Aluminum becomes more soluble as the soil pH decreases.
Aluminum constitutes about seven percent of Earth's crust by mass, so it's an abundant element. Aluminum is easily oxidized, however, so it's almost invariably found combined with other elements -- in the aluminosilicate minerals found in clays, for example. As the soil becomes more acidic, aluminum becomes more soluble and can become toxic to plants exposed to these conditions.
Effects of Acidity
Aluminum hydroxides like the mineral gibbsite are very insoluble at neutral or alkaline pH. As the pH decreases, however, the excess of hydrogen ions can combine with the hydroxide ions to form water, yielding water and aluminum +3 ions in solution; consequently, the aluminum becomes much more soluble. The same is true for aluminum in other forms in the soil. These aluminum +3 ions in solution are the form that may be toxic to plants.
Symptoms
The roots exhibit the first signs of aluminum toxicity; as the concentration of aluminum ions increases, root growth is inhibited. The roots become stubby and brittle and lose much of their ability to take up nutrients and water. Aluminum is especially effective in reducing uptake of calcium, a nutrient vital to plant growth. Once the plant can no longer obtain all the nutrients it needs, it may begin to exhibit other signs of nutrient deficiency, including stunted shoot growth and yellowing or chlorosis of the leaves.
Mechanisms
The way in which high aluminum levels poison plants is an area of ongoing investigation. Relatively little aluminum penetrates as far as the plants' leaves, so the mechanism primarily involves the root cells. One area of debate involves the degree to which the aluminum ions enter the symplast, the cytoplasm of root cells. Among other effects, aluminum ions can potentially make the plant cell walls more rigid, inhibit DNA replication, interfere with various enzymes, modify the structure of the cell membrane and interfere with nutrient uptake.
Aluminum Tolerance
Plant tolerance for high aluminum levels varies between species and cultivars; genetically, some plants are more resistant than others. Some plants secrete acids from their roots that bind the aluminum ions, rendering them inactive. Wheat, for example, can release malates (salts of malic acid), while many legumes can release citrate and buckwheat can secrete oxalate. Increased levels of aluminum can trigger responses in the plant that cause the secretion of these acids so that the plant need not squander energy by producing them continuously.
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