Aluminum (Al) toxicity is a serious constraint to crop production in many regions around the world. The first step in correcting this problem is to identify it. Should we use exchangeable (KCl-extractable) Al, Al-saturation percentage, or soil-solution Al concentration (or activity of the various Al species)?
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.
As you know, 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.
1. Available Aluminum: 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: It 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.
Aluminum Toxicity
Excess soluble/available aluminum (Al+++) is toxic to plants and causes multiple other problems. Some of the more important problems include:
1. Direct toxicity, primarily seen as stunted roots
2. Reduces the availability of phosphorus (P), through the formation of Al-P compounds
3. Reduces the availability of sulfur (S), through the formation of Al-S compounds
4. 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 the attached 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.
1. The total amount of Al present in a particular soil type
2. The soil pH
3. The types and amounts of clay in the soil
4. 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.
Acid Subsoil
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.
In brief, 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.
Soil Aluminum Saturation
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. 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, and finally:
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 is more acidic (pH is below 5.0), soluble Al is almost certainly a problem.
3. When the soil pH is increased to be (between 5.0 and 5.5), soluble Al likely a small problem.
4. When the soil pH is (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.
http://www.agriculturejournals.cz/publicFiles/52771.pdf
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.
As you know, 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.
1. Available Aluminum: 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: It 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.
Aluminum Toxicity
Excess soluble/available aluminum (Al+++) is toxic to plants and causes multiple other problems. Some of the more important problems include:
1. Direct toxicity, primarily seen as stunted roots
2. Reduces the availability of phosphorus (P), through the formation of Al-P compounds
3. Reduces the availability of sulfur (S), through the formation of Al-S compounds
4. 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 the attached 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.
1. The total amount of Al present in a particular soil type
2. The soil pH
3. The types and amounts of clay in the soil
4. 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.
Acid Subsoil
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.
In brief, 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.
Soil Aluminum Saturation
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. 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, and finally:
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 is more acidic (pH is below 5.0), soluble Al is almost certainly a problem.
3. When the soil pH is increased to be (between 5.0 and 5.5), soluble Al likely a small problem.
4. When the soil pH is (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.
http://www.agriculturejournals.cz/publicFiles/52771.pdf
In volcanic ash soils (I guess this is your case in Hawaii), KCl-extractable Al or Al-saturation are not suitable indicators of Al-toxicity because it results mostly from Al-humus complexes and not from hydroxy-Al inter-layered 2:1 layer silicates. I suggest you to assess Al-availability by extracting soil with a 0.02 M EDTA and 0.5 M ammonium-acetate solution at pH 4.65 (soil:extractant ratio of 1:5) for 30 minutes (Lakanen & Erviö, 1971). You can find additional information in my paper in Eur. J. Soil Sci. 63, 674–684.
Article The role of metals in soils and accumulation in the laurel f...
Many thanks Fulvio M. Rivano for your answer and thanks a lot Juan Luis Mora for your valuable paper! Additional information in the attached file.
Acid soil infertility is a major limitation to plant growth worldwide, especially in the humid tropics, subtropics and temperate zones. Infertility results primarily from toxicities of Al. Kindly see the following link:
http://plantsinaction.science.uq.edu.au/edition1/?q=content/16-5-2-aluminium-and-manganese-toxicities
Please read an article by USEPA about aluminium in soils and its effect, and will help you. this is a 34 pg document and can be obtained through this link: http://rais.ornl.gov/documents/eco-ssl_aluminum.pdf
Here at Brazilian Cerrado Aluminun is a big issue because we use lime and correct only 20 cm. Moredeep than 20 cm we have to use gypsum. The natural vegetation here is Alumminum tolerant and the mechanism are variable. However, the introduced crops need to hae the soil corrected witrh lime and gypsun.
Exchangeable and available Al is the major problem in acidic soil and most important effect on plant is the root deformation and poor growth. However many reviews mention that aluminium effect can be reduced by adding or increasing the organic matter level in soil. Freely available Al will be bind with organic compound making it inactive.
Thank you, Dr Hue, for giving us the opportunity of thinking and sharing contributions on this subject.
We have applied several Al toxicity indices to forest soils from NW Spain (Alvarez E, Fernandez-Marcos M L, Monterroso C and Fernandez-Sanjurjo M J 2005 Application of aluminium toxicity indices to soils under various forest species. Forest Ecology and Management 211, 227-239).
