Hi Everyone new to chemistry and learning lots but I had a question that I am having trouble working out.

I work for a vegetable grower and we make our own fertilizer.

My question is around the fish fertilizer. The brief description of the process is we use fish meal and mix it with phosphoric acid and enough water to make a paste. Let it sit a week then add, depending on volume a good amount of KOH. We also add a some skim milk and a small amount of molasses. This process among other things converts the protein in the fish to plant available nitrogen and gives us phosphorus and potassium.

I would love to understand all the chemical processes that are happening here and would like to understand how we are converting the protein in the fish meal to nitrogen. Is it phosphoric acid or KOH that is doing the work and how does the amount of these acids effect the process? Why is such a small amount of acid but a large amount of base used?

I have done my own research but coming up short in understanding what is going on and what the acid and base are doing.


Cells of fish contain proteins, and proteins are made of long chain of amino-acids, linked together by peptide bonds. In the presence of an acid (like phosphoric acid in your experiment or hydrochloric acid in our stomach), the peptide bonds are destroyed and the free aminoacids are released, so that they can be immediately assimilated by living organisms, like roots of plants if these acids are added to the ground as fertilizers.

But the phosphoric acid is not destroyed in the process. It still remains in the aminoacids mixture. As most acids, it is a corrosive substance that must be neutralized before being disposed in the ground. This neutralization is made with KOH, by one of the reactions $$\ce{H3PO4 + 2 KOH -> K2HPO4 + 2 H2O}$$ $$\ce{H3PO4 + 3 KOH -> K3PO4 + 3 H2O}$$ Whatever the reaction chosen, phosphoric acid is now transformed into a potassium phosphate, which can be assimilated. The advantage of this process is that now the fertilizer contains amino acids (bringing Nitrogen N to the soil), plus a salt bringing two important elements for fertilizing a soil : phosphorus P and potassium K. The three fundamental elements (N, P, K) for the agriculture are present in this fertilizer

To go back to the use of KOH, the equation shows that the amount of KOH to be added is twice or three times the amount of the initial phosphoric acid. That is why you have to add a huge amount of KOH to your mixture.

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  • $\begingroup$ What role do the molasses play? Seems to serve no purpose. $\endgroup$ – M. Farooq Sep 27 at 14:18
  • $\begingroup$ Apparently the molasses are useless. $\endgroup$ – Maurice Sep 27 at 15:02
  • $\begingroup$ Seems like that including the skimmed milk :-) $\endgroup$ – M. Farooq Sep 27 at 15:12
  • 3
    $\begingroup$ Plants and many bacteria can metabolize simple sugars (such as in sucrose or galactose) directly. This is why plants manufacture sugars in the first place (good solar energy battery). Possibly these are like "energy drinks" for the plants, or fuel for symbiotic soil bacteria. $\endgroup$ – Lawnmower Man Sep 27 at 22:06

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