ABSTRACT
Nickel (Ni) ion forms in soil solution are both cationic and anionic, depending mainly on soil solution pH. Ni is now generally accepted as an essential nutrient for plant growth, although the knowledge of the whole biological significance for plant productivity is still limited. Total soil Ni, as it stands for the rest of the heavy metals, reflects both geological and anthropogenic inputs and is not a good indicator of its mobility. There are different approaches to determine Ni forms in soils, and they are summarized as follows: column leaching techniques, another technique, and sequential extraction. Bioavailable fraction is considered the fraction of the total amount of a Ni existing in the soil that is available to receptor organisms. C. P. Jordao finds that all the fractions determined were significantly correlated with Ni concentration in lettuce tissues, concluding that all these fractions are easily available to plants.
