Audionut Posted March 8, 2016 Report Share Posted March 8, 2016 You don't need a degree in chemistry to understand the basics needed in creating your own fertiliser mixes. In this post I'll try and run through everything you need to know to create specific nutrient mixes, and I'll do my best to KISS. Inorganic salts (nutrients) are a combination of (positive) cations, and (negative) anions. Cations have a positive charge, and are nutrients such as Potassium (P+), Ammonium (NH4+), Calcium (Ca2+), Magnesium (Mg2+) and Iron (Fe2+). Anions have a negative charge, and are nutrients such as Nitrate (NO3-), Sulfate (SO42-), Phosphate (PO42-) and Chloride (Cl-). In life, things must maintain a neutral charge, meaning that you can't have some solution of just Potassium (P+), or just Nitrate (NO3-). Instead, these two elements can bond to form a salt (solid) as KNO3 (Potassium Nitrate). So that Potassium Nitrate is a salt of potassium and nitrate, and if this potassium nitrate is dissolved in a solution (water), the bond would break leaving free Potassium cations (P+) and free Nitrate anions (NO3-) in the solution, with the solution remaining as a neutral charge (equal positive and negative ions). Note that things like Potassium (P+) have a single positive charge, with Sulfate (SO42-) having a double negative charge. In order to have a salt of Potassium and Sulfate (Dipotassium sulfate (K2SO4)) there needs to be two Potassium (P+) ions, that only contain a single positive charge each, to bond with the double negative charge of Sulfate (SO42-). This is probably only really useful if you want to check the accuracy of your sample analysis, since we know that the sample must contain a neutral charge and an analysis of a sample that contains an imbalance of positive and negative charges would indicate measurement error. But anyway, I'll move on. What it does show is that we can't just add Nitrogen, or just Potassium, or just Sulfate to our growing medium, since there is always another ion attached. So if we want to increase the Nitrogen concentration in the medium, we must also add Potassium (KNO3) or Sulfate ((NH4)2SO4)) or Calcium (Ca(NO3)2) for example. If we want to add some nutrient to our growing medium, we can determine the concentration of the salt added as simply the amount of salt added in grams, times 1000, divided by the number of liters of water the salt is added to ((amount added in grams * 1000) / liters of water). So lets say we add 10 grams of an inorganic nutrient to 100 liters of water. 10 * 1000 = 10000 (amount added in grams * 1000)10000 / 100 = 100 (result of above / liters of water) So adding 10 grams of this inorganic nutrient to 100 liters of water would result in a nutrient concentration of 100 ppm (parts per million). If we want to determine how much of this nutrient concentration is a specific nutrient we can use a molar mass calculator. Plugging in KNO3 (Potassium Nitrate) to the molar mass calculator gives the following percentages. Potassium = 38.6717%Nitrate = 61.3284% That is to say, that KNO3 contains 38.67% by mass of Potassium, and 61.32% by mass of Nitrate. So if we take the nutrient concentration we determined above (100 ppm) and times that by the percentage of the specific nutrient we can find the concentration of that specific nutrient. In this case, 100 * 38.67% = 38.67 ppm of potassium and 100 * 61.32% = 61.32 ppm of nitrate. That is to say that if 10 grams of KNO3 was added to 100 liters of water, there would be 100 ppm of the nutrient solution, of which the Potassium concentration would be 38.67 ppm and the Nitrate concentration would be 61.32 ppm. We can extend this further to find just the Nitrogen concentration which is 13.85% by mass in KNO3, and thus would equal 13.85 ppm. A dosing solution is also relatively easy to determine. Lets say you have a one liter container, and you want each ten milliliter dose to add 10 ppm of Nitrate to 100 liters of water. Lets use KNO3 as the nutrient source. First lets work out some solution maths. 1000 ml / 10ml = 100, or in other words there are 100 doses of 10 ml solution in each liter of solution. To add 10 ppm of Nitrate from KNO3 to 100 liters of water, we have to do some of the above maths in reverse to make it easier in the long run. We can't just add the Nitrate, we have to add all the other stuff that comes with KNO3 also, so lets work this out. Nitrate is 61.32% by mass of KNO3, and we have to find the inverse. 100% / 61.32% = 1.63 So lets do a bunch of maths in reverse. 10 ppm (Nitrate) * 1.63 (Inverse percentage of Nitrate in KNO3) = 16.3 ppm (total concentration of KNO3 needed to be added to water)16.3 ppm * 100 (liters of water) = 1630.1630 / 1000 = 1.63. Or in other words, adding 1.63 grams of KNO3 to 100 liters of water will raise Nitrate concentration by 10 ppm. For our solution we simply multiply the grams by the number of doses in the dosing solution. 1.63 * 100 = 163 grams. Or in other words, adding 163 grams of KNO3 to one liter of water, will give a 10 ppm dose of Nitrate to 100 liters of water with a 10 ml dose. Pretty easy right............ Worse case scenario you should be able to purchase KNO3 for $25 per kg. 1000 grams / 163 grams = 6.13. $25 / 6.13 = $4.08. So this dosing solution would deliver 100 doses for $4.08. You might pay $30 or so for something like this with a pretty label. Best case scenario you purchase KNO3 for $1 per kg. $1 / 6.13 = $0.16. So this dosing solution would deliver 100 doses for 16 cents. Companies purchase tons of nutrients at a time for an even better cost per kg, so it becomes quite clear that when you purchase nutrients in liquid form, basically you pay top dollar for water, a bottle, a label, laziness and a couple of grams of the stuff that actually does something for the plants. Doing the maths long hand like this all the time is good for practice, but boring and repetitive. I suggest to add the maths to a spreadsheet, I have a spreadsheet for aquarium use that is rather complicated, I plan to prepare a spreadsheet for this specific purpose with better ease of use at some later date. If you're ready to go like a bull out of a gate, be mindful that inorganic nutrients have solubility limits. In the case of KNO3 this is 316 grams per liter at 20 degrees Celsius. If you try and add more it just will not dissolve, . Also, some of the nutrients do not play nice with each other. I'll discuss this and some other topics as time permits. 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