Water Soluble Fertilizer Calculator

Fertigation doses are specified as a concentration of nitrogen in the delivered solution, in parts per million, which for dilute aqueous solutions is identical to milligrams per litre. The conversion used here follows directly from unit definitions: one US gallon is 3.785 litres, so a target of X ppm N requires 3.785X milligrams of nitrogen per gallon, and dividing by the nitrogen fraction of the product gives the mass of fertilizer. Expressed for a percentage label the result is X times 3.785 divided by ten times the percent N, which puts a 20-20-20 product at 3.79 grams per gallon for 200 ppm N. Phosphorus and potassium are labelled by long-standing convention as the oxides P2O5 and K2O rather than as elements, so a nominal 20-20-20 actually supplies 20 percent N but only 8.7 percent elemental phosphorus and 16.6 percent elemental potassium, using the conversion factors 0.4364 and 0.8302. Typical targets are 50 to 100 ppm N for plugs and young transplants, 100 to 200 ppm N for constant liquid feed of most greenhouse crops, and 200 to 300 ppm N for heavy feeders such as tomato in full production. Electrical conductivity is the better whole-solution check, with most crops managed between about 1.0 and 2.5 mS per cm, seedlings lower and fruiting crops tolerating more. The conductivity figure produced here is only an approximation, since the common rule that 1 mS per cm corresponds to roughly 640 to 700 ppm of dissolved salts averages over ions of very different mobility. Two further omissions matter in practice: the calculation ignores the mineral content and alkalinity already present in the irrigation source, which must be measured and subtracted, and concentrated stock solutions containing calcium will precipitate if mixed with phosphates or sulphates, which is why injector systems use two separate tanks.

Mineral nutrition became a science with Justus von Liebig, whose 1840 treatise argued that plants draw their mineral requirements from inorganic salts rather than from humus directly. Defining what those requirements were took another generation of water culture experiments, notably by Julius von Sachs and Wilhelm Knop in the 1860s, who grew plants to maturity in solutions of known composition and thereby identified the major essential elements. The reference formulation of the modern era was published by Dennis Hoagland and Daniel Arnon at the University of California, Berkeley, first in 1938 and in the widely cited revised form in 1950; Arnon and W. E. Stout had set out formal criteria for calling an element essential in 1939, and Hoagland solution remains the standard nutrient recipe in plant research laboratories. The oxide convention on fertilizer labels is a fossil of nineteenth-century analytical chemistry, when phosphorus and potassium in a sample were determined gravimetrically by ignition and weighed as P2O5 and K2O; the analysis method changed long ago but the reporting basis was retained in law and trade. Complete water-soluble products formulated for dissolving and applying through irrigation appeared commercially in the mid twentieth century, the Peters line being among the best known in North American greenhouse practice. Applying them continuously became routine only after drip irrigation was commercialized, developed in Israel from the work of Simcha Blass and his son Yeshayahu and manufactured by Netafim from 1965, which supplied the low-volume, high-frequency delivery that constant liquid feeding assumes. Proportional injectors that draw stock solution at fixed ratios such as 1:100 or 1:200 completed the system.