Homemade Fertilizer Nutrient Content Compared: Only One of These 4 Recipes Delivers Real Potassium

Walk into enough gardening forums and you’ll find four jars competing for counter space: comfrey tea, banana peel water, rice water, and eggshell water. Each has its own die-hard fan base, and each gets called “free fertilizer” as if the label meant something specific. It doesn’t. When these four liquids have actually been measured in a lab — not guessed at, not extrapolated from the dry ingredient, but tested as the liquid you’d actually pour on a plant — the nutrient concentrations aren’t close. One recipe carries genuinely useful potassium. Two are so dilute they’re closer to plain water with a rumor attached. One is trying to solve a problem it’s chemically incapable of solving. Here’s what the numbers actually show, recipe by recipe.

Four Different Chemistries Wearing One Name

“Homemade fertilizer” isn’t one category — it’s four unrelated extraction methods that happen to end up as liquid in a jar. Comfrey tea and banana peel water work by breaking down plant tissue: cell walls rupture as the material decomposes, and nutrients that were locked inside living cells leach into the surrounding water. Rice water isn’t tissue breakdown at all — it’s rinse-off, mostly starch that separated from the grain during washing, carrying only whatever trace minerals happened to be on the surface. Eggshell water is different again: it’s an attempt to dissolve a mineral, calcium carbonate, that’s chemically built to resist dissolving in water — that resistance is precisely what makes a shell strong enough to protect an embryo for three weeks. Three mechanisms, three very different yields, and one of them barely produces a yield at all.

Comfrey Tea: The Potassium Heavyweight

Of the four, comfrey tea is the one that actually earns its reputation — though not for the reason most articles give. A two-year, USDA-funded field trial (SARE FNE20-967) lab-tested six chopped-leaf teas steeped in water for several days, and Russian comfrey came out with the highest potassium of the entire group: roughly 890 ppm K after a 3-day steep, ahead of stinging nettle, dandelion, red clover, lambsquarters, and redroot amaranth [1]. USU Extension confirms the underlying composition — comfrey leaf supplies protein, calcium, potassium, phosphorus, and even vitamin B12 [6]. That potassium number is real and it’s usable: at roughly 890 ppm, a 1:10 dilution before applying still lands well within the range most flowering and fruiting plants can use without burning roots.

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What isn’t well supported is the folklore usually offered to explain why comfrey is so potassium-rich — the claim that its deep taproot “mines” minerals from the subsoil that shallow-rooted plants can’t reach. One analysis comparing root-excavation studies and dry-weight nutrient data found comfrey’s roots concentrated in the top 30 cm of soil, not a deep taproot at all, and its NPK profile (roughly 3-1-5) isn’t meaningfully richer than ordinary high-biomass plants like alfalfa (2.5-1-2) or clover (2-0.5-2) [7]. A genuine “dynamic accumulator,” by the definition researchers use, would need nutrient levels around ten times the average plant — comfrey doesn’t clear that bar. It’s simply a nutrient-dense plant that happens to produce a lot of biomass, which is a perfectly good reason to grow it — you don’t need the mining story to justify the jar. If you’re growing your own supply, our comfrey growing guide covers Bocking 14 propagation and cutting schedules.

One caution worth stating plainly: comfrey contains pyrrolizidine alkaloids that are toxic to the liver if ingested repeatedly or in quantity [6]. That’s a human-ingestion warning, not a reason to avoid using the tea on soil, but it’s a good reason to wear gloves when handling fresh leaves and never eat them.

Rice Water: Feeds the Soil, Not the Plant

Rice water is the recipe most likely to be misunderstood, because the actual chemistry runs backward from how it’s marketed. A peer-reviewed study measured washed rice water directly: plain, unfermented rinse water carried about 160 ppm nitrogen, 91 ppm phosphorus, and 118 ppm potassium; letting it ferment for three days raised those figures to roughly 220, 210, and 136 ppm — a 7 to 56 percent increase across nutrients [2]. Those numbers aren’t nothing, but they’re not the main event either. Rice rinse water is overwhelmingly starch, and starch isn’t a nutrient a plant’s roots absorb the way they absorb nitrate or potassium ions — it’s a carbohydrate that soil microbes consume as food. The practical effect of rice water is feeding the microbial population in the soil around the roots, which can loosen structure and support nutrient cycling indirectly, rather than delivering N-P-K directly to the plant the way a labeled fertilizer does. If you want the fermentation chemistry in more depth — including exactly how long to let it sit before the microbial population peaks — see our rice water for plants guide.

