Many researchers stated that Low bio-availability is restricting factor to uptake metals. On the other hand side, people try to control high bio-availability or mobility of metals.if they try to control then it will be limited, then phytoextraction potential will be less now which one is good?
It depends on the toxic metal concentration of the soil and plant type, if the toxic metal content was below than tolerable level of certain plant, no need to control the availability of toxic metals.But, In the case of toxic metal surpassed tolerable level, you need to control the mobility in order to let the accumulation plants live in the substrata. no need to much worry about metal availability after the treatment, only the accumulation plant grow relatively well, plant rhizosphere will mobilize the metal
It depends on the end outcome of the project. If the task is to rehabilitate heavy metal contaminated soils, conditions should be made conducive for effective plant absorption. For the above purpose, soils can be managed under acidic conditions to promote the mobility of heavy metals (e.g. application of FeSO4 could reduce soil pH) for plant absorption.
On the other hand, if plants are produced for food production purposes from agricultural soils with greater levels of heavy metals (e.g. Cadmium accumulation is agricultural soils because of regular use of processed phosphorus fertilisers), soil conditions must be managed to ensure less plant uptake. One such method is to increase soil pH by liming to reduce heavy metal mobility in agricultural soils. If plants (e.g. pasture) are grown as livestock feed in high pH soils, such plants may lack many trace elements because of high soil pH, in which case animals have to be fed supplements/minerals to ensure livestock health and performance.
Selva Selvarajah, thanks for your nice explanation. We have mine soil, where pH is very low (4.2) are not favorable for plant growth. We added lime and wood ash ( pH =8.8) to clean the soil by willows. -Soil is contaminated heavily with Cu and Zn.To increase the mobility of metals soil should be managed under acidic conditions for plant absorption.Now we are clear, if you know some results that support the statement please provide us to explore more.
Thanks
Mir Md Abdus Salam, it is basic soil chemistry that soil acidity (low pH) enhances heavy metal mobility. High mobility due to acidity can leach heavy metals into the receiving environment. For the above reasons, typically acid extractants are used to study heavy metals in soils (see my refereed paper under ResearchGate on paddy soils and Cu and Zn availability). Chelates (such as EDTA) can also mobilise heavy metals without altering pH, which are used in phytoremdiation.
Liming or flyash application can increase soil pH, thus stabilise heavy metals resulting in less heavy metal mobility, ; less leaching or less plant absorption.
Your soil pH 4.2 is very low and not conducive for phytoremediation since plant performance will be affected adversely. However, increasing pH >6.5 will result in poor uptake by plants owing to poor availability of heavy metal for plant absorption. Therefore, there is a balancing approach required to promote plant performance while improving heavy metal mobilisation.
There are many open access papers available on phytoremediation. I have provided a few below:
The following link refers to Zn and Cu in mining soils and phytoremediation
https://www.researchgate.net/publication/293300362_Phytoremediation_of_heavy_metal_contaminated_soil_and_its_mechanism
The following link is one example of how pH changes can affect plant absorption/heavy metal availability (you can find more by search engine):
https://pubag.nal.usda.gov/download/15471/PDF
Good luck, Selva
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