Oranges [photo] 


Solubility of the OMNI™ range

The OMNI™ range of products is made up of highly soluble raw materials. These high grade products have certain physical and chemical properties that influence the solubility and compatibility with other products. The raw materials used in these products react differently when mixed with water. One of the consequences is that the products can bring about a change in temperature. On the one hand the temperature can rise, which is known as an exothermic reaction, while it can also be responsible for an endothermic reaction where temperature will drop. The question arises: Why is this important when fertilizer is dissolved? The resulting temperature during the dissolving of these products has a direct influence on the inherent solubility thereof.

Table 1 shows how the solubility of certain raw materials changes with an increase in temperature. The key to success when dissolving speciality fertilizers lies in the starting temperature. The higher the starting temperature and the longer it can be maintained will eventually determine the actual solubility.

Table 1: Solubility of different speciality products at different starting water temperatures

  0 10 20 30
OMNI K™ 13.9 21.9 31.6 45.3
VITA K™ 7.4 9.3 11.1 13.0
MgSO4 22.0 28.2 33.7 38.4

Several methods exist to stabilise the temperature at a higher level. Physical heating is done in fertilizer factories under highly controlled circumstances. The most practical method is to heat the water by means of heaters using solar energy, electricity or fuel – including diesel, coal and wood. The use of air bubbling through the water is employed with great success at farm level. This method is slower, but improves the agitation of the mixture to further increase the dissolving tempo.

The water quality is just as important, as the chemical composition of the water during the dissolving process of certain salts (such as fertilizer) can lead to precipitation. Besides the content of dissolvable salts in the water, it is also important to maintain a relatively low pH before starting the dissolving process. The total neutralisation of carbonates and bicarbonates can be accomplished at a pH of 5.5.

Table 2 shows the salt indexes which are not acceptable for use in irrigation water and inevitably, the dissolving of fertilizer.

Table 2: Salt indexes not acceptable for use in irrigation water

  mS/m mmol/L
EG / EC >270  







(ARC 2004 – Institute for Agricultural Sciences)

Every product in the OMNI™ range holds an inherent maximum solubility. Products containing sulphur and potassium as part of the base materials have a very limited solubility. On the other hand, calcium and nitrate containing products are more soluble. It is obvious that farmers wanting to dissolve soluble fertilizers at farm level should proceed very cautiously to ensure that the best value is derived from the product. If the basic rule of thumb is adhered to where no more than 10% of the product is added to the water, problems should not arise when using the OMNI™ range of products.

Herewith a few additional rules when products are dissolved in a tank:

  • the higher the temperature, the better;
  • determine water quality and rectify the pH of the water to approximately 5.5 or lower;
  • fill the tank halfway with water;
  • start circulating or agitating the water – the higher the temperature, the better;
  • add fertilizer to the water at approximately 3 kg per minute;
  • when 50% of the water has been added, fill the tank to 90% capacity;
  • add the last fertilizer to the mixture at the same rate of 3 kg per minute;
  • circulate the mixture for approximately two hours or until the fertilizer has dissolved; and
  • fill to capacity and circulate for another 30 minutes.

For further information and assistance, contact your local Omnia agronomist.

By Uys Meiring, Senior Technical Advisor: Specialities