Calculating the amounts of fertilisers to use when writing a solution recipe to a specification
Nutrient solutions are usually made by diluting two stock solutions, and if necessary adding acid or alkali to correct the pH of the diluted solution. It is necessary to use two stock solutions to avoid interactions between ions which might result in precipitation of insoluble salts. The very common reaction is between calcium and sulphate ions resulting in the precipitation of insoluble calcium sulphate. Calcium can also react with phosphate and precipitate insoluble calcium phosphate. These situations are avoided by using two stock solutions, by convention known as the A & B solutions. The A solution contains calcium nitrate or calcium chloride, with or without potassium nitrate and possibly potassium chloride and usually with iron chelate. The B solution contains all the phosphate and sulphates and may also contain nitrates or chlorides and all the trace elements except iron.Preparing the stock solutions requires a list of the quantities of the salts to be used in stock solution, and this list is usually referred to as the Recipe.
Sonneveld has shown that it is relatively easy to write a recipe to a molar solution specification. His method is based on the principle that any nutrient solution must have a balance of cations and anions and that each of the fertilisers used is a salt, that is molecule consisting of anions and cations in simple proportions, and that the solution strength or conductivity is the sum of the cation equivalents. Fertilisers used for making stock solutions are not always pure chemicals, but the weight of each fertiliser which contains 1 gram mol of the principle nutrient can be calculated from the specification of the fertiliser. When this is known it is a simple matter to calculate how much fertiliser is needed to provide so many mols of a nutrient in a solution. The stock solutions are usually concentrated, often 100 or 200 times the dilute solution, so the stock solution recipe will contain 100 or 200 times the amount indicated by the initial calculation.
Recipe calculation using Sonneveld's method can be tedious, but is made very easy if a spread sheet is used in a computer.
Many growers will not be as interested in making recipes directly from mol specifications as they are in modifying and existing recipe to correct some symptoms noted in a crop. Adjusting a recipe by changing the amounts of one or more of the fertilisers and calculating the change in nutrient concentration in ppm is more simple than Sonneveld's calculation as only the percent nutrients in each fertiliser is required to be known. These calculations are also very easily performed using a spread sheet in a computer.
Web site readers who have paid up subscriptions to the web site can download a package of spread sheets called NUTRICALC from this website. Nutricalc contains a recipe calculator for adjusting recipes in terms of ppm nutrients at various dilutions with a page to use as a data base for storing recipes, calculators for costing recipes, calculators for adjusting stock solutions from one recipe to another, a printer for check lists to use when weighing fertilisers, and a calculator constructed on Sonneveld's principles.
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