Soil Micronutrients

Micronutrients are present in soils mainly as their oxides,
sulphides and silicates. They are inherited from the
soilforming rocks and minerals through transformation during
various stages of soil development. The distribution of
micronutrient minerals is not uniform in a soil but spatial
variations could be identified through a suitable soil test
method. The elemental analysis consists of two distinct
parts, viz extraction and determination. Determination of
micronutrient is proved to be better with Atomic Absorption
Spectrophotometer (AAS), while there are many factors to
be standardized in the extraction part. The influence of two
different extractants namely EDTA and DTPA at different
pH on the extraction of micronutrients was tested in the
laboratory as a first step to standardize a suitable analytical
method which could be used to determine soil available
nutrients like zinc (Zn), manganese (Mn), iron (Fe)
and copper (Cu).

Soil samples were collected from Munnar, Vandiperiyar
and Nilgiris. They were air dried, passed through 2 mm
sieve and stored at room temperature. Equimolar solutions
of EDTA and DTPA were prepared and adjusted to various
pH of 2.5, 7.0, 8.5 and 10. The soil samples were shaken
for an hour and centrifuged. The supernatant liquid was
analysed in AAS for Zn, Mn, Fe and Cu. The soils from
Munnar were richer in Cu and Zn followed by those from
Vandiperiyar and Nilgiris. Content of iron was higher in
Nilgiris soils when compared to that of the other two locations.
Manganese content was generally lower in Vandiperiyar soils
in comparison to the soils of Nilgiris and Munnar. It was inferred
that extraction capacity of EDTA was on par with that of DTPA
for the extraction of Zn, Mn and Cu. However, EDTA is cheaper
than DTPA. On the other hand, extraction of Fe was found
better with DTPA.

The amount of Zn, Fe, and Cu extracted using both the
extractants were higher at higher pH, while maximum
quantity of Mn was extracted at acidic pH (2.5). This is
because Mn is mostly present in +2 oxidation state in
tea soils, which is relatively basic when compared to the
other elements studied. Hence, there could be more
chance for Mn to form a complex with extractants at
acidic pH resulting in more yield. Similarly, the other
elements, which are relatively acidic were extracted
better at basic pH. In most of the cases, their availability
was found in the order of Fe>Cu >Zn. Acidity of the above
nutrients is also lying in the same order, i.e, Fe is more
acidic than Cu and Zn. Hence it is concluded that higher
the acidity of a particular ion, higher will be the quantity
extracted at basic pH. Extraction of large quantity of iron
at strongly acidic conditions also reveals that it is
predominant in +2 oxidation state in tea soils as the
other oxidation sites are relatively basic.