Correcting to get the 5-1-3 ratio from various fertilizer formulations
Assumptions: Just straight forward recipe calculations, no regards to other factors such as chemical reactions with the other mixes, solubility, the actual ratios of slow releases and other variables.
While I have the boring actual mathematical formula derivations for various fertilizers and desired formulations aside from the 5-1-3 ratio and various sources of N and K, those more generalized equations would be unnecessarily complicated and the purpose of simplification and usability would have only been defeated. Thus the formulas here is the simplest that I can reduce them to.
Given:
You can get any fertilizer sources that has a certain N-P-K ratio to be corrected, preferably those with minor and micro nutrients.
where N,P,K are non-zero, the N to P ratio is no more than 5, and the K to P ratio is no more than 3.
Ammonium sulfate (21-0-0) and Sulpomag (0-0-22) are used to adjust the ratio.
Solution:
Amount of original NPK fertilizer is 1 unit (could be lbs, kg, as long as same units are used. Volume like cups, can be used but it becomes approximate)
Amount of Ammonium sulfate needed for correcting 1 unit of original fertilizer is:
Ammonium sulfate = (5P - N)/21 units
Amount of Sulpomag needed for correcting 1 unit of original fertilizer is:
Sulpomag = (3P - K)/22 units
where N, P, and K are the raw (not simplified) ratios of original fertilizer.
(If you would have used urea(46-0-0) for correction then the amount would be:
Urea = (5P - N)/46 units
If you would have used Muriate of Potash (KCl: 0-0-52) for your source of K then the amount would be:
KCl = (3P - K)/52 units
You will have to change the formula accordingly to adjust to the actual formulation of the N and K.
I would recommend the sulpomag over muriate of potash because you will have excessive Cl which is bad for sodic soils)
example 1: Correcting Romeo's 24-14-14 fertilizer
For each lb of Romeo's fertilizer we must add
Ammonium sulfate needed = (5*14 - 24)/21 = 2.19 lbs
Sulpomag needed = (3*14 - 14)/22 = 1.27 lbs
When properly mixed, the effective raw ratio would be: 15.68 - 3.14 - 9.41 which is exactly a 5-1-3 ratio
example 2: correcting Triple 16
For each lb of triple 16 we must add
Ammonium sulfate needed = (5*16 - 16)/21 = 3.05 lbs
Sulpomag needed = (3*16 - 16)/22 = 1.45 lbs
When properly mixed, the effective raw ratio would be: 14.54 - 2.91 - 8.72 which is exactly a 5-1-3 ratio
example 3: correcting Complete 20-20-20
For each lb of Complete fertilizer we must add
Ammonium sulfate needed = (5*20 - 20)/21 = 3.81 lbs
Sulpomag needed = (3*20 - 20)/22 = 1.82 lbs
When properly mixed, the effective raw ratio would be: 15.09 - 3.02 - 9.05 which is exactly a 5-1-3 ratio
example 4: correcting the osmocote 18-9-10
for each lb of this Osmocote you would need to add
Ammonium sulfate needed = (5*9 - 18 )/21 = 1.29 lbs
Sulpomag needed = (3*9 - 10 )/22 = 0.77 lbs
When properly mixed, the effective raw ratio would be: 14.71- 2.94 - 8.83 which is exactly a 5-1-3 ratio, but beware, the N and the K would have spiked availability during the first watering because of slow release of the original 18-9-10.
example 5: correcting my favorite BEST fertilizer with minors: 6-20-20 XB
for each lb of Best fertilizer I would add: 4.48 lbs of Ammonium sulfate and 1.82 lbs of sulpomag. They turn out really cheap and have plenty of magnesium, sulfur and other minors. Probably the best and cheapest mix all of which I can locally and cheaply buy. No shipping and handling hassles and can always wait on sale promotions from big box stores.
