Soil Aluminum and Soil Test Interpretation

Aluminum is not an essential element for either plants or animals. Most plant producers have heard that too much aluminum (Al) can be harmful to plants. However, many may not be aware that there are multiple forms of Al in the soil and most of them are not directly harmful to plants. There are also multiple methods of testing the soil for these various forms of Al and several different ways to use these soil test results. This paper will discuss these aspects of soil Al and using soil Al test results.

Aluminum is the most abundant metal in the earth's crust. It makes up about 7% of the mass (essentially the weight) of the earths crust. If you apply this number to an acre of soil 6 2/3 inches deep (2 million pounds of soil), that 7% “Total Al” would equal about 140,000 lb Al/acre or 70,000 ppm. Those of us involved in producing plants, whether those plants are agricultural, turf, or ornamental, should understand how Al can affect these plants.

As mentioned earlier, there are multiple forms of Al in the soil and multiple tests that have been used to identify these different forms. Only two of the tests are useful for growers.

1. Available Al: This test determines the amount of the “available” or easily soluble Al (Al⁺⁺⁺). Growers can use this result to evaluate the potential of Al toxicity to their crop. This is not a routine test and must be specifically requested.
2. Mehlich 3-Al: Mehlich 3-Al (M3-Al) is extracted with the same chemical solution that is used in determining many plant nutrients. Mehlich 3 is a stronger extracting solution than used for available Al, so the results are much larger values. The M3-Al result has no relationship to “available” Al. It is used by several north eastern states when converting the Mehlich 3-P result to the equivalent Morgan-P test result. We automatically run the M3-Al test on all soil samples that are received from those states in which the University lab runs the Morgan-P test, so that they are able to make this conversion if they want.

Excess soluble/available aluminum (Al⁺⁺⁺) is toxic to plants and causes multiple other problems. Some of the more important problems include…

• Direct toxicity, primarily seen as stunted roots
• Reduces the availability of phosphorus (P), through the formation of Al-P compounds
• Reduces the availability of sulfur (S), through the formation of Al-S compounds
• Reduces the availability of other nutrient cations through competitive interaction

The primary damage caused by excess Al⁺⁺⁺ is in damage to plant roots, as seen in these wheat seedlings. Diagnosing this type of damage requires that growers inspect the root systems of their crops or other plants. Of course, when plants have damaged root systems, many other above-ground symptoms are likely. One of the most common will be P-deficiency. However, since Al-toxicity occurs in strongly acid soils, plants may also exhibit deficiency symptoms of calcium (Ca), magnesium (Mg), or other nutrients. They might also show symptoms of manganese (Mn) toxicity, which is common when the soil pH is too low. Finally, poor root development reduces the plants ability to absorb water. Plant problems that damage the roots are difficult to diagnose with leaf analysis. This is because the uptake of these toxins is somewhat self-limiting, due to the root damage that they cause. This is most common with Al and copper (Cu) toxicities.

Very little Al⁺⁺⁺ in the soil solution is required to cause damage to most plants. Few, if any plants grown for commercial purposes in this country will tolerate more than 1.0 ppm of soluble Al⁺⁺⁺, and most will have some problems at levels greater than 0.5 ppm.

Since Al is the most abundant element in the soil, but the soluble Al⁺⁺⁺ is the toxic form, we need to know how much Al⁺⁺⁺ is present in the soil and what controls its availability to plants. The availability of Al⁺⁺⁺ is not completely understood, but certain soil factors are known to have a significant effect.

• The total amount of Al present in a particular soil type
• The soil pH
• The types and amounts of clay in the soil
• Soil organic matter

The soil pH is probably the single most important management factor controlling the amount of Al⁺⁺⁺ in the soil solution. Soluble Al is present in the soil when the pH begins to drop below pH 6.0. However, it is inconsequential in the vast majority of soils until the pH drops below pH 5.5. Even then, it is rarely a problem until the soil pH drops below pH 5.0. However, the amount of soluble Al increases dramatically in nearly all soils as the soil pH drops below pH 5.0. In these extremely acid soils, only those species adapted to acid soils (such as blueberries, cranberries, and acid-loving ornamentals) or the few crop species bred to tolerate high soil Al levels can be expected to do well.

