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New Contributor

Fish Fertilizer acid / base reactions

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! but at the moment I would really like to understand how we are converting the protein's 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 such a small amount of acid but a large amount of base.

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

7 Replies
Contributor III

Re: Fish Fertilizer acid / base reactions

Dear Daniel,

Fish meal by itself is a common organic fertilizer. It already contains a great deal of nitrogen, phosphorus, and potassium. It is usually applied directly to soils as a fertilizer. Hydrolyzed Fish fertilizer is a natural fermented liquid form of fish by-product fertilizer. It sounds like your process is really just making the hydrolyzed form from the fish meal. The phosphoric acid may aid in the breakdown of proteins in the meal to the amino acids desired in application. The KOH also is a common chemical used to dissolve proteins, so it may be used to get the proteins from the meal into the application solution while adding additional potassium (possibly unnecessary) to the blend. The addition of molasses also indicates a variation in the mixture of brown sugar and fish wastes used in the production of hydrolyzed fish fertilizer.

NOTE that no nitrogen is taken up by a plant as the element nitrogen (gas phase), but it is usually acquired as ammonium or nitrate ions from amino acids from proteins in the soil. “Organic” fertilizers like fish fertilizer eliminate the potential for nitrogen “burn” in a plant that can occur with an excessive application of inorganic fertilizers like ammonium nitrate (although it can happen with any over-fertilization, including manure).

Best regards,

Steven

New Contributor III

Re: Fish Fertilizer acid / base reactions

Dear Daniel,

I am glad Steven is familiar with fish meal as a fertilizer - I would not have known where to start with an answer for you without his lead.

If you were interested a little more in the fundamental chemistry, you may know that proteins are molecules made of sub-units known as amino acids (as Steven commented). You could go to Wikipedia to see the many different kinds of amino acid that there are.  The important part is that one end of each is an amine (which is a basic or alkaline) group and the other end is a carboxylic acid (which, naturally, is acidic).  Proteins are formed by the alkaline end of one amino acid reacting with acid end of another.in a typical acid-base reaction to form what is called a "peptide" bond with a molecule of water as a side product.  Amino acid - Amino acid reactions | Britannica   The reaction product of a carboxylic acid with an amine is called an amide - the peptide unit formed by two amino acids reacting is a special case of an amide.  Amide bonds are usually pretty strong and only break down when you make the conditions a little more aggressive - for instance by adding a strong acid in the presence of water.  The acid catalyzes the break of the amide bond and adds the molecule of water back in the reverse reaction of forming the peptide to give back the original two amino acids.  See this page 21.9: Chemistry of Amides - Chemistry LibreTexts  and skip to the bottom where they describe "hydrolysis". (from the Latin for hydro "water" and lysis "loosening")

Acid also helps to break down bone (if you've ever done the chicken bone in vinegar reaction you've seen this).  If there are any fish bone in the meal this would also help to soften the bone particles and extract the calcium.  I don't know if more available calcium helps the plants or not.

That would explain why, from a chemical reaction point of view, you're adding the phosphoric acid; you need a strong acid but you also want one that will leave a salt that is beneficial for the plants.  You could use hydrochloric acid but then the result (after adding the KOH) would be potassium chloride which would not benefit the plants like the phosphate would. 

 

As for the potassium hydroxide as Steven points out it can also be used to catalyze hydrolysis of the protein, as counterintuitive as that seems.  I have to wonder, if there is any fat in the fish meal, whether the KOH is used to help break that down in a reaction know as soponification (there's plenty of sources on the web to read up on that reaction because it's the one that converts fats and oils into soap.)  The KOH would also neutralize any residual phosphoric acid and convert it to potassium phosphate.  This also adjusts the pH of the overall mixture - I don't know if you have a particular pH that you're also trying to achieve.   

