Determination of Exchangeable Potassium

Potassium Exchangeable K . (Feldspars, mica etc) 90-98% of Total K The final type of macro-nutrient is represented by potassium, which is available as a cation – positively charged ion   In some regions, this nutrient limits plant growth Most potassium comes from weathering of local minerals K in soil water Exch. + K in soil Soln 1 – 2 % of Total K (Exch. = 90 % & K in soil soln. = 10 %) Fixed K 2

Forms of K in the soil Relatively Unavailable K Slowly available K Readily available K Plant available K = readily available k Plant available K = Exchangeable K + soil solution K However, Plant available K is measured by Exchangeable K only

Types of K analysis Total K: Exchangeable K: Determines all the three pools of K. The procedure usually involves a digestion of the soil with HF. Exchangeable K: Exchangeable K is replaced by a cation such as NH4+. This analysis correlates well with plant available K for many soils and plants.

Procedure Preparation of Standard Curve Determine the flame photometer reading for the standard solutions (0, 2, 4, 6, 8 and 10 ppm K) Construct a standard curve with the readings.

Procedure Preparation of Soil Extract Weigh 10 g of soil into extraction bottle and add 100ml of 1.0 N NH4OAc solution. Place the bottle with contents in a shaking machine and shake for one hour. Filter the supernatant solution through No. 42 Whatman filter paper.

Procedure Determination of K Determine the flame photometer reading for soil extract. Using the meter reading and standard curve, determine the concentration K in the soil extract.

Results Table of values Standards (ppm K) % Emission 2 4 6 8 10 2 4 6 8 10 Soil extract y Blank z 8

Results A standard curve of emission (%) against concentration (ppm)

Calculations Equation for a linear curve y = mx +c From the calibration curve ; y = mx But y = % E x = Conc (ppm K) Hence % E = m x (conc ppm k) Conc K (ppm) in soil extract = % E m 10

Calculations Conc of K in soil (ppm) = Conc K in soil extract (ppm) x volume of extractant (ml) mass of soil (g) Conc of K in soil (ppm) = Conc K in soil extract (ppm) x 100 ml 10g Conc of K in soil (ppm) = Conc K in soil extract (ppm) x 10 11

Calculations mg K / kg to cmol K /kg soil 1cmol K = 390mg eg. 81mg/kg x 1 cmol/390 mg =0.208 cmol K/ kg soil

Sources of K in Soils K-containing minerals. Eg: Potash feldspars (KAlSi3O8) Muscovite KAl3 Si3O10(OH)2 Biotite KAl (Mg, Fe)3 Si3O10 (OH)2 Micas

Fate of K+ in soil K+ ion liberated by weathering may be lost through leaching taken up by plants or other living organisms in the soil be held on the cation exchange positions of the soil colloids; or be converted to less available forms.

Factors affecting K Equilibrium in Soils Conversion of soil and added potassium to less available forms is influenced by: 1. Types of Colloid Humus and clay are the two major soil colloids Humus has high capacity to retain cations in the exchangeable form has very low capacity for the fixation of K.

Factors affecting K Equilibrium in Soils Clay mineral Clays of the 2:1 type such as montmorillonite and illite fix K very readily and in large amounts. Clays of the 1:1 type such as kaolinite fix little K 2. Wetting and Drying The 2:1 type minerals fix K only upon drying (contraction). Some release of the ions occurs upon rewetting of the soils (expansion) 3. pH K fixation increases with application of lime or higher pH.

Luxury Consumption Absorption of nutrient elements in excess of the amount required for optimum growth. Luxury K K content of plant K required for optimum growth Required K Available K in soil

Leaching Losses of K More K is lost by leaching in contrast to N & P (particularly P). Leaching is common in heavily fertilized sandy soils.

Agricultural significance of K equilibrium) A very large proportion of K at a given time is relatively unavailable to plants. K is subject to wasteful leaching losses since its available form is very soluble. The removal of this element by crops is high, especially when high levels are available in the soil.