Buying straight fertilizers and mixing your own is the cheapest. For me, most economical forms of fertilizers for each macro element are as follows:
N: calcium nitrate, ammonium nitrate, potassium nitrate, nitric acid
P: phosphoric acid, mono potassium phosphate (these two are the most expensive ones)
K: potassium nitrate
Ca: calcium nitrate
Mg: magnesium sulfate
and for Fe, Fe-EDTA. Other trace elements are usually more expensive than macros but since they are used in very small amounts, their contribution to total cost is insignificant.
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