It may feel like you're adding a lot more of the KOH than the phosphoric acid but know three things:

   1.   the chemicals you're working with are not 100% acid or base but water solutions of them.  You may add a kilo of the "phosphoric acid" but only 80 or 85% of the weight is actually the acid and the rest is water. So in a kilo (1000 g) of the phosphoric acid as you buy it is actually 850 g pure phosphoric acid. The same may be true of the potassium hydroxide if it comes as a solution.  You have to take that into account

   2   reactions are molecule with molecule not gram to gram.  In other words, 1 gram of KOH does not neutralize 1 gram of phosphoric acid.  Each molecule has a molecular weight based on the atoms that make it.  You have to convert the weight of the stuff you're using into a number of molecules (using the molecular weight) to determine how many molecules there are to react, and then figure the number of molecules of the other compound to see if they are the same number.  In this case phosphoric acid molecule is almost twice the weight of the KOH molecule (1.75x to be more accurate) so a kilo of pure KOH would have 1.75x more molecules in it than a kilo of phosphoric acid. (It's like if you have a pound of feathers compared to a pound of glass beads.  It takes a higher number of the lighter feathers to make a pound than the number of glass beads that would take to weight a pound.  So there are many more KOH molecules in a kilo than there are phosphoric acid molecules in a kilo.)  If there was one acid group per molecule then it would take one molecule of KOH to neutralize it.  That means it would take (in weight) a little more than half (1 divided by 1.75 = 0.57) the weight of phosphoric acid in KOH to neutralize the phosphoric acid. BUT

 3    the other complication to #2 is that phosphoric acid has THREE acid groups in a single molecule to neutralize while KOH only has one alkaline group to do it with.  So you need to triple that amount that was calculated in #2: 3 x 0.57 or 1.7x in weight.   

If the KOH was pure KOH then you'd need 1.7x the weight of pure phosphoric acid to fully neutralize all of it.  If the KOH you add is only 45% pure KOH then that number goes up to 3.8x (1.7 divided by 0.45).  So if you're adding what seems like a lot more of the KOH that checks out, chemically.  That's for full neutralization to take the pH to neutral (7).  If you want the whole mess to be acidic when you finish you add less of the KOH than needed for full neutralization and more KOH if you want it to end up alkaline.  Neutralizing the phosphoric acid makes the potassium phosphate salt that is mildly alkaline all by itself in water but is also something called in chemistry  a "buffer" - a compound that helps keep the pH of a mixture stable.  That may have advantages in terms of storing or using your final product.

You can look up the calculations for an acid-base titration to see how the calculations for converting from weight to number of molecules are done.  The word they use that you may not be familiar with is "mole" - and not the garden variety!  You may have to look up the chemistry mole to understand the calculation.

I know this answer is much longer than the nice succinct one Steven gave you but I wanted to answer your question about the amount of KOH not seeming proportional and there's just no short way to do that.

Hope some of these references can help you continue your chemistry education.  Good for you for being curious about what's happening and what you're doing with all these different chemicals.

Karen

New Contributor

Re: Fish Fertilizer acid / base reactions

Daniel,  Speaking to the fertilizer manufacture and application elements in this issue, is your goal to make a certified 'organic' fertilizer?  If so, the chemical processing with phosphoric acid could likely disqualify the mixture under some 'organic' definitions and 3rd party certification programs.  Even if no certification is involved, branding food products as grown only with organic fertilizers if the fertilizers are not really organic could be a circumstance of green washing and has put a few companies I know of in trouble.   The comment about using hydrochloric acid instead of phosphoric acid and ending up with KCl is a legitimate pathway to a potassium nutrient source.   The vast majority of potash (potassium) fertilizer mined and used in the world is actually KCl, chemically known as Muriate of Potash. BUT, using HCl instead of H3PO4 means you now do not have a phosphate source of nutrient.  Depending on what you are growing, a plant needs a certain balance of nitrogen, phosphorous and potassium (known as the NPK ratio in the fertilizer and ag world).  Keeping the phosphoric acid in the mixture is the better approach.  Phosphorous is especially important for seedling root growth and development.  I would also have the ratio of your N-P-K inputs analyzed.  I expect the nitrogen ratio could be low.  It typically needs to be the highest number over P and K.  The ratio is important for best plant development and crop yield potential.  If nitrogen is the limiting value, then the plant will use all the nitrogen it can and not use all of the phosphorus and potassium available so there is a wastage of crop nutrient.  All nitrogen sources need to be in the nitrate form for the plant to absorb it.  So urea, ammonia, ammonium, amino acids all have to be converted to NO3- ether chemically or in soil process before the plant can use it.