The study only measured out to a 3-day ferment, so there’s no lab data here for whether a longer, uncontrolled ferment on your counter keeps adding nutrients or just starts growing mold instead — treat 3 days as the tested window, not a floor to push past. Either way, rice water is best used as a mild soil-biology supplement poured on as part of a routine watering, not as a substitute for an actual feeding schedule.

Banana Peel Water: Real Potassium, Heavily Diluted

Banana peels really do contain potassium — that part of the folklore is true. The problem is how little of it ends up in the water. Analysis of home-soaked banana peel water has put the potassium concentration at roughly 50 to 200 ppm, compared with 3,000 to 5,000 ppm in a commercial liquid potassium fertilizer — a 15- to 100-fold dilution [5]. That range overlaps with plain rice water on the low end and can exceed it on the high end, but it sits nowhere near comfrey tea’s measured 890 ppm; even at its best-case 200 ppm, comfrey tea still carries four to five times more potassium, and at the low end of the banana peel range the gap widens to nearly eighteen-fold.

The viral claim that banana peels have an NPK ratio of “0-25-42” doesn’t hold up to basic plant biology — no plant tissue naturally contains zero nitrogen, since nitrogen is required to build the proteins in every living cell, peels included. The real limitation with banana peel water isn’t the peel’s composition; it’s extraction. Most of what actually leaches into a short soak is sugar and soluble amino acids, not the potassium bound in the fibrous cell structure, which is why longer soaks (one to four weeks, allowing microbial breakdown) release more than a quick 24- to 48-hour steep. For a deeper look at why raw peels underperform their reputation — including the pest and anaerobic-rot risks of tossing whole peels straight into soil — our article on banana peels, coffee grounds, and eggshells breaks down the mineralization timeline.

Eggshell Water: Solving a Problem It Can’t Solve

Eggshell water is the weakest of the four recipes, and the reason is basic chemistry rather than a matter of degree. An eggshell is about 95 percent calcium carbonate, and calcium carbonate has a solubility product (Ksp) of just 3.36 x 10⁻⁹ at room temperature — in plain terms, it’s built to barely dissolve in water at all [4]. One detailed practitioner test found that soaking or even boiling crushed eggshells in plain water released only about 0.2 percent of the shell’s roughly 2,000 mg of calcium content — a dilute solution the tester called “essentially useless” [3]. Compare that with soaking the same shells in vinegar: the acetic acid converts calcium carbonate into calcium acetate, a genuinely water-soluble compound, and that method extracted roughly 1.6 percent calcium into solution — an eightfold improvement over plain water for the same shells [3].

Even if you switch to the vinegar version, it’s worth asking what problem you’re actually solving. Blossom-end rot, the symptom eggshell water is most often reached for, is overwhelmingly a water-delivery problem, not a soil calcium shortage: calcium moves through the plant in the transpiration stream, and fruit cells developing during uneven soil moisture get shorted on calcium even when the soil itself has plenty. Consistent watering and mulch fix that mechanism; pouring more calcium — in a form the plant can barely absorb, at that — doesn’t address the actual bottleneck. If you want calcium in a form plants can use quickly, a labeled calcium product or gypsum solves both the solubility problem and the mineral form; plain eggshell water solves neither.

The Numbers Side by Side

Read the potassium column and one thing jumps out: comfrey tea isn’t marginally ahead, it’s in a different range entirely. Rice water and banana peel water occupy roughly the same dilute territory as each other — both closer to “a very light feeding” than a real fertilizer application — while eggshell water isn’t competing on potassium at all; it was never trying to be a potassium source. Its entire claim to relevance is calcium, and that’s exactly where plain water lets it down.

