Rice water does contain small amounts of nitrogen, phosphorus, and potassium, the three nutrients tomato plants need most. But the concentrations are so low that rice water works more like a very mild supplement than an actual fertilizer, and using it carelessly can invite mold and fungus gnats to your garden.
What’s Actually in Rice Water
The leftover water from rinsing or soaking rice picks up trace nutrients from the grain’s surface. Lab analyses have measured unfermented rice water at roughly 80 to 160 mg/L of nitrogen, 65 to 91 mg/L of phosphorus, and 118 to 131 mg/L of potassium. It also carries small amounts of calcium, magnesium, and micronutrients like zinc, copper, and boron.
To put those numbers in perspective, a standard tomato fertilizer delivers nutrients at concentrations hundreds of times higher. Rice water’s nutrient levels are closer to what you’d find in lightly enriched tap water than in any product labeled as plant food. That doesn’t make it useless, but it means you shouldn’t expect it to replace regular fertilizing. Think of it as a minor bonus when watering, not a feeding strategy.
The Starch Problem
The biggest practical concern with rice water isn’t the nutrients. It’s the starch. Rice water contains easily digestible carbohydrates that feed soil fungi and bacteria the moment they hit the ground. In moderate amounts and healthy outdoor soil, this microbial boost can be fine. But applied too often or in heavy doses, that starch creates a layer of organic material on and near the soil surface that encourages mold growth.
That mold, in turn, attracts fungus gnats. Despite a popular online claim that rice water repels these pests, the opposite is true: fungus gnats feed on the mold that thrives on starch-rich soil. Adding rice water regularly gives them exactly what they need to multiply. This is especially problematic for container-grown tomatoes or indoor seedlings where airflow is limited and the soil surface stays moist longer.
How Rice Water Affects Soil pH
Tomato plants prefer slightly acidic soil, typically in the 6.0 to 6.8 range. Research tracking repeated applications of rice water across multiple growing cycles found that unfermented rice water gradually raised soil pH, making it less acidic over time. Fermented rice water, on the other hand, lowered pH, making the soil more acidic. The effect was modest in either case but became measurable after several consecutive applications.
For tomato growers, this means occasional use of plain rice water is unlikely to throw your soil chemistry off. But if you’re applying it every week for months, you could slowly shift the pH in a direction that isn’t ideal for your plants, depending on whether you’re using fermented or unfermented water. If your soil is already on the alkaline side, fermented rice water could actually nudge things in a helpful direction. If it’s already quite acidic, stick with plain (unfermented) rice water or skip it altogether.
Fermented vs. Unfermented Rice Water
You’ll see three preparation methods recommended online: rinsing rice and collecting the cloudy water, boiling rice and saving the cooled cooking liquid, or fermenting the rinse water for several days before use. Each method produces a different starch concentration. Rinse water is the mildest. Boiled rice water contains more dissolved starch. Fermented rice water has been partially broken down by bacteria, which changes its nutrient profile and lowers its pH.
Fermented rice water is sometimes promoted as the most beneficial option because fermentation can make nutrients slightly more available to plant roots. One study found that rice water fermented with specific bacteria contained nitrogen, phosphorus, and potassium at roughly 0.04%, 0.028%, and 0.1% respectively, which improved growth in pepper plants. Those percentages are still very low compared to commercial fertilizers, but they’re a meaningful step up from plain rinse water. The tradeoff is that fermentation takes a few days and produces a sour smell that some gardeners find unpleasant.
How to Use It Without Causing Problems
If you want to try rice water on your tomato plants, the key is moderation. Apply it no more than once a month to reduce the risk of mold buildup, pest attraction, and gradual soil chemistry changes. Use it as a soil drench rather than a foliar spray, since starchy water sitting on leaves can promote fungal diseases that tomatoes are already prone to, like early blight and septoria leaf spot.
Dilution helps too. If you’re using boiled rice water, which is starchier, mix it with an equal part of plain water before applying. Rinse water is dilute enough to use straight. Either way, let the water cool to room temperature first. Pouring hot cooking water onto roots will damage or kill them.
For tomatoes growing in containers, be more cautious. The confined soil volume means starch accumulates faster, drainage is limited, and mold has fewer natural checks. Outdoor garden beds with good airflow and active soil biology handle rice water much better.
Is It Worth the Effort?
Rice water won’t hurt your tomato plants in small, occasional doses, and it does deliver trace nutrients that contribute (in a very minor way) to soil fertility. But it’s not a substitute for compost, balanced fertilizer, or proper watering practices. The nutrient concentrations are simply too low to make a noticeable difference in how your tomatoes grow or produce fruit. If you’re already rinsing rice for cooking and want to pour that water on your garden instead of down the drain, go ahead. It’s a reasonable way to avoid waste. Just don’t expect it to transform your harvest, and don’t apply it so often that you’re feeding mold instead of tomatoes.