What norms (mg/kg) are used to determine Al toxic and optimum levels in soils?
Aluminum is the most abundant metal in the earth's crust, so setting limits does not make much sense!
The most important parameter to check is soil pH, which affects availability and mobility of Al ions. As a rule of thumb, if pH=>6 no problems are foreseable, whichever Al amounts are in soil.
Available Al can be determined as an additional test on borderline soils (see attached file as a starter)
Portia Phohlo,
The toxicity depends on the crop you have in that soil. Small amounts of aluminum could be very toxic to a crop, but the same amount could be non-toxic to other crops. Therefore, toxicity should be evaluated together with the crop.
Aluminum is the most abundant metal in the earth's crust, so setting limits does not make much sense!
The most important parameter to check is soil pH, which affects availability and mobility of Al ions. As a rule of thumb, if pH=>6 no problems are foreseable, whichever Al amounts are in soil.
Available Al can be determined as an additional test on borderline soils (see attached file as a starter)
@Andres, thank you very much for your response, to be more specific, I am looking for toxicity levels under rye grass, kikuyu and clover pasture in sandy soils.
@Salvatore thank you for your response your insight was very helpful, which pH are you referring to i.e.KCl,H2O or CaCl2?
It is not easy to set a clear level of Al toxicity because as the others have mentioned, different plants have different Al tolerance levels. However, if KCl-extractable Al is above 2 cmolc/kg (in sandy or silty soils), percentage exchangeable Al > 25%, or soil-solution Al concentration (actually activity) > 10 micro-moles/L, then Al toxicity is likely.
Hi Portia,
I think Salvatore's reference to pH is very important. Lowering the pH of the soils will increase the availability of the aluminium to plants The principle effect of acidic soils on plants is aluminium toxicity so there is a pretty close relationship between the two. Soils with neutral or alkaline pH are unlikely to be affected by aluminium regardless of the levels in the soil whereas the aluminium level becomes more important as the soil pH decreases. Aluminium is precipitated out of soil solution at about pH 5 (CaCl2) so it isn't a problem when the soil pH is above 5.
Making a sweeping generalisation, sandy soils are more prone to acidification because of their lower buffering capacity and higher leaching rates. On the other hand, aluminium is primarily present in the soil as a cation and cations are primarily found in clay particles and organic matter, both of which tend to be reduced in sandy soils.
So, firstly see if the soil pH (CaCl2) is less than 5. Only worry about Al toxicity if those results confirm low soil pH. Even if they are, sandy soils are very responsive to liming (principally due to the buffering capacity and leaching rates mentioned above) which could remediate them pretty quickly.
In acid soil solution [soil pH(H2O) < 5,5], small amounts of Al-P complexes probably stimulate growth of crops. But when the pH drops lower than 5,0, toxic effects occur for most plants. Tea however may accumulate Al under these very acid soil conditions and continue to benefit of Phosphorus co-absorption.
Hi Portia
When you have to consider the capacity toxic aluminum in some crops must take into consideration that this level is not equal among plant species since it depends on mechanisms of evasion or tolerance that they possess. While it is true that the pH conditions long determine the amount of soluble aluminum and potentially toxic, equally important is to know or determine the amount of each species can withstand.
Can you please tell Threshold limits of Al and Ni in soils and plant (Brassica juncea)
as far as I know, For legumes. the toxicity threshold for exchangeable Al is 3 mg/kg of soil_Using CaCl2 method.
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