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. Results are provided in Table 1. 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.
(Table 1 is available on request drsvenkatesan@gmail.com)
Authors : S. Venkatesan, K.V. Hemalatha and A. Jeyakumar
