Volume 3 Number 5 March 2018 Fertilizer Calculation Basics .

Neil Mattsonnsm47@cornell.eduVolume 3 Number 5 March 2018Fertilizer Calculation Basicsfor Hydroponics2018 SponsorsI’m often asked by growers andstudents alike to walk themthrough the calculations forsetting up a hydroponic fertilizerrecipe. While the math itself ispretty straightforward, there areseveral key points to take intoaccount, including: percentelemental composition of afertilizer, injector ratios, size ofstock tank, and compatibility offertilizer salts in stock tanks. Thisalert will cover the basics offertilizer calculations when usingdry fertilizersStep one how much fertilizer toachieve a ppm target?We’re going to begin using metricunits (milligrams, grams, liters)but I’ll show how to convert toU.S. standard units. Let’s say wewant to use commercial calciumnitrate to supply 100 ppm nitrogen(N). There are different forms ofcalcium nitrate but let’s assumethe form we are using contains19% calcium (Ca) and 15.5%nitrogen (N) (these values comefrom the product label). Let’scalculate how many milligrams(mg) of calcium nitrate we needto achieve 100 ppm N in 1 liter (L)of water.www.e-gro.org1

Fertilizer Calculationse-GRO Edible Alert - 2018The basic principle we follow is that 1 ppmequals 1 mg/L. (This is an inherent property ofphysics. 1 L of water at room temperatureweights 1 kg or 1,000,000 mg, therefore 1 mg ofa given element in 1,000,000 mg of waterbecomes 1 ppm). Then we have to take intoaccount that a given fertilizer contains only somefraction of element. In our case calcium nitratecontains 15.5% nitrogen. To calculate how manymg needed per 1 L of water divide the targetvalue by the percent of the element.Conversions1 gallon 3.785 Liters1 ounce 28.35 grams1 gram 0.03527 ounces1 pound 454 grams1 pound 0.45 kilograms1 kilogram 1,000 grams1 gram 1,000 milligramsExample: use commercial calcium nitrate tosupply 100 ppm N.1 ppm 1 mg/L100 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 waterGreat! Now, remember the fertilizer salts we usecontain two elements of interest. In our case, thefertilizer also contains calcium, so let’s calculatethe ppm (or mg/L) of calcium supplied by using645 mg of calcium nitrate in 1 L of water. This iscalculated by multiplying the total mg/L offertilizer salt used by the percent calcium (19%Ca):645 mg/L calcium nitrate x 0.19 (% Ca) 122.6mg/L (ppm) Ca.Therefore using 645 mg of calcium nitrate in 1 Lof water, provides 100 ppm N and 122.6 ppm Ca.Let’s try one more practice example. How manymg of magnesium sulfate (9.7% Mg, 13% S) do youneed to provide 40 ppm Mg? And how many ppm Sdoes this also supply?40 mg/L Mg / 0.097 412 mg of magnesiumsulfate in 1 L of waterHow much S does this supply? 412 mg/Lmagnesium sulfate x 0.13 53.56 mg/L S (ppm S)Conversion factorsNow let’s say we want to know how many ouncesof magnesium sulfate we need to prepare 100gallons of water with the above target of 40 ppmmagnesium. We start by doing the above math,which tells us we need 412 mg of magnesiumsulfate per liter of water. Then we just take intoconversion factors.For convenience let’s start by converting 412 mgto grams. Remember 1 gram has 1,000 mg. So,412 mg 0.412 g. Next, remember 1 gallon ofwater contains 3.785 liters. Therefore 100 gallonsof water contains 100 x 3.785 378.5 L. So theamount of magnesium sulfate we need for 378.5 Lof water is:0.412 g magnesium sulfate per liter x 378.5 L 155.9 gramsFinally to convert to ounces, note that 1 ounce 28.35 grams. So we divide the number of gramsby 28.35 to calculate the number of ounces offertilizer required.155.9 grams magnesium sulfate / 28.35 g/oz. 5.5 oz. of magnesium sulfate per 100 gallons ofwaterwww.e-gro.org2

