Dry fertilizer math - The Planted Tank Forum
Old 09-12-2012, 06:47 PM Thread Starter
Algae Grower

Join Date: Aug 2012
Posts: 42
Dry fertilizer math

Hi all - I was doing some dry fertilizer math this last weekend and thought I'd share it with the group, figuring someone else might find it useful.

To find grams of a fertilizer compound given a certain number of US gallons:

G = (P * N) / (264.17*E)

where
P = change in ppm desired (ppm new - ppm old)
N = number of US gallons
E = element % of compound (in decimal form - eg .449 not 44.9)
G = number of grams of compound

Using this same equation but solving for ppm instead to find out the ppm increase of a given weight of compound:

P = (264.17 * E * G) / N

For E, the breakdown of the fertilizers I use are below:

K2SO4 -> 44.9% K, 55.1% SO4
KNO3 -> 61.3% NO3, 38.7% K
KH2PO4 -> 69.8% PO4, 28.7% K
MgSO4 -> 20.2% Mg, 79.8% SO4

Example: to find the ppm change of 1 g of KNO3 in 20 US gals of water

P = (264.17 * .613 * 1) / 20 = 161.9 / 20 = 8.1 ppm

NOTE: 264.17 is the number of US gallons in 1 KL (1 million mL), so if you want to use units to prove the formula, you need to expand ppm to g/KL and the constant 264.17 to gal/KL
JustAGuy716 is offline

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