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Table 1 shows a comparison between potassium nitrate, potassium sulphate and potassium chloride on various physico-chemical characteristics.

Table 1. Physico-chemical characteristics of potassium nitrate, potassium sulphate and potassium chloride.

Physico-chemical characteristic




% K2O




% K




% Accompanying anion

57,6-60,7 as NO3

51,0-54,6 as SO4

45,2-46,7 as Cl

13,0-13,7 as N

17,0-18,2 as S

45,2-46,7 as Cl

Salt index




EC (1 g/l at 25°C; mS/cm)




pH standard grades (10% sol.)




pH acidic grades (10% sol.)


Maximum solubility at 10°C




Maximum solubility at 20°C




Figure 1 clearly shows the advantage of potassium nitrate (KNO3) over other K-carriers regarding its solubility in most typical agricultural conditions.


Figure 1. Solubility of potassium nitrate (KNO3), potassium chloride (KCl) and potassium sulphate (K2SO4) at different temperatures (Wikipedia, 2010). (Click on the figure to enlarge, Click here to open and print the figure)

Salt Index

Salt content is one of the most critical characteristics of fertilisers used for row-seed placement. The Salt Index (SI) is a measure of the salt concentration that a fertiliser induces in the soil solution (Mortvedt, 2009).

The SI of a material is expressed as the ratio of the increase in osmotic pressure of the salt solution produced by a specific fertiliser to the osmotic pressure of the same weight of sodium nitrate (NaNO3), which is based on a relative value of 100. Sodium nitrate was chosen as the standard because it is 100 % water-soluble and because it was a commonly used nitrogen fertiliser when the SI concept was first proposed in 1943.

When applied near the seed, fertilisers with lower SI values will better ensure seed germination and undisturbed proliferation. At equal product weight, potassium nitrate has a lower SI value (73,6) than KCl (116,3).

Besides the SI there is another indicator, used to express the potential risk of soil salinity, which is related to the effect of fertiliser solutions on the electrical conductivity (EC).  In Potassium nitrate and saline conditions the effect on the EC of various fertiliser combinations, compared to KNO3, when the N and K input are kept constant, is demonstrated. Proper fertiliser selection will help to reduce the risk of soil salinity and associated yield reduction, in particular when salts cannot be washed down by irrigation.

EC (Electrical Conductivity)

The electrical conductivity (EC; or specific conductance) of an electrolyte solution is a measure of its ability to conduct electricity. The EC-value of a fertiliser salt is expressed in milliSiemens/cm or deciSiemens/m, measured at a temperature of 25 °C after dissolving 1 gram of fertiliser salt per litre of water. Potassium nitrate has the lowest EC value (Table), compared to the alternative K sources.

Maximum solubility

The maximum solubility of potassium nitrate is 2 to 3 times higher than potassium sulphate (SOP). This allows for the preparation of more concentrated mother tank solutions with potassium nitrate. It should be noted that the temperature of the water will drop, once a fertiliser salt is being dissolved in water. This drop in water temperature will temporarily limit the maximum dissolution rate, until a temperature equilibrium is obtained with the environment.

Speed of dissolution

Another advantage of potassium nitrate is its high speed of dissolution: potassium nitrate dissolves much faster than potassium sulphate. It will take less time to fully dissolve a similar amount of fertiliser salt containing potassium nitrate than one that contains potassium sulphate.


Haifa, 2009. Leaflet.

Mortvedt, J.J. 2009. Calculating salt index.

Wikipedia, 2010. Solubility table.


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