Aluminium is used by some plants but it is not an essential nutrient and it is toxic to many plants. Your soil will contain some aluminium as about 7% of the Earth's crust is aluminium but ideally, your plants shouldn't take up much of this aluminium.
High levels of aluminium are toxic to some plants and are associated with acidic soil. Indeed, most of the problems associated with acidic soil are due to aluminium toxicity. Aluminium has the following affects on plants:
- Roots - aluminium decreases the amount of roots a plant produces and it also reduces the function of roots that are produced. This means that plants are not able to uptake as much water or as many nutrients as they require.
- Phosphorus - aluminium prevents plants from being able to uptake phosphorus.
- Calcium - aluminium prevents plants from being able to uptake and use calcium.
- Magnesium - aluminium also prevents plants from being able to uptake and use magnesium.
- Sulfur - aluminium reduces the amount of sulfur that can be taken up by plants.
Symptoms of Aluminium Toxicity
Plants will become stunted if they absorb too much aluminium. They may also show symptoms of phosphorus deficiency, calcium deficiency, magnesium deficiency or sulfur deficiency. Plants suffering from aluminium will frequently also suffer from manganese toxicity. This is usually due to a low soil pH and is not believed to be a result of excess aluminium itself.
Treating Aluminium Toxicity
If the pH of soil is greater than 5.5 (when measured in calcium chloride which is the most common way pH is measured by laboratories performing soil tests - this is roughly pH 6-6.5 when measured in water or with a probe) aluminium is not as available to plants so is less likely to cause toxicity. As such, if a laboratory soil test reveals that the pH of your soil is less than 5.5 or a home pH test reveals that the pH is less than 6, lime can be added to soil to prevent or treat aluminium toxicity. Organic matter, when it accounts for greater than 5% of the soil, can tie up aluminium making it inaccessible by plants, so adding organic matter can be a supplemental means of reducing aluminium toxicity. If raising soil pH is ever difficult, aluminium tolerant plants can be grown. For instance, hydrangeas need aluminium in order to produce blue flowers so they are a good choice for soils that contain elevated levels of aluminium.