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Aluminum in soil: avoiding aluminum toxicity

Aluminum is one of the most abundant elements on the planet: roughly 7% of the earth’s mass is made up of aluminum.

Non-essential for plant growth, available or soluble aluminum can be toxic to plants, whereas other forms—such as aluminosilicates and precipitates, or forms of this elemental metal bound up in ligands—are decidedly not phytotoxic.

Thus while there are a variety of soil tests available for determining the level of aluminum in the soil, one that is relevant to agricultural productivity will look for available or soluble aluminum (Al+++).

The most telling sign of aluminum toxicity in plants themselves is diminished root growth. Generally, the growth of roots is reduced to about half of what is normal, but this varies from crop to crop.

Reduced root mass and length means reduced uptake of virtually every other nutrient, as well as the capacity to take in enough water. Additionally, vital plant nutrients like sulfur and phosphorus tend to bond with available aluminum.

Due to these two factors, excess aluminum in the soil can lead to the appearance of other nutrient deficiencies: pale or faded color, wilted or stunted growth, thin or weak stems, and necrotic spots.

Aluminum is also more available to plants in acidic soils (soils with a very low pH), and thus acid-loving crops like blueberries and cranberries are among the more aluminum tolerant selections.

A soil with pH below 6 (and especially below 5) is much more likely to have an excess of soluble aluminum, and thus contain levels that are toxic to most crops: however the cause of such a low pH can only truly be ascertained with the right soil tests.

The threshold at which available or soluble aluminum (Al+++) in soil can begin to do damage is about 0.5 ppm, with few commercial crops tolerating more than 1.0 ppm.

Reducing aluminum in the subsoil is sometimes done with gypsum, which makes it more soluble. The calcium (Ca) cations in gypsum are competitive with aluminum (Al) cations, making them less absorbable to plants. This strategy for subsoil neutralization, however, runs the risk of contaminating nearby watersheds.

Reducing toxic aluminum content in the topsoil is usually done with an addition of agricultural lime, raising the pH to 5.5 or above.

Whether it is overly-abundant in the subsoil or topsoil, however, aluminum poses a serious risk to overall crop productivity.

While test for aluminum isn’t necessarily a part of the cadre of standard soil tests, in soils with a low pH, a test for soluble or available aluminum in the soil is prudent.

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