Available micronutrients  - fe, mn, cu, zn

ESTIMATION OF AVAILABLE MICRONUTRIENTS  (LINDSAY AND NORVELL, 1978)

         Several extractants have been tried to extract and estimate the available micronutrient status of the soil. Of these Diethylene Triamine Penta Acetic Acid (DTPA) was found to be the most suitable extractant for estimating the content of available Zn, Cu, Mn and Fe by the use of atomic absorption spectrophotometer.

Principle

         DTPA forms stable complexes with Zn, Cu, Mn and Fe. Its capacity to complex each of the micronutrient cation is 10 times its atomic weight.

Materials required

(i). DTPA 0.005 M, (ii) CaCl2.2H2O – 0.01 M solution and (iii) Triethanolamine : 0.1 M solution. (TEA suppresses the solubility of CaCO3 which will release the occluded micronutrients unavailable to the plant).

         DTPA extractant solution is prepared by dissolving 13.1 ml of TEA, 1.967 g of AR grade DTPA and 1.47 g of  CaCl2 in 100 ml glass distilled water. The contents are allowed for some time so that the DTPA will dissolve and then dilute to about 900 ml. The pH of the solution is adjusted to 7.3 ± 0.05 with 1:1 HCl by stirring and the volume made upto 1 litre.

Preparation of standard solution

Zinc

         0.4415 g of AR grade ZnSO4.7 H2O is dissolved in 200 ml glass distilled water in a beaker to which 5 ml of 1:5 H2SO4 is added. The contents are transferred to a one litre volumetric flask and the volume made up. This gives 100 ppm of Zn. From this stock solution working standards of 0, 0.1, 0.2, 0.4 and 0.6 µg/ml (or) ppm solutions are prepared.

Iron

         0.702 g of AR grade ferrous ammonium sulphate dissolved in 5 ml of 1:5 H2SO4 and made up to 1 litre with distilled water gives 100 ppm of Fe stock solution. From this stock solution various concentrations of working standards viz., 0,1,2,4,6 ppm are prepared.

Manganese

         0.288 g of AR grade potassium permanganate is dissolved in 300 ml of glass distilled water. To this 20 ml of conc.H2SO4 is added and warmed to 60oC. Then oxalic acid is added drop by drop to make the solution colourless. The contents are cooled and made upto 1 litre with gives 100 ppm stock solution. From this working standards of 0,1,2,4,6, 8 ppm are prepared.

Copper

         0.392 g of AR grade CuSO4 5H2O dissolved in glass distilled water and made upto 1 litre gives 100 ppm of Cu. From this working standards of 0,1,4,6 ppm solutions are prepared.

         From the working standard solutions, standard curve for each nutrient is prepared. While preparing the standard, start from the lowest concentration just after standardizing the instrument with blank solution. In between two standard solutions, introduce the blank and ensure that there is no change in the zero point. Prepare a graph by plotting the absorbance values against concentrations.

Procedure

·            Transfer 10 g of air dried soil sample to 150 ml conical flask / polythene bottle and add 20 ml of the DTPA extractant solution.

·            Close the bottle and shake for 2 hours in a horizontal shaker.

·            Filter through Whatman No.42 filter paper.

·            Take the reading of the filtrate in the atomic absorption spectrophotometer and by referring to the standard curve calculate the concentration of each micronutrient in the sample.

·             Shaking time, concentration and pH of the DTPA extractant and temperature will
  influence the quantity of nutrients extracted.

Critical levels of micronutrients

Zinc

1.2ppm

Copper

0.72ppm

Iron  (Non Calcareous Soil)

3.7ppm

(Calcareous Soil)

6.3ppm

Manganese

2.0ppm