As for volcanic soils, Hargrove and Thomas (1984) found that LaCl3 is more effective than KCl in extracting the less hydroxylated and less polymerised organically-bound Al and correlates well with titratable acidity. Garcia-Rodeja et al (2004) showed that, for andic horizons, CuCl2-extractable Al correlated strongly to the soil components determining CEC (organic matter and allophone).
Article Application of aluminium toxicity indices to soils under var...
Thank you, Dr. Maria Luisa Fernandez-Marcos. I must admit that I have written a few papers and chapters about Al toxicities in acid soils, yet I still cannot find a clear identification parameter of this problem. Perhaps, there is none (or there are several): It varies with specific soil and situation! Thanks, NVH
Hi I have this test in an Acrisol in Ghana with soil acidity and would like to understand how to interpret results considering very variable values in (H+)
KCl cmol(c)/kg ranging from 0,03 to 2,73. Al as shown in table resulted in lab analysis from 7400 to 20000 ppm
We are considering 2t/ha liming rate and also application of biochar. Any thought ?
Dear Emiliano:
If I'm not mistaken, the Al values in your Table are TOTAL Al, which means little in terms of soil acidity or lime requirement (for correcting Al toxicity). Just assume that your KCl-extractable Al averages 2 cmol(c)/kg as you reported, then 2t/ha of lime would be reasonable. Some biochar application may help, but biochar quality, especially its pH and CaCO3 equivalent need be known.
Best, NVH.
Nguyen Hue thanks so much for your reply. I guess both Fe and Al will be corrected with rates 2t/ha considered. Biochar will be produced with Japanese kilns (Kon tiki, as suggested by Ithaka Institute in Switzerland) and we will soak it and charge it with inoculants, manure and compost. Application rates considered for biochar in reclamation land will be around 2t/ha. P Rock is being considered too but we want first to recover land by cultivating crotoalaria junco /sunn hemp (N fixing legume as cover crop) and incorporate aboveground biomass to soil before planting commercial crops.
A soil pH of over 5 and a soluble Al of under 5 ppm are the threshold which will ensure no aluminum toxicity in sensitive plants. Soils under 5 pH should be limited to 5.5 or greater this should ensure a soluble Al of less than 5 ppm. Other antidotes as noted would be phosphorus as starter fertilizer the application of organic amendment and attention to possible Boron and other micronutrient deficiencies. For liming Calcitic limestone will be fine unless the soil Mg is less than 100 then dolomite recommended. Finally in the acid soil condition biochar can have a liming effect and can be useful.
By observing what kind of wild plants are growing in such a soil. For example Minuartia verna is a heavy metal hyperaccumulating plant (Al, Pb, Fe, Cu, Zn) capable of growing on a soil contaminated with Al.
I have ameliorated Al toxicity by liming and found that there was very little response to liming on manured soils. The chelation of Al by humus must be detoxyfying the Al.
Once the pH is above 5.1 there is very little issue with Al toxicity. That is the root of the curative effect of liming acid soils.
Soil organic matter also works to chelate and take Al out of solution and is therefore a way of counteracting the issues with acid soil infertility.
Manuring and the addition of compost work together will help other nutrient deficiencies and excesses to resolve acid soil infertility constraints.
When ammoniated nitrogen sources are used under acid soil conditions they result in increased issues as ammonium absorption in the plant maintains neutrality by releasing hydrogen protons to the soil worsening the soil acid infertility and increasing Aluminum toxicity.
Soil acid infertility is a complex of toxicities and deficiencies and is found prevailing particularly in humid areas of old red and yellow soils which can also suffered for poor physical properties.
Nguyen Hue has observed that the total Aluminum has little to do with aluminum toxicity. The toxic environment is the function of soluble Aluminum and if that is over 1 ppm the Aluminum soluble in solution will express a disturbance of the root tips. This solubility of free Aluminum will occur when soil is less than 5.1 pH. The blasting of the tips of the roots are a function of induced Calcium deficiency and the application of soluble Calcium as well as organic amendment work to counteract the toxicity. Beside pH, Calcium, organic matter there is benefit of Boron in some instances. The use of complete soil and foliar analysis can be useful in overcoming the issue.
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