Fish fertilizer is very popular. It is reported to be a good source of nutrients and a good source of proteins, amino acids and oils – for your plants. Can plants use fish proteins and oils? Is fish fertilizer a good source of nutrients?

Before I go any further let me say that there is nothing wrong with using fish fertilizer. It will help make your plants grow. I have two problems with fish fertilizer; it is extremely expensive compared to other sources of fertilizer and many of the claims for it have no basis.

Fish Emulsion vs Fish Hydrolysate

What is the difference between fish emulsion and fish hydrolysate? The difference from a plants point of view is minor, but if you are trying to sell product–there are big differences.

Fish emulsion and fish hydrolysate start with the left over bits from the fish industry–the parts no one else wants. These are then treated with various chemicals and enzymes to break down larger organic molecules into nutrients and other small organic molecules. Further treatment can take one of two paths; it is either heated or cold processed. Fish emulsion is the end product if the heating process is used. Fish hydrolysate is the result of using cold processing.

There is great debate between the benefits of emulsion vs hydrolysate–which is better? The reality is that plants can’t use most of the large or even small organic molecules from either process. Normally microbes in the soil degrade these to nutrients plants can use. So the argument that heat in the emulsion process is detrimental, makes no sense. it is true that heat will denature proteins, but they need to be denatured for the plants to use them.

I think the arguments for or against either process is just marketing hype. I have seen no scientific evidence to support the superiority of either process.

Fish Fertilizer Benefits

Fish fertilizer is an organic product–for the most part. So it does have the benefits other organic soil additives have. It feeds plants, microbes and improves soil structure.

But proponents of fish fertilizer make claims that do not apply to other organic fertilizers. Most seem to be centered around the fact that the liquid fertilizer contains proteins and oils. We all know fish oils are very important for our health and so they must be good for plants, right? Wrong!

Plants can’t make use of large molecules such as oils and proteins; see Organic Fertilizer – What is it’s Real Value? for more details. When these molecules are added to soil, microbes digest them and turn them into small molecules like nitrate, and phosphate. It is only then that plants can make use of these molecules.

Since the large molecules need to be degraded before plants can use them, there is little difference – to the plant – between proteins and oils from fish, cows (manure), or even plants. I have found no support for the claim that fish fertilizer is better than any other organic fertilizer.

The main thing plants need from fertilizer is a source of nitrogen. Garden soils usually have enough P and K and the other minor nutrients. Nitrogen is the thing that is missing in soils. Given this fact, fish fertilizer is no better or worse than other types of fertilizer.

Fish fertilizer has about 2% nitrogen, which is the same as most organic fertilizers; compost, manure, and coffee grounds.

Is Fish Fertilizer Organic?

This probably seems like a dumb question–fish are organic so why would fish extracts not be organic? Here’s why. In the process of turning fish scraps into fertilizer companies add a number of chemicals, including phosphoric acid, and odor inhibitors. Apparently, as long as these ingredients form less than 1% of the finished product, the product can still be called organic. Who knew–organic fertilizer only needs to be 99% organic!

Cost of Nitrogen

I checked several fish fertilizers and a common analysis is 2-4-2 and if you buy in large containers you can get 9 lb (3.8 Kg) for $25. Small quantities are even more expensive. This fertilizer has 2% nitrogen, and so the cost for the nitrogen is $33 for 100 g of nitrogen. Wow! Even fresh caught Atlantic salmon doesn’t cost that much!!

What is the cost of 100 g of nitrogen if you buy a commercial fertilizer? Scotts sells a 30-0-9 at $17 for 6.2 Kg, or $0.91 for 100 g.

Fish fertilizer is 35 times more expensive than commercial fertilizer and plants can’t tell the difference between the two sources of nitrogen.

I can hear your objection – BUT … fish fertilizer is organic. That is true, and organic fertilizers do more than just provide nutrients. They also help build better soil by feeding microbes. Fish fertilizer is about 14% protein which is the same as manure. A 30 lb bag of manure will cost you $4 compared to $75 for the same amount of fish protein. Manure bought in bulk is even cheaper.

I really can’t think of any good reason to buy fish fertilizer if other sources of fertilizer are available.

References:

1) Photo Source: Cheryl’s Garden Goodies