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How to prepare fertilizer solutions in a two-tank system

When preparing a fertilizer stock solution, there are several details that should be noted to get stability, avoid precipitations, and maintain the best nutritional values for the plants.
 

The tanks

The tanks should be of opaque material that does not transmit light, and well-sealed.
This is necessary to avoid dirt, development of biotic organisms, and UV damage to chelates.

 

Water source

Knowing the quality of your irrigation water allows you to create a better fertilization program while saving unnecessary costs. EC, pH, and bicarbonates are critical factors that you should consider.
It is advisable to get a lab analysis if lab services are available in your region.

  • Alkalinity (high pH) is closely related to the presence of bicarbonates (HCO3-). High levels of bicarbonates increase the risk for precipitation in your tank.

  • Add acids to counter bicarbonates and to adjust the pH. Added acids will first neutralize the bicarbonates, and only then they’ll lower the pH.

  • Bicarbonates increase the electrical conductivity (EC) of the solution. EC can't be reduced by using acids.

  • The acids add nutrients (nitrogen, phosphorus or sulfur) to the stock solution. Take this into account in your calculations.


Compatibility

The two -tank system enables separation of fertilizers that might form precipitates when dissolved together.  

Note:

  • calcium and magnesium fertilizers should be dissolved separately from phosphorus and sulfur fertilizers.

  • In a single-tank system, make sure that the pH is between 3-4.

When magnesium sulfate is mixed in the same tank with phosphorus fertilizers, the pH of the solution must not exceed 5. Use the table to set fertilizer composition in each tank.

Micronutrients

Micronutrients can be added to the solution either as sulfur-based compounds or as chelates.

  • In the form of chelates, micronutrients are protected from precipitation, thus more available for plant uptake. However, the stability of chelates depends on pH. When choosing chelates, the following must be considered:
    • stock solution & drip water pH
    • soil or substrate pH
    • Water disinfection equipment (UV or acidification) might affect certain chelates
  • Sulfur-based compounds are pH indifferent and cost less, but they form precipitates easily, thus becoming unavailable for plant uptake.

 

Preparing the fertilizer solution

Temperature:  As solubility increases with temperature, higher water temperature enables higher concentration of nutrients in the stock solution.  
Maximum recommended concentration is 20% (e.g., 200 kg fertilizers per m3)
At low water temperatures, use concentrations between 10-15%.

  • Fill 1/4 - 1/3 of the spray tank with water.

  • Add the fertilizer(s) gradually while filling the rest of the tank with water.

  • Use an electric stirrer or an educator nozzle to improve dissolution rate.

Note: fertilizers of lower solubility will dissolve better if applied first. Follow the order advised below.

 

TANK A (calcium and magnesium) TANK B (phosphorus and sulfur)
Adjust the pH to 5-7.  If acid is needed, use only nitric acid.
Stir and wait for the solution to stabilize.
 
Set the pH level below 5, ideally around 4.
Stir and wait for the solution to stabilize.
Polyphosphate may be applied at this stage. If you use Haifa VitaPhos-K™, readjust pH. With Haifa GrowClean™, pH adjustment is unnecessary.
Advised order of fertilizer mixing into the solution
  1. Ammonium nitrate (AN)
  2. Magnesium Nitrate *
  3. Calcium Nitrate
  4. Potassium Nitrate
* MgNO3 is always preferred in tank A. It can be applied to tank B if pH is lower than 5.
** Ammonium or urea-based fertilizers may often give the solution a smell of rotten egg and create mild white precipitations in the tank. It can be solved by lowering the pH.
  1. Polyphosphates (if not already added)
  2. Potassium sulphate (SOP)*
  3. Monopotassium phosphate (MKP)
  4. Monoammonium phosphate (MAP)
  5. Potassium nitrate
  6. Magnesium sulphate
* Different SOP products affect the pH differently. Consider this when adjusting the pH.
* Haifa SOP™ Bio doesn’t affect the pH.
Micronutrients
  • Readjust pH and wait for the solution to stabilize before adding micronutrients.
  • Start adding the chelates after all the fertilizers are well dissolved.
  • In most cases, it is recommended to add chelated micronutrients in tank A.
  • Stability of iron chelates is more sensitive to the pH. Free iron can react with phosphoric acid (H3PO4) at pH lower than 3, so it’s always preferable to add it in tank A.
  • See the table below for more details



































 

Suitability of micronutrients in fertilizer solutions:

  pH stability range Suitable for Tank Comments
Sulphur based: 
MnSO4
ZnSO4
CuSO4
FeSO4
pH Indifferent Foliar application
Acidic soils
A, B
  • Low cost
  • Inferior plant availability
  • Contribute to soil salinity
  • Mn & Fe precipitate with P2O5 at pH below 4.5 – advised to use in tank A
Fe-EDTA 1-6.5 Foliar application
Acidic soils
A
  • Limited effectiveness in alkaline soils
  • Highly soluble
Fe-DTPA 2-8 Soilless greenhouses
Recirculating systems
A
  • Superior chelate stability compared to Fe-EDTA
Fe-EDDHA 4- 12 Soilless greenhouses
Recirculating systems
Alkaline soils
Calcareous soils
 
A
  • Prevents precipitation in alkaline soils.
  • Biodegradable.
  • Products with higher rate of ortho-ortho are considered of better quality.
  • UV susceptible - manage carefully in low pH disinfestation systems.
  • Enhanced risk of Mn deficiency at high pH
Fe-HBED 4-12 Soilless greenhouses
Recirculating systems
Alkaline soils
Calcareous soils
A
  • Iron chelate with the highest stability.
  • Considered as 100% ortho-ortho
FE-HEEDTA 2-8 UV disinfection systems
Acidic soils
Soilless greenhouses
A
  • Suitable for use with UV disinfection systems
Cu-EDTA
Zn-EDTA
1.5-10
 
All soils & Soilless Greenhouses
Open Field
A, B
  • High adaptability fits most agricultural cropping systems. 
Mn-EDTA 3-10 Soil & Soilless Greenhouses
Open Field
A, B
  • High adaptability fits most agricultural cropping systems.