There are several different sources of N, P, and K used to make fertrilizer formulations. Generally for dry fertilizer materials, the P comes from diammonium phosphate (DAP), and the K comes from potassium chloride (KCl). The N is supplied from the DAP as well. Since DAP is 18-46-0 and KCl is 0-0-60 or 0-0-62, they are blended to make the formulation you want. The additional N may come from Urea (45-0-0) added to the mix, or ammonium sulfate or some other N source. For liquid fertilizers, other sources of N, P or K may be used, depending on the solubility and on the desired use. Urea-ammonium nitrate solution is often used as the N source and potassium hydroxide (KOH) for the K source. Phosphoric acid is the raw ingredient for the P source. It comes in different concentrations.
For foliar applications, the materials used as N, P, K sources need to have a low salt level to avoid injury to the leaves. This usually means a higher cost raw material. But for small amounts used in gardens, this is not a serious cost difference. The garden and lawn fertilizers generally have a lower analysis than fertilizers used for farm fields, so it takes more material to get the same amount of nutrients. This makes it easier to get a uniform rate of application and reduces potential injury to the plants. These low analysis products are less practical for field application because of the volume needed to provide adequate nutrients to the crops. Handling and transportation would be very expensive relative to the more concentrated fertilizer materials.
If you would like more detail on this, feel free to send me an e-mail.
Harold Reetz
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