ESTIMATION OF AVAILABLE NITROGEN IN SOIL BY  ALKALINE PERMANGANATE METHOD (SUBBIAH AND ASIJA, 1956)

Aim

         To estimate the quantity of available (mineralisable) nitrogen in the given soil sample.

Principle

         The available nitrogen in the soil is estimated by distilling the soil with 0.32% KMnO4 and 2.5% NaOH. The evolved ammonia is absorbed in 2% boric acid and titrated against standard sulphuric acid. The amount of nitrogen that is mineralized in this estimation is considered as available for crop uptake.

Reactions                            

A.       Distillation                 

                                            alkaline

   2KMnO4 + H2O        →    2MnO2 + KOH + 3[O]

                                   Medium (NaOH)

                           Oxidative

RCHNH2COOH + (O) →    RCOCOOH + NH3

                                Deamination

                              distillation

 NH3 + H2O              →   NH4OH

                              absorption

NH4OH + H3BO3        →       (NH4)2 B4O7 + H20

B.     Titration

(NH4)2 B4O7   +   H2SO4          →     (NH4)2 SO4 + H3BO3

Reagents

1. 0.32% KMnO4 solution

2. 2.5% sodium hydroxide

3. Liquid paraffin

4. 0.02 N H2SO4

5. 2% Boric acid solution

6. Double indicator (prepared by dissolving 0.066 g of methyl red and 0.099 g of bromocresol  green in 100 ml of ethyl  alcohol)

Procedure

·         Transfer 20 g of 2 mm sieved soil into 1 litre distillation flask.

·         Add 20 ml of water, 1 ml of liquid paraffin and a few glass beads.

·         Add 100 ml each of freshly prepared 0.32% KMnO4 solution and 2.5% sodium hydroxide solution. 

·         Distill the contents and collect about 100 ml distillate in a 250 ml beaker containing 20 ml of 2% boric acid solution with 2 drops of double indicator.

·         Calculate the amount of mineralisable (available) nitrogen present in the soil.

Calculations

Weight of soil taken

=

20 gm

Volume of 0.02 N H2SO4 used in the titration

=

x ml

1 ml of 0.02 N H2SO4

=

0.00028 g of N

Therefore x ml of 0.02 N H2SO4

=

0.00028 x X g of N

This is present in 20 g of soil

In 2 x 106 kg soil i.e., the weight of 1 hec furrow slice soil

=

Rating of soil with reference to available N status

Low             =   Less than 280 kg ha-1

Medium       =   280 - 450 kg ha-1

High             =   More than 450 kg ha-1