How Aluminum Affects Plant and Soil Health
Aluminum in the Soil

The aluminum content of soil varies greatly depending on its parent minerals and weathering history. Total soil aluminum may range from 1-30%, but the average content is about 7-10%. Aluminum is a structural constituent of clays and minerals such as feldspar and zeolites and is biologically inert in its mineral forms. In most soils, the primary aluminum forms are clay mineral crystals and aluminosilicate minerals. Aluminum phytotoxicity is not determined by the total aluminum content but by soluble aluminum content.

Below pH 4, aluminum is leached from mineral structures and released as Al3+. Few of these Al ions remain in the soil solution, and most are adsorbed to cation exchange sites in the soil. In the presence of phosphate, soluble aluminum forms an insoluble precipitate of aluminum phosphate, which can prevent plants from accessing soil phosphorus. 

Al ions may also adsorb and then polymerize on the negatively-charged surface of clay minerals and soil organic matter. This may help stabilize the clay minerals and improve aggregate stability. However, an accumulation of these polymerized layers mechanically obstructs the cation exchange sites of the soil and prevents soils from retaining and exchanging cation nutrients.

Aluminum in the Plant and Toxicity Symptoms

Aluminum is not recognized as an essential nutrient for plants; thus, there are no related deficiency symptoms. Soil acidity is the primary determining factor of mobile Al concentration, and soil pH < 5 significantly increases Al uptake and toxicity. On acid soils, Al toxicity is often the primary limiting factor. Aluminum toxicity has been correlated with decreased chlorophyll content and inhibited photosynthesis. The main physical symptoms of aluminum toxicity are inhibited root growth and roots with short, thick, brittle tips. This poor root architecture makes plants susceptible to drought stress and inhibits nutrient uptake, often resulting in low cytoplasmic calcium concentrations. Aluminum has also been shown to react with phosphate is taken up by the plant, rendering it unavailable.

Source: Bojorquez-Quintal, 2017

Figure 1. The metabolic, physiological, and biochemical consequences on the plant from the uptake of aluminum.

Aluminum isn’t all Bad

Since the early 1900s, numerous studies have reported the beneficial effects of aluminum on plant growth when applied in low concentrations. While a clear mechanism has never been established, the evidence cannot be ignored. Many reported beneficial effects have been in tropical plants adapted to acidic soils high in Al, such as coffee (Coffea arabica) and tea (Camellia sinensis). In several plants adapted to these conditions and a few crop species of economic importance, low doses of Al may increase root proliferation, lateral root expansion, and root biomass growth. It is theorized that in low concentrations, Al promotes the uptake of other nutrients; a 2014 study found that Al increased Ca and K uptake in C. arabica. Some species also hyper accumulate Al in their tissues to discourage herbivory.

Further Reading

Bojorquez-Quintal et al. (2017). Aluminum, a friend or foe of higher plants in acid soils. Frontiers in Plant Science.

Chauhan et al. (2021). Aluminum toxicity and aluminum stress-induced physiological tolerance responses in higher plants. Critical Reviews in Biotechnology.

Panda et al. (2009). Aluminum stress signaling in plants. Plant Signaling & Behavior.McLean, E. O. (1976). Chemistry of soil aluminum. Communications in Soil Science and Plant Analysis.