You can make probiotics for plants at home by culturing beneficial microbes from your local environment or from common kitchen ingredients like rice and milk. These living microorganisms, once applied to soil or sprayed on leaves, help plants access nutrients, fight off disease, and grow stronger root systems. The process is simple fermentation, similar to making yogurt or sauerkraut, and most methods require only a few dollars in materials.
What Plant Probiotics Actually Do
Soil is full of bacteria and fungi that form partnerships with plant roots. The most beneficial groups, including Bacillus, Pseudomonas, and Azotobacter species, promote plant growth through several mechanisms that work simultaneously. They convert nitrogen from the air into forms plants can absorb. They produce organic acids that dissolve phosphorus locked up in soil minerals, freeing it for root uptake. They even manufacture plant hormones called auxins that stimulate root growth, increase root mass, and improve the overall architecture of the root system.
A large meta-analysis of studies from 2010 to 2020 found that microbial inoculants significantly enhance crop productivity. Stress relief was the primary mechanism, accounting for about 54% of yield improvements, while better nutrient availability explained another 22%. Pseudomonas-based inoculants boosted yields by nearly 50%, and certain combinations of nitrogen-fixing bacteria with compatible host plants increased yields by as much as 196%. Only about 2.5% of studies found no significant effect.
Beneficial fungi also play an important role. Mycorrhizal fungi extend the reach of plant roots and produce proteins that act like glue for soil particles, improving the soil’s ability to hold water, store carbon, and resist erosion.
Culturing Lactobacillus From Rice Wash
This is the simplest method and produces a concentrated serum rich in Lactobacillus, the same bacteria used in bokashi composting. You need rice, water, milk, and a sweetener.
- Step 1: Make rice wash. Rinse about a cup of rice in two cups of water. Swirl it vigorously so the starchy water turns cloudy. Strain out the rice and pour the liquid into a jar, filling it no more than two-thirds full. Cover the jar with a breathable cloth and secure it with a rubber band. Leave it in a cool, shaded spot for three to five days. The liquid will start to smell slightly sour.
- Step 2: Feed with milk. Pour the fermented rice wash into a larger container and add milk at a ratio of roughly 10 parts milk to 1 part rice wash. Cover with cloth again and leave it for five to seven days. The mixture will separate into curds floating on top and a yellowish liquid (serum) beneath.
- Step 3: Strain and preserve. Remove the curds (you can compost them or feed them to animals). The yellow serum is your Lactobacillus concentrate. Mix it with an equal weight of brown sugar or molasses to preserve it. This sugar-stabilized serum can be stored at room temperature in a sealed jar for several months.
To apply, dilute the serum at about 15 to 30 mL per gallon of water for soil drenching. For foliar sprays, use roughly 15 mL per gallon. You can apply it weekly or biweekly throughout the growing season, either on its own or mixed into your regular watering routine.
Collecting Indigenous Microorganisms (IMO)
This method, developed within the Korean Natural Farming tradition, captures the specific microbes already thriving in your local ecosystem. Because these organisms evolved in your climate and soil conditions, they’re well adapted to the environment your plants grow in.
IMO-1: Capture
Cook rice using less water than normal so the grains are firm and dry. Fill a wooden or breathable container about one-third full with the cooked rice. Cover the container with a porous lid or breathable cloth that allows air in but keeps out insects and debris. Place the container near the base of healthy, undisturbed plants, ideally in a wooded area or along a forest edge where diverse microbial life thrives. Cover it loosely with leaf litter and an overturned bucket with holes to protect it from rain and animals.
Leave the container undisturbed for five days. When you retrieve it, a successful capture looks like a