Part of why online advice on this topic is so contradictory is that most of it cites the dry ingredient’s composition — banana peel powder, dried comfrey leaf, ground eggshell — as if that number describes the liquid you actually apply. It doesn’t. Drying and grinding concentrate nutrients into a small mass; soaking in water for a day or two extracts only whatever is water-soluble and near the surface. A claim that’s accurate for the dried material can be off by one or two orders of magnitude once you’re talking about the liquid in the jar, which is exactly the gap this comparison is built to close.

Match the Recipe to the Actual Goal

None of these four recipes is “bad” — they’re just narrow. The mistake is reaching for whichever one is trending without checking whether it actually addresses the deficiency or goal you have in mind.

How to Actually Use These Without Wasting Your Time

If you’re using comfrey tea for its potassium, dilute the concentrate roughly 1:10 to 1:15 before applying, since the steeped liquid is concentrated enough to risk root burn at full strength. Apply it during flowering or fruiting stages, when potassium demand is highest, rather than on young foliage-stage plants that need nitrogen more. Rice water and banana peel water are both dilute enough to use as a plain watering-can substitute once or twice a week without a dilution step — treat them as a habit that does no harm rather than a fertilizer program. Skip plain-water eggshell soaks entirely; if you want the calcium, either use the vinegar-extraction method or work crushed, finely ground shells directly into the soil months ahead of planting, since fine grinding is what actually speeds up the otherwise glacial breakdown of calcium carbonate. And resist the urge to combine all four into one “everything tea” — you gain nothing by diluting your one genuinely useful recipe (comfrey) with three that barely register on a nutrient test. This comparison is one piece of our full Fertilizing the Garden guide, which walks through a complete season-by-season feeding plan beyond just these four DIY recipes. If a plant is showing a specific deficiency symptom rather than just needing routine feeding, a $15–20 soil test from your local extension office will tell you far more than any of these four recipes can — it’s the only way to know whether you’re actually short on potassium or calcium in the first place, rather than guessing from a symptom and reaching for whichever jar is trending that week.

Frequently Asked Questions

Is comfrey tea as good as a commercial liquid fertilizer?
For potassium specifically, its measured concentration is in a usable range once diluted, but it isn’t a complete, balanced feed — it’s short on nitrogen and phosphorus relative to a labeled all-purpose product. Use it as a potassium supplement during flowering, not a full replacement.

Does fermenting rice water longer make it stronger?
Only up to a point. The measured gains in the peer-reviewed trial occurred within a 3-day fermentation window; there’s no evidence that extending it further keeps adding nutrients, and a much longer, uncontrolled ferment risks turning sour and harboring mold instead.

Why does banana peel water seem to work for some people if it’s this dilute?
Mostly because the placebo of a watering routine, plus the trace sugars and micronutrients it does contain, rarely hurts a plant — and confirmation bias does the rest. It’s not harmful; it’s just not the potassium powerhouse the folklore claims.

Can I speed up eggshell calcium release without vinegar?
Grinding the shells to a fine powder increases surface area and helps, but it still relies on soil acids and microbial action over months, not a quick water soak. Vinegar extraction remains the fastest route to a calcium form your plants can actually use quickly.

Sources

[1] SARE (Sustainable Agriculture Research and Education) Final Report FNE20-967, “A Dynamic Accumulator Database and Field Trials for Six Promising Species”
[2] Peer-reviewed study on fermented washed rice water and NPK mineral fertilizer, PMC10559983
[3] Robert Pavlis, “DIY Cal-Mag Fertilizer, vinegar and eggshells – does it work?”, Garden Myths
[4] Chemistry LibreTexts, “Solubility Product Constant, Ksp”
[5] Growperma, “Banana Peel Fertiliser: Myth vs Science”
[6] USU Extension, “Comfrey in the Garden”
[7] Robert Pavlis, “Comfrey – Is it a Dynamic Accumulator?”, Garden Myths