Jim Jenkins  

New Contributor III

Re: Fish Fertilizer acid / base reactions

James,

That was a really great tutorial on fertilizer.  Also excellent point about the possible legal/ethical dilemmas of "organic" fertilizer.

I made the comment about using HCl only as illustration of alternate chemical reactions and not as useable substitute for the fertilizer mix.  Thanks for the exposition on KCl.

Karen

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New Contributor

Re: Fish Fertilizer acid / base reactions

Hi Steven, Karen and James Thank you so much for all your replies. They are a big help in gaining more understanding. James I understand where your coming from in regards to organics. I actually work in certified organics and been through multiple audits etc. Green washing is a big problem in the industry and is a consumer and legal education process. 

Karen that was such a long detailed explanation. My sincere thanks for taking the time to put it together!

Just wanted to follow up on my question. I have put a kelp meal through the same process and had some very interesting results. The kelp meal has actually turned into a jelly not a liquid substance and require a significant amount of water to thin it out. I am guessing it could have been the heat generated from adding the KOH had a gelatin like reaction.... Just wildly guessing here! 

Kind Regards

Dan

Contributor III

Re: Fish Fertilizer acid / base reactions

Dear Dan,

Yes, kelp is a different organism and nutrient source from fish!  A large type of algae, it is more like a plant than an animal.  Nutritional break-downs show a much lower protein content than fish, with higher iron (and other minerals?) content and a slightly lower phosphorous content.  Thus, the specific fertilizer application may be a little bit different, or simply cost-based.

This does indicate that "digestion" will take place differently.  You may be seeing something more like paper pulping instead of protein dissolution in your process when using kelp instead of fish meal.

Best regards,

Steven

Fish and Shellfish Nutrient Composition Chart | Seafood Health Facts 

https://www.nutritionvalue.org/Seaweed,_raw,_kelp_nutritional_value.html 

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New Contributor III

Re: Fish Fertilizer acid / base reactions

Daniel,

I am not sure of the chemistry (not knowing enough about all the chemical components of seaweed) but it's worth noting that various seaweeds are the source of very commercially important gelling agents such as carrageenan, agar, and alginate.  These are polysaccharides, polymers (long chains of repeating units) of sugar molecules.

 

I learned from the Wikipedia article on kelp that it is a brown seaweed and a commercial source of alginate.  Having alginate released from the kelp in a water solution could very well lead to a gel, but then it's also possible that the acid could catalyze the breakage of the bonds in the alginate.  Alginate (or any water soluble polymer) thickens things because it is a very large molecule dissolved in the water - fundamentally it just makes it really hard for the water to flow because the large polymer molecules are taking up a lot of space in the solution.  If the glycoside bonds (that link the sugar units together to make the polymer) were broken it would shorten the polymer.  The shorter the polymer the less thick the gel it forms, until (in the ultimate case) if the alginate was completely degraded to its individual sugar units it would not thicken significantly.

So it's possible that the alginic acid in the kelp might be liberated from the kelp by the acid treatment (I'm not sure whether this is a reasonable idea or not) and then form potassium alginate on the addition of the KOH which would form  a gel if it was concentrated enough.  This article doesn't answer my uncertainty about whether the phosphoric acid would liberate the alginate from the kelp or mostly degrade it but it is interesting to read. Alginic acid - Wikipedia

  

Steven and James might be more knowledgeable about this chemistry than I;  I just know that many gelling agents come from seaweeds so that it might not be surprising that when you are digesting kelp you might get a gel as a result.

Karen

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