March 14, 2023

Aluminum Toxicity

Chris Augustin Director/Soil Scientist Dickinson Research Extension Center 701-456-1100 chris.augustin@ndsu.edu

Aluminum toxicity reduces crop yields by reducing and malforming root growth and phosphorus tie-up. If your soil pH is greater than 5.5, aluminum toxicity will not be an issue. Aluminum toxicity can occur when soil pH is less than 5.5 and extractable aluminum is greater than 25 ppm. Our soils contain high amounts of aluminum that is predominately found within clay minerals. Aluminum is essentially chemically inert until the soil pH is less than 5.5. When the soil pH is less than 5.5, aluminum enters the soil solution and negatively impacts root growth. Aluminum is a cation with a plus three charge. Plant available phosphorus is negatively charged and varies based on soil pH. The negatively charged phosphorus binds to the positively charged aluminum and is rendered plant unavailable. This is why phosphorus deficiencies can be more prevalent in acidic soils. Some research shows that high levels of banded phosphorus can tie-up aluminum which reduces aluminum toxicity. However, this phosphorus in not plant available. For more information on this research, please refer to pages 53 and 54 of this link: https://www.ndsu.edu/agriculture/sites/default/files/2022-12/2022%20WDCD%20book%20for%20webpage.pdf Aluminum has the ability further acidify soil. When aluminum enters the soil solution it can split three water molecules (hydrolyze) into hydrogen (plus one charge) and hydroxyl (negative one charge). Three negative hydroxyls bind to one aluminum (plus three charge) and free hydrogen is left in the soil solution. The free hydrogen further acidifies the soil solution. Cultivar/crop selection, and phosphorus applications are band aid acid treatments. They can reduce the adverse impacts of aluminum toxicity, but the only way to truly manage soil acidity is to apply soil amendments like lime (calcium carbonate) that react and neutralize the positively charged acid causing hydrogen. The figure below shows how neutralizing beet lime increased soil pH and decreased extractable aluminum. The aluminum is still in the soil. However, the negatively charged carbonate portion of lime neutralized a large portion of the acid and rendered the aluminum insoluble. Therefore, the aluminum cannot negatively impact crops.

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