Neil Mattson [email protected] Volume 3 Number 5 March 2018 Fertilizer Calculation Basics 2018 Sponsors for Hydroponics I’m often asked by growers and students alike to walk them through the calculations for setting up a hydroponic fertilizer recipe. While the math itself is pretty straightforward, there are several key points to take into account, including: percent elemental composition of a fertilizer, injector ratios, size of stock tank, and compatibility of fertilizer salts in stock tanks. This alert will cover the basics of fertilizer calculations when using dry fertilizers Step one how much fertilizer to achieve a ppm target? We’re going to begin using metric units (milligrams, grams, liters) but I’ll show how to convert to U.S. standard units. Let’s say we want to use commercial calcium nitrate to supply 100 ppm nitrogen (N). There are different forms of calcium nitrate but let’s assume the form we are using contains 19% calcium (Ca) and 15.5% nitrogen (N) (these values come from the product label). Let’s calculate how many milligrams (mg) of calcium nitrate we need to achieve 100 ppm N in 1 liter (L) of water. www.e-gro.org 1 e-GRO Edible Alert - 2018 Fertilizer Calculations The basic principle we follow is that 1 ppm equals 1 mg/L. (This is an inherent property of Conversions physics. 1 L of water at room temperature 1 gallon = 3.785 Liters weights 1 kg or 1,000,000 mg, therefore 1 mg of a given element in 1,000,000 mg of water 1 ounce = 28.35 grams becomes 1 ppm). Then we have to take into 1 gram = 0.03527 ounces account that a given fertilizer contains only some fraction of element. In our case calcium nitrate 1 pound = 454 grams contains 15.5% nitrogen. To calculate how many mg needed per 1 L of water divide the target 1 pound = 0.45 kilograms value by the percent of the element. 1 kilogram = 1,000 grams Example: use commercial calcium nitrate to 1 gram = 1,000 milligrams supply 100 ppm N. 1 ppm = 1 mg/L 100 mg/L (ppm) N / %N 100 mg/L N / 0.155 [this is the percent N in calcium nitrate] = 645 mg of calcium nitrate in 1 L of water Conversion factors Now let’s say we want to know how many ounces Great! Now, remember the fertilizer salts we use of magnesium sulfate we need to prepare 100 contain two elements of interest. In our case, the gallons of water with the above target of 40 ppm fertilizer also contains calcium, so let’s calculate magnesium. We start by doing the above math, the ppm (or mg/L) of calcium supplied by using which tells us we need 412 mg of magnesium 645 mg of calcium nitrate in 1 L of water. This is sulfate per liter of water. Then we just take into calculated by multiplying the total mg/L of conversion factors. fertilizer salt used by the percent calcium (19% Ca): For convenience let’s start by converting 412 mg to grams. Remember 1 gram has 1,000 mg. So, 645 mg/L calcium nitrate x 0.19 (% Ca) = 122.6 412 mg = 0.412 g. Next, remember 1 gallon of mg/L (ppm) Ca. water contains 3.785 liters. Therefore 100 gallons of water contains 100 x 3.785 = 378.5 L. So the Therefore using 645 mg of calcium nitrate in 1 L amount of magnesium sulfate we need for 378.5 L of water, provides 100 ppm N and 122.6 ppm Ca. of water is: Let’s try one more practice example. How many 0.412 g magnesium sulfate per liter x 378.5 L = mg of magnesium sulfate (9.7% Mg, 13% S) do you 155.9 grams need to provide 40 ppm Mg? And how many ppm S does this also supply? Finally to convert to ounces, note that 1 ounce = 28.35 grams. So we divide the number of grams 40 mg/L Mg / 0.097 = 412 mg of magnesium by 28.35 to calculate the number of ounces of sulfate in 1 L of water fertilizer required. How much S does this supply? 412 mg/L 155.9 grams magnesium sulfate / 28.35 g/oz. = magnesium sulfate x 0.13 = 53.56 mg/L S (ppm S) 5.5 oz. of magnesium sulfate per 100 gallons of water www.e-gro.org 2 e-GRO Edible Alert - 2018 Fertilizer Calculations Note! Beware P and K By convention in the U.S. the fertilizer label lists the percentage P2O5 instead of the percent P (phosphorus). And similarly the labels lists the percentage K2O instead of the percent K. Sadly this means we have to take into account conversion factors to calculate the percent elemental P and K. To convert P2O5 to P multiply by 0.4364 To convert P to P2O5 multiply by 2.2915 To convert K O to K multiply by 0.8301 2 