Potassium Dynamics
in Soils
 Introduction
 The Potassium Cycle
 Forms of Potassium in soil
 Availability of Potassium in soil
Potassium Management of soils
Introduction
K is the third most likely element after
nitrogen and phosphorus to limit plant
productivity.
Therefore it is commonly applied as fertilizer
It is present in soil solution as positively
charged cation, K+
It does not form any gases that is lost to air
Introduction
Its behavior in soil is influenced more
by cation exchange reactions than by
microbiological processes
It does not cause off site environmental
problems
It is not toxic and does not cause
eutrophication problems
-Potassium in Plant and
Animal Nutrition
Potassium activates many enzymes in
plants and animals that are
responsible for energy metabolism,
photosynthesis and other processes
As a component of the cytoplasm of
plant cells, it helps to lower cellular
osmotic water potential thereby
increasing the ability of root cells to
take up water.
-Potassium in Plant and
Animal Nutrition
K is important for nitrogen fixation in legumes
Good K nutrition has been shown to help
plants adapt to environmental stress
In animals including humans, K is important
in regulating the nervous system and
maintenance of good blood vessels.
-Deficiency Symptoms of
Potassium
Reduced ability to adapt to
environmental stress, e.g., drought,
lodging, etc
Tips and edges of leaves become yellow
(chlorosis) and then die (necrosis).
In some crops, K deficiency produces
white necrotic spots that looks like
insect damage.
The Potassium Cycle
The Potassium Cycle
Original source of K is primary minerals
 Micas (biotite and muscuvite)
 Potassium feldspar (orthoclase and
microcline)
Potassium is released from the minerals
during the weathering process
When available for plant uptake, it is taken
up by plants in large quantities
The Potassium Cycle
In natural ecosystems, K is returned to the
soil by rainwater leaching of foliage and by
plant residue or as waste from animals that
feed on plant
Potassium is lost by soil erosion and runoff,
and leaching to the groundwater
Most potassium in agricultural ecosystems are
lost through removal of crops and crop
residues from soil
Forms of Potassium in Soil
The four forms of K are as follows:




K in primary mineral structure
K in nonexchangeable positions of secondary
minerals
K in exchangeable form on soil colloid surfaces
K ions soluble in water
Total K in soil and its distribution as shown in
the K-cycle is a function of the clay minerals
present in soil
Availability of Potassium
Forms
K in primary mineral
structure
K in nonexchangeable
positions of secondary
minerals
K in exchangeable form
on soil colloid surfaces
K ions soluble in water
Unavailable (90-98% of
all soil K)
Slowly available
-fixed K is not easily exchangeable
-In equilibrium with more available
forms
Readily available (1-2% of
all soil K)
-90% of readily available
Readily available
-Subject to leaching
-Equilibrium with exchangeable form
K–Fixation by Soil Colloids
Nonexchangeable-K  Exchangeable-K  Soil Solution-K
Factors Affecting K
Availability
Types of soil colloids

Ability of various colloids to fix K varies (2:1)
Wetting and drying of soils

Physically affects the structure of colloids
Freezing and thawing

Physically affects the structure of colloids
Soil Acidity (pH)

High pH increases the fixation of K because of less
H+ and Al3+ at exchange sites.
K Problem in Soil Fertility
Potassium Management
The main problem with managing soil K is
that of converting the unavailable forms of
the element to available forms
 Available K is usually supplemented by fertilization
Another problem is removing a lot of crops
from the soil without returning the crop
residues
 Attempts should be made to return as much
residue as possible for the natural plant-soil
cycling of K to continue
Potassium Management
Growing high K content plants places demand
on the soil supply of potassium
 To have high yields of such crops, e.g., alfalfa, it needs to
planned in advance to supply soil with enough K to last the
cropping season –fertilization
Light and frequent application of fertilizer is
better than heavy and infrequent application
to reduce luxury consumption
Treating soils with lime have been found to
increase K retention in soils.