Luckily, the solution to excessive Al⁺⁺⁺ in most soils is simple and inexpensive… apply the proper amount of lime to bring the soil pH into the proper range for the plant species being grown and Al toxicity will not be a problem..

Some soils have extremely acid subsoils. These soils present special problems. While normal lime applications and tillage will easily correct the topsoil, lime is not mobile enough to have a significant or quick effect on subsoil acidity.

When tillage is not an option, acid subsoils become more of a problem to deal with. Lime that is surface applied or applied with only shallow incorporation affects only the top couple of inches of soil, or however deep the shallow incorporation was.

If lime cannot be incorporated throughout the rooting zone of the upcoming crop, then another approach must be taken. Gypsum (CaSO₄) is the best solution to reduce the toxic effects of sub-soil Al⁺⁺⁺ in these situations. Gypsum is not a liming agent, because it cannot neutralize acid. However, the excess Ca applied with the gypsum is a competitive cation to the toxic Al⁺⁺⁺ and causes the Al⁺⁺⁺ to be leached into greater soil depths (assuming enough water passes through the subsoil). Neither the lime nor the gypsum is an instant solution to excess Al⁺⁺⁺. Depending on the nature and particle size of lime, it could require up to 18 months for the lime to completely react and neutralize the acid soil. Gypsum could work faster, depending on how fast it can be leached through the subsoil.

As mentioned earlier, soils differ in the amount of potentially soluble Al they contain. Some soils can contain different amounts and types of clay, and different amounts of organic matter (OM). Different clay types can affect both the potential amounts of Al available to go into solution, as well as the amount of Al⁺⁺⁺ that can be “fixed” or tied-up, after it is formed. Certain compounds in soil OM have the ability to form Al-chelates which are unavailable to plants, thus removing some of the Al⁺⁺⁺ from the soil solution. All of this simply means that some growers will have a more or less difficult time in reducing the amount of Al⁺⁺⁺ in their soils.

Since Al⁺⁺⁺ is a soluble cation, it can be evaluated by percent saturation of the soil CEC, in the same way as the major nutrient cations. Like these other cations, Al⁺⁺⁺ is held on the negative sites of clay and OM (adsorbed). This adsorbed Al⁺⁺⁺ is called exchangeable Al. Some of the exchangeable Al⁺⁺⁺ is released into the soil solution. This “free” Al⁺⁺⁺ in solution is the form that damages plants. However, the adsorbed Al⁺⁺⁺ provides a ready source of additional Al⁺⁺⁺ to recharge the soil solution. Like the nutrient cations, the percent of the soil CEC that is occupied by exchangeable Al⁺⁺⁺ is called the percent Al saturation and it is an indicator of the reserve Al⁺⁺⁺ that must be counteracted if toxicity is to be reduced or eliminated. The accompanying table listing the effects of Al saturation on corn yields in a Wharton and Merrill silt loam soil was taken from Clemson University data. The yields aren't very high, but the effects of soil pH and Al saturation demonstrate the effects of Al saturation on yields. Clemson states that when the exchangeable Al (% saturation) is greater than 60%, there is a large increase in the soil solution Al⁺⁺⁺. This and previous information illustrate how both methods of evaluating soil Al have a value, and may be needed. However, they give us somewhat different information.

1. Do Not Use Mehlich 3-Al To Evaluate Potential Al Toxicity. It is only used to convert Mehlich 3-P into Morgan-P
2. When the soil pH is below 5.0, soluble Al is almost certainly a problem.
3. When the soil pH is between 5.0 and 5.5, soluble Al likely a small problem
4. When the soil pH is between 5.5 and 6.0, soluble Al is not likely to be a significant problem
5. When the soil pH is above 6.0, soluble Al is almost certainly not a problem.
6. Lime is the solution to excess soluble Al in the topsoil
7. Gypsum may be needed to correct excess soluble Al in the subsoil