Potassium Dynamics in Soils

1
Potassium Dynamics in Soils

• Introduction
• The Potassium Cycle
• Forms of Potassium in soil
• Availability of Potassium in soil
• Potassium Management of soils

2
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

3
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

4
-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.

5
-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.

6
-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.

7
The Potassium Cycle
8
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

9
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

10
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

11

Availability of Potassium Forms
K in primary mineral structure Unavailable (90-98 of all soil K)
K in nonexchangeable positions of secondary minerals Slowly available fixed K is not easily exchangeable In equilibrium with more available forms
K in exchangeable form on soil colloid surfaces Readily available (1-2 of all soil K) -90 of readily available
K ions soluble in water Readily available Subject to leaching Equilibrium with exchangeable form
12
KFixation by Soil Colloids

• Nonexchangeable-K ? Exchangeable-K ? Soil
  Solution-K

13
Factors Affecting K Availability

• Types of soil colloids
• Ability of various colloids to fix K varies (21)
• 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.

14
K Problem in Soil Fertility
15
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

16
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.