Fertilizer Calculationse-GRO Edible Alert - 2018Note! Beware P and KBy convention in the U.S. the fertilizer label liststhe percentage P2O5 instead of the percent P(phosphorus). And similarly the labels lists thepercentage K2O instead of the percent K. Sadlythis means we have to take into accountconversion factors to calculate the percentelemental P and K.To convert P2O5 to P multiply by 0.4364To convert P to P2O5 multiply by 2.2915To convert K2O to K multiply by 0.8301To convert K to K2O multiply by 1.2047Two fertilizer stock tanks each with their own fertilizerinjector are used because some fertilizer areincompatible with each other in concentrated amounts.For example, let’s assume we are using acommon hydroponic fertilizer 5-12-26 which isused to provide most of the macronutrients andmicronutrients except for Ca and enough N(therefore, it is typically used in conjunction withcalcium nitrate).Let’s calculate the percentage P and K in 5-12-26.Remember the 3 numbers mean %N - %P2O5 and %K2OSince the percent P2O5 is 12%, then we multiply12% by 0.4364 5.24% PAnd, since the percent K2O is 26%, then wemultiply 26% by 0.8301 21.58% KInjectors and Stock TanksMost commercial hydroponic facilities preparefertilizers in concentrated stock tanks and thenuse fertilizer injectors to dilute fertilizer down tothe level needed by the plants. This allows theliquid fertilizer stocks to be stored in relativelylow volume tanks. A common injector ratio is1:100, meaning the fertilizer in the concentratedstock tank is at a 100 times higher concentrationthan what the plant receives. The fertilizerinjector then takes 1 part fertilizer stock andadds 99 parts tap water to prepare the dilutefertilizer that plants receive. Injector ratios inthe range of 1:50 to 1:200 are commonly used. Aratio of 1:200 means that are stock solutionneeds to have twice as much fertilizer as whenusing a 1:100 ratio, because in this case thefertilizer injector takes 1 part fertilizer and adds199 parts tap water.This means when we are conducting fertilizercalculations we will need to multiply the mg offertilizer salt per 1 L of water times the volumeof the stock tank and times the injector ratio.Injectors and Stock TanksMost commercial hydroponic facilities preparefertilizers in concentrated stock tanks and thenuse fertilizer injectors to dilute fertilizer down tothe level needed by the plants. This allows theliquid fertilizer stocks to be stored in relativelylow volume tanks. A common injector ratio is1:100, meaning the fertilizer in the concentratedstock tank is at a 100 times higher concentrationthan what the plant receives. The fertilizerinjector then takes 1 part fertilizer stock andadds 99 parts tap water to prepare the dilutefertilizer that plants receive. Injector ratios inthe range of 1:50 to 1:200 are commonly used. Aratio of 1:200 means that are stock solutionneeds to have twice as much fertilizer as whenusing a 1:100 ratio, because in this case thefertilizer injector takes 1 part fertilizer and adds199 parts tap water.www.e-gro.org3

Fertilizer Calculationse-GRO Edible Alert - 2018Tomato StageElement1 (ppm)NO3-NNH4-NPKCaMgSClFe 0.05TomatoStage 2(ppm)TomatoStage 47350200601168920.550.330.050.340.05Tank A Calcium nitrate ½ of potassiumnitrate Iron chelate (Nitric acid)Tank C Acid, used to drivedown pH (sulfuric,nitric, phosphoric,citric, etc.)Tank B ½ of potassium nitrate Potassium sulfate Monopotassium phosphate Magnesium sulfate Monoammonium phosphate Ammonium nitrate All micronutrients exceptiron chelate (sulfuric acid) (phosphoric acid)Table 2. Commonly used procedure for separatingfertilizer salts in 2-3 stock tanks to avoidincompatibility.Table 1. Nutrient targets for tomato at stage 1 (seedling to2nd truss anthesis), stage 2 (2nd truss to 5th truss anthesis),and stage 3 (after 5th truss anthesis). Recipe comes from theUniversity of Arizona Controlled Environment AgricultureCenter (CEAC)This means when we are conducting fertilizer calculations we will need to multiply the mg of fertilizersalt 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 dissolvein water or be taken up by the fertilizer injector. The most common incompatibilities involve calciumnitrate, which cannot be combined with fertilizers container sulfates or phosphates. Note! Always checkwith the fertilizer manufacturer/supplier to check for incompatibility issues before preparingconcentrated stock solution.Because of the incompatibility issues, commercial growers typically use 2-3 fertilizer injectors (eachwith their own stock tank) connected in series. By convention Tank A contains calcium nitrate (and anyfertilizers compatible with calcium nitrate) and Tank B contains the fertilizers not compatible withcalcium nitrate. A third stock tank (Tank C) is sometimes used and typically contains an acid which isused to decrease pH. (Most often we have to worry about decreasing pH in the hydroponic root-zonedue to the alkalinity in tap water and the majority of N used in the in nitrate form which drives pH upover time).Steps to Calculate a Complete Nutrient SolutionNow let’s put the pieces together to discuss how to calculate the components required to put together acomplete nutrient solution. This is somewhat complex because each of the fertilizer salts that we usedcontains two different elements of interest. That means if we want to add say calcium, that dependingon the fertilizer source we use we will also be adding chloride, nitrate, sulfate, etc. that we also needto account for. Therefore, a fairly specific order is followed to make sure we do not add an excess ofwww.e-gro.org4