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Main roles of nitrogen (N) in plants

Nitrogen is a major component of all proteins (including all enzymes), amino acids, nucleic acids and chlorophyll. Nitrogen is continuously sourced from the soil and converted from the mineral to the organic form. The most important inorganic sources involved in this conversion are NO3-, NH2 and NH4+. Typical values of nitrogen contents in plants are 3-5 % in their dry matter.

Nitrogen deficiency imbalances symptoms in plants

Nitrogen deficiency symptoms are expressed by the plant as:

  • Reduced growth rate.
  • Reduced size of plant, flower and fruits.
  • Chlorosis (general yellowing of the plant starting in the older parts of the plant).

Main roles of potassium (K) in plants

Potassium cation has no structural purpose but it is the most common cation in the plant biochemical processes. Essential roles of potassium are to be found in the protein synthesis, the photosynthetic process and the transport of sugars from the leaves to the fruits and in the production and accumulation of oils. A good potassium supply shall therefore sustain the leaf function all along fruit growth and shall contribute to the positive effect of K on yield and on a high content of soluble solids (more sugars) in fruits at harvest time. For example in tomato, about 60-66 % of K absorbed by the plant is found in the fruit. The action of potassium on protein synthesis enhances the conversion of absorbed nitrate into proteins which contributes to an enhanced efficiency of the supplied N fertiliser (SQM, 2006). Typical values of potassium contents in plants are 3-4 % in their dry matter.

The potassium cation is involved in the maintenance of plant osmotic potential (cell turgescence), one implication of this being the regulation of the opening and closure of stomata, which allow plants to exchange gas and water with the atmosphere. This enables plants to maintain an adequate hydric status under stress conditions such as salinity or water shortage. Indeed, crops with a high potassium content generally show a better water use efficiency, that is, they consume relatively less water than K deficient crops to produce the same amount of biomass. In addition, potassium is involved in maturation processes of the fruit such as the synthesis of the pigment lycopene, which is responsible for the red colour of fruits, such as tomato fruits. Potassium also promotes a high acid content, which is essential for a good fruit flavour (SQM, 2006).

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Figure 1. K intensifies the transport and storage of assimilates from the leaf to the fruit.
(Click on the figure to enlarge, Click here to open and print the figure)

Summary of the main roles of potassium in the plant:

  • Promotes the production of proteins (faster conversion of inorganic N to proteins).
  • Promotes the photosynthesis (more CO2 assimilation, which will result in more sugar production).
  • Intensifies the transport and storage of assimilates (from leaf to fruit) (Figure 1).
  • Prolongs and intensifies assimilation period (which results in higher fruit quality).
  • Improves the efficiency of N fertilisers.
  • Improves the water use efficiency (less water needed per kg plant biomass).
  • Regulates the opening and closure of stomata (guard cells).
  • Is responsible for the synthesis of lycopene (red colour pigment in fruits).

Potassium deficiency symptoms in plants

  • Leaf and stem
    • Young plants have dark green leaves, small stems and short internodes.
    • Necrosis of the leaf border in the oldest leaves, the leaf curls upwards.
    • Interveinal necrotic spots in the oldest leaf.
    • Reduced plant height and leaf area.
  • Fruits and storage organs
    • Fruits drop easily during ripening.
    • Blotchy ripening.
    • Fruits are insipid (tasteless), lacking acidity.
    • Green and yellow areas merging into the red colour of the surface of tomato.
    • Uneven ripening.
    • ‘Glassy’ blotches.
    • Reduced number of fruits per truss.
    • Reduced proportion of fruits set.
    • Reduced average weight per fruit.
    • Storage organs (e.g. potato tubers) with low dry matter content.


SQM, 2006. SQM Crop Kit Tomato.


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