🎃 Potasium

Forms, Function, Deficiency, Toxicity

  • The potassium ion (K+) is actively taken up soil solution by plant roots.
  • The concentration of K+ in vegetative tissue ranges from 0.1 to 4% on dry matter basis.

Sources of K

  • The micas and fieldspars constitute the major K bearing minerals which on weathering slowly release K to the soil. K-Feldspar: KAlSi3O10

Forms of potassium in soils

  • Water soluble K
  • Exchangeable K
  • Fixed or Non exchangeable K
  • Lattice or Mineral K

👉🏻 The different forms are in dynamic equilibrium with one another and represented as follows.

1. Water soluble K

  • The Concentration of H2O soluble K in soil depends on
    • (a) Type of Clay
    • (b) H2O content
    • (c) Intensity of leaching
    • (d) Amount of exchange K
    • (e) Kind and concentration of other ions
  • The dilution of the soil, increases the concentration of H2O soluble K and drying decreases it.
  • The uptake of K is influenced by the presence of other cations, i.e. Ca2+ and Mg2+, Al3+ (acidic soils) and Na+ in salt affected soils.

Activity Ratio

  • Measures of the intensity of labile K the soil.
  • It represents K that immediately available to crop roots.
  • P & K is absorbed by plant mainly through diffusion.

2. Exchangeable K

  • The K adsorbed on soil clay complex and replaceable with neutral salts in relatively short time.
  • The Cl reacts with soil Ca and form CaCl2 which is leached by high rainfall. Hence K is called as Decalcifier.

3. Non exchangeable or Fixed K

  • Added K which is firmly bound by the soil and not immediately replaceable with neutral salts.

4. Lattice K or Mineral K

  • Muscovite, biotite and K feldspar. The capacity of soils to release lattice K by weathering depends on the content of K minerals and soil texture.

Potassium fixation

  • In the dynamics of soil Potassium, the phenomenon of fixation of exchange K and the liberation of non-exchange K play an important role.
  • K ions are relatively small to enter the silica sheets where they are held firmly by electrostatic forces.
  • The presence of K+ ions can block the release of fixed NH4+ and vice visa.

Functions of K

  • Unlike the other elements, K does not enter into the composition of any of the important plant constituents. It is present in a plant in a state of solution in cell sap but it is not effective without its co-nutrients such as N & P.
  • It regulates the opening and closing of stomata.
  • Plays a major role in transport of water and nutrients throughout the plant in xylem.
  • Imparts disease resistance and it improves drought tolerance. (Reduce transpiration rate & increase photosynthesis) AFO-2021
  • Neutralizes organic anions and other compounds and maintains the pH of cytoplasm in the range of 7-8.
  • Activates > 60 enzymes, including starch synthetase, K is responsible for the activation of nitrate reductase enzyme.
  • Enhances crop quality (increases size of grain or seed), size and shelf life of fruits and vegetables & improves the fiber quality of cotton and quality of tobacco leaf.
  • In citrus fruits, however an excess of K has a bad effect on quality.
  • Because potassium improves the overall health of growing plants and helps them fight against disease, it is known as the “quality” nutrient.
  • Produce strong stiff straw especially in Paddy & Wheat which reduces lodging of crops, enhances winter hardiness.
  • It regulates the movement of ions with in the plants and hence it is called traffic policeman of the plant.
  • Essential in the formation and transfer of starches and sugars thus required in large quantity for potato, sweet potato, turnip, banana.
  • Regulates H2O imbalance within the plant.

Deficiency of K

  • Potassium deficiency does not manifest immediately in the form of visible symptoms. First growth rate decreases and later deficiency symptoms appear. Deficiency symptoms first develop on the older leaves.
  • Chlorosis along the margins followed by scorching and browning of tips of older leaves which gradually progresses inwards giving burning appearance. Slow and stunted growth of the plant and crop lodging.
  • Shrivelled fruits and seeds.
  • Reduced crop yields without the appearance of definite symptoms; the phenomenon is called hidden hunger.
  • Decrease in resistance to certain plant diseases
  • Decrease in the quality of fruits and vegetables.
  • Potassium deficiency disturbs the overall physiological activity within the plant system by altering the activities of enzymes like invertase, catalase in crops like sugarcane.
  • Rice: The leave tips will dark brown in colour and blades will blueish green, chlorotic and necrotic are seen.
  • Banana: Deficiency is seen in the margin and bottom of leaves.
  • Grapes: Leaves are yellow with brown spots which are necrotic, brittle with uneven ripening.
References 
- Tisdale, S.L.,Nelson,W.L.,Beaton,J.D.,Havlin,J.L.1997.Soil fertility and Fertilizers. Fifth edition, Prentice hall of India Pvt. Ltd, New Delhi.
Singh, S.S.1995.Soil fertility and Nutrient Management. Kalyani Publishers, Ludhiana.
- http://ocw.mit.edu
- https://www.cropnutrition.com/efu-potassium

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