Two fertilizer stock tanks each with their own fertilizer To convert K to K2O multiply by 1.2047 injector are used because some fertilizer are incompatible with each other in concentrated amounts. For example, let’s assume we are using a common hydroponic fertilizer 5-12-26 which is needs to have twice as much fertilizer as when used to provide most of the macronutrients and using a 1:100 ratio, because in this case the micronutrients except for Ca and enough N fertilizer injector takes 1 part fertilizer and adds (therefore, it is typically used in conjunction with 199 parts tap water. calcium nitrate). This means when we are conducting fertilizer Let’s calculate the percentage P and K in 5-12-26. calculations we will need to multiply the mg of Remember the 3 numbers mean %N - %P2O5 and % fertilizer salt per 1 L of water times the volume K2O of the stock tank and times the injector ratio. Since the percent P2O5 is 12%, then we multiply Injectors and Stock Tanks 12% by 0.4364 5.24% P Most commercial hydroponic facilities prepare And, since the percent K2O is 26%, then we fertilizers in concentrated stock tanks and then multiply 26% by 0.8301 21.58% K use fertilizer injectors to dilute fertilizer down to the level needed by the plants. This allows the Injectors and Stock Tanks liquid fertilizer stocks to be stored in relatively Most commercial hydroponic facilities prepare low volume tanks. A common injector ratio is fertilizers in concentrated stock tanks and then 1:100, meaning the fertilizer in the concentrated use fertilizer injectors to dilute fertilizer down to stock tank is at a 100 times higher concentration the level needed by the plants. This allows the than what the plant receives. The fertilizer liquid fertilizer stocks to be stored in relatively injector then takes 1 part fertilizer stock and low volume tanks. A common injector ratio is adds 99 parts tap water to prepare the dilute 1:100, meaning the fertilizer in the concentrated fertilizer that plants receive. Injector ratios in stock tank is at a 100 times higher concentration the range of 1:50 to 1:200 are commonly used. A than what the plant receives. The fertilizer ratio of 1:200 means that are stock solution injector then takes 1 part fertilizer stock and needs to have twice as much fertilizer as when adds 99 parts tap water to prepare the dilute using a 1:100 ratio, because in this case the fertilizer that plants receive. Injector ratios in fertilizer injector takes 1 part fertilizer and adds the range of 1:50 to 1:200 are commonly used. A 199 parts tap water. ratio of 1:200 means that are stock solution www.e-gro.org 3 e-GRO Edible Alert - 2018 Fertilizer Calculations Tomato Tomato Tomato Stage Element Stage 2 Stage 3 Tank A Tank B 1 (ppm) (ppm) (ppm) • Calcium nitrate • ½ of potassium nitrate • ½ of potassium • Potassium sulfate NO3-N 90 120 190 nitrate • Monopotassium phosphate NH4-N 0 0 0 P 47 47 47 • Iron chelate • Magnesium sulfate K 144 350 350 • (Nitric acid) • Monoammonium phosphate Ca 144 160 200 • Ammonium nitrate Mg 60 60 60 Tank C • All micronutrients except S 116 116 116 • Acid, used to drive iron chelate Cl 89 89 89 down pH (sulfuric, • (sulfuric acid) Fe (EDTA) 2 2 2 nitric, phosphoric, • (phosphoric acid) citric, etc.) Mn 0.55 0.55 0.55 Zn 0.33 0.33 0.33 Cu 0.05 0.05 0.05 Table 2. Commonly used procedure for separating B 0.34 0.34 0.34 fertilizer salts in 2-3 stock tanks to avoid Mo 0.05 0.05 0.05 incompatibility. Table 1. Nutrient targets for tomato at stage 1 (seedling to 2nd truss anthesis), stage 2 (2nd truss to 5th truss anthesis), and stage 3 (after 5th truss anthesis). Recipe comes from the University of Arizona Controlled Environment Agriculture Center (CEAC) This means when we are conducting fertilizer calculations we will need to multiply the mg of fertilizer salt per 1 L of water value times the volume of the stock tank and times the injector ratio. Beware Fertilizer Incompatibility! When prepared in stock tanks in a concentrated form, many fertilizer components are not compatible – meaning that when combined they form a precipitate (or a water insoluble sludge) that will not dissolve in water or be taken up by the fertilizer injector. The most common incompatibilities involve calcium nitrate, which cannot be combined with fertilizers container sulfates or phosphates.