Available potassium

ESTIMATION OF AVAILABLE POTASSIUM IN SOIL (STANFORD AND ENGLISH, 1949)

Aim 

         To estimate the available potassium status in the given soil sample.

Principle

         The method is based on the principle of equilibrium of soils with an exchanging cation made of the solution of neutral normal NH4OAc, in a given soil solution ratio. During the equilibrium, ammonium ions exchange with the exchangeable K ions of the soil. The K content in the equilibrium solution is estimated with a flame photometer. Since NH4+ holds highly charged layers together just as K, the release of the fixed K, in an exchangeable form, is retarded during NH4OAc extraction.

Reactions:

K+- CLAY + NH4+→ NH4+ - CLAY +  K+ + Excess of NH4+

Reagents

i.                    Neutral 1 N NH4OAc solution

ii.                  Standard K solution

Procedure

         Take 5 g of the soil sample in a 250 ml shaking bottle.  Add 25 ml of the neutral N NH4OAc extractant and shake for 5 minutes in a horizontal shaker. Filter the contents through Whatman No.1 filter paper collecting the filtrate in a suitable container.  Estimate the K concentration in the extract by aspirating the solution into the flame of the flame photometer and measuring the reading.  Prepare a standard curve for K concentrations ranging from 0 to 100 ppm at 10 ppm interval.  Find out the K concentration of the soil extract from the standard curve and calculate the available K status in the soil.

Calculations

Weight of soil taken

=

5 g

Volume of 1 N NH4OAc extractant added

=

25 ml

K concentration in the soil extract

=

X ppm

Available K status in the given soil

=

Rating of soil with reference to available K status

         Low             =      Less than 118 kg ha-1

         Medium       =      More than 118 and upto 280 kg ha-1           

         High             =      More than 280 kg ha-1 

Things to Learn

i)      Why K+ are replaced with NH4+ in available K estimation?

ii)    How will you prepare 1000 ml of 1000 ppm K by using KCl?

iii)    What is the principle involved in flame photometer?