Fertilizer Solubility
When applying fertilizers through the irrigation water (fertigation), it is essential to be familiar with some important facts regarding fertilizers solubility.| Solubility g/l | ||||||
| Fertilizer / Temperature (C˚) | 5 | 10 | 20 | 25 | 30 | 40 |
| Potassium nitrate | 133 | 170 | 209 | 316 | 370 | 458 |
| Ammonium nitrate | 1183 | 1510 | 1920 | . | . | . |
| Ammonium sulfate | 710 | 730 | 750 | . | . | . |
| Calcium nitrate | 1020 | 1130 | 1290 | . | . | . |
| Magnesium Nitrate | 680 | 690 | 710 | 720 | . | . |
| MAP (Mono Ammonium Phosphate) | 250 | 295 | 374 | 410 | 464 | 567 |
| MKP (Mono Potassium Phosphate) | 110 | 180 | 230 | 250 | 300 | 340 |
| Potassium chloride | 229 | 238 | 255 | 264 | 275 | . |
| Potassium sulfate | 80 | 90 | 111 | 120 | . | . |
| Urea | 780 | 850 | 1060 | 1200 | . | . |
Dissolving a Fertilizer
When you dissolve a fertilizer, you should not exceed its solubility. Otherwise, a precipitate may form and might clog the irrigation system. Moreover, the nutrients you intend to provide through the solution may not be fully available.For example, according to the data in the table above, the solubility of Potassium Nitrate in 20oC is 209 g/l and the fertilizer contains 38% Potassium. If you attempt to dissolve 300 g/l in the fertilizer stock tank, you will not get 114 g/l of Potassium (38% of 300g), but only 80g. The remaining 34g will precipitate and will not be available.
Mixing Fertilizers
When mixing fertilizers that contain a common element (for example potassium nitrate together with potassium sulphate) the solubility of the fertilizers is decreased. In such case, we cannot refer to the fertilizer solubility data alone. The same happens when the water used for dissolution is highly rich with minerals, e.g. calcium, magnesium or sulphate.
In such cases, additional chemical reactions come into play, and calculations become more complex. Usually, these are not calculated in the field and instead, trial-and-error practices are common.
Mixing Fertilizers - Fertilizer Compatibility
Some fertilizers should not be mixed together in one stock tank because an insoluble salt might form very quickly. An example for such incompatibility is mixing fertilizers that contain calcium with those that contain phosphate or sulphate.
Use this fertilizers compatibility chart:
The Jar Test
We've established that when mixing fertilizers, one must be familiar with the solubility data of the fertilizers used, as well as with the chemical reactions that may take place. In order to avoid unwanted precipitates, a common recommendation is to perform a "jar test". In this test, the fertilizers are initially mixed in a jar containing the same water used for irrigation.
Complete dissolution Formation of a precipitate
The fertilizers should be mixed exactly in the same concentration as intended to be used in the stock tanks. If a precipitate forms or if the solution has a "milky" appearance, the test should be repeated with lower concentrations of the fertilizers.
Meq/l - ppm conversion table
The fertilizers should be mixed exactly in the same concentration as intended to be used in the stock tanks. If a precipitate forms or if the solution has a "milky" appearance, the test should be repeated with lower concentrations of the fertilizers.
Conversion Tables
Element Forms| From | To | Multiply by | From | To | Multiply by | |
|---|---|---|---|---|---|---|
| NH4 | N-NH4 | 0.777 | N-NH4 | NH4 | 1.285 | |
| NO3 | N-NO3 | 0.226 | N-NO3 | NO3 | 4.427 | |
| P2O5 | P | 0.436 | P | P2O5 | 2.291 | |
| PO4 | P | 0.326 | P | PO4 | 3.066 | |
| K2O | K | 0.830 | K | K2O | 1.205 | |
| CaCO3 | Ca | 0.400 | Ca | CaCO3 | 2.497 | |
| CaO | Ca | 0.714 | Ca | CaO | 1.399 | |
| MgCO3 | Mg | 0.288 | Mg | MgCO3 | 3.467 | |
| MgO | Mg | 0.603 | Mg | MgO | 1.657 |
Meq/l - ppm conversion table
| From | To | Multiply by | From | To | Multiply by | |
| Meq/l N-NO3 or N-NH4 | ppm N | 14 | ppm N-NO3 or N-NH4 | Meq/l N | 0.07143 | |
| Meq/l H2PO4 | ppm P | 31 | ppm P (Phosphorus) | Meq/l H2PO4 | 0.03226 | |
| Meq/l K (Potassium) | ppm K | 39 | ppm K (Potassium) | Meq/l K | 0.0256 | |
| Meq/l Ca (Calcium) | ppm Ca | 20 | ppm Ca (Calcium) | Meq/l Ca | 0.05 | |
| Meq/l Mg (Magnesium) | ppm Mg | 12 | ppm Mg (Magnesium) | Meq/l Mg | 0.0833 | |
| Meq/l SO4 (Sulfate) | ppm SO4 | 48 | ppm SO4 (Sulfate) | Meq/l SO4 | 0.02083 | |
| Meq/l Na (Sodium) | ppm Na | 23 | ppm Na (Sodium) | Meq/l Na | 0.04348 | |
| Meq/l Cl (Chloride) | ppm Cl | 35.5 | ppm Cl (Chlorides) | Meq/l Cl | 0.02817 |