like calcium and potassium) they release hydrogen to maintain the electrical balance. If we didn’t harvest, those alkaline cations would be returned to the soil, largely balancing the acidity. But, as harvesting is a rather essential part of farming, those cations are exported from the paddock and the soil gradually acidifies.
Organic matter can contribute to soil acidification. Hydrogen is released as the organic matter breaks down. The benefits of having more organic matter in the soil outweigh the minor acidification it can cause. Plus, organic matter will not build up indefinitely, see Q. 89 ‘How much carbon can I add?’. Acidification stops when the soil reaches equilibrium.
In cropping, most soil acidification is linked to nitrogen fertiliser use, product removal and nitrogen use
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