Is Fish Poop Good for Your Plants?

Helpful Products from Gardens Alive!
Liquid Kelp Concentrate
Handheld Garden Sprayer
Watergarden & Pond Beneficial Bacteria Tablets
Watergarden & Pond Phosphate Binder

You Bet Your Garden
Question of the Week © 2023 Mike McGrath

Note: Hundreds of Mike's informative articles are available (in alphabetical order!) right here at the Gardens Alive website. To find Mike's answers to your specific garden problem, Click here and find your topic (like Weeds, Worms, Rhododendrons...) in our complete alphabetical archive of Questions of the Week.

Q. Jim in Harleysville PA writes: "I'm a fairly new listener of your podcast and I first want to say that I love listening to you on my way to and from work--and I have a question regarding using my pond waste as fertilizer. I have a 700-gallon pond in my back yard; and every few weeks I flush out the filter. What comes out is decaying fish poop, fish food and plant waste. It is an extremely nasty smelling thick liquid. Anyway, I have been pouring this nasty liquid at the base of all my rose bushes with unbelievable results. It has been making them grow and bloom like crazy! So: my question is: would this liquid be safe to use in my vegetable garden? I am a little leery to try it because of how bad this stuff smells and I don't want it to affect the taste of my veggies. Lastly, I am a homebrewer trying my hand at growing hops for the first time and was also wondering if I could use this nasty liquid on the hops without affecting their ability to flavor beer?"

A. Bottom Line? That nasty smelling decoction of fish poop, decaying fish food, rotting plants and more than likely guano from the birds circling overhead dreaming of a fish dinner might just be the single best plant food imaginable, Jimbo!

Everyone knows about "fish fertilizer", a nutrient dense product made from the left-overs when fish are processed for food; and while it can be a good fertilizer, it has its limitations. One: it can be very high in nitrogen, which grows big plants but can inhibit flowering and fruiting; that makes it perfect for growing sweet corn but not tomatoes. Second, some batches of one of the first brands of fish fertilizer that was available in stores have been found to have very high levels of chlorine--as in chlorine bleach--and that's not good for gardens.

(If you are not lucky enough to have your own poopy pond and want to incorporate some kind of fishy fertilizer in your landscape, look for combination products that include both fish and seaweed--and only organically approved ones of course.)

But I digress. Hey I also breathe. And read comic books. And play pinball. And...where was I?

Oh yeah. The wonderful muck that collects at the bottom of your pond and in your filtration system is not fish, per se--it is the fecal matter of fish, which is rich in a well-balanced variety of essential plant nutrients plus all kinds of micronutrients and a huge amount of biological activity. Its also in liquid form, which means that those nutrients become available to your plants much faster than any kind of granular material. And it will increase the biological life of your soil as well.

But wait; there's more! There is no one type of the "fish food" you reference, but most contain ingredients (like soybean and fish meal) that are used directly as organic fertilizers. They're high in protein and contain all kinds of groovy vitamins and minerals and such to keep your fish healthy. And then there's your muck--that wonderful combination of fish poop, uneaten fish food, rotting plant material; and well, just plain muck.

(Side note: The creator of ORGANIC GARDENING magazine, J. I. Rodale, felt that compost made utilizing aquatic plants was tremendously beneficial to plant life.)

It is not much of an exaggeration to suggest that harvesting and re-using your muck is an imitation of the agricultural richness that results when the Nile floods, depositing all those rich nutrients onto the surrounding land.

The response of your roses--lush growth and enthusiastic flowering--strongly suggests that your muck is the perfect food for other fruiting plants, like tomatoes and peppers. And no, they're not going to smell like The Swamp Thing on a humid day. Easy test--cut some roses and bring them inside. Rinse off the stems to get rid of any splashed-on muck, put them in water and sniff the blooms. They will smell like roses--not fishy flowers. Same for your vegetable garden and same for your ambitious project of growing hops. (Good luck with that and send me a double-hopped IPA if you are successful.)

I would caution against using your muck as a fertilizer if the pond were surrounded by a treated lawn, but I doubt that's the case, as the lawn herbicides would kill your fish long before they could poop enough to fill a thimble.

Utilization: I would rinse out the filter in a large container of water and immediately water the diluted result around the roots on your plants, ideally first thing in the morning, when the living organisms in the muck will be able to get down in the ground before being degraded by sunlight. Just be sure to use it right away. As with compost tea, the beneficial life forms in the liquid depend on oxygen to remain beneficial, so use your fishy poopy water as soon as possible.

And if its easy to do so, mix in some muck from the bottom of the pond from time to time. That's some fine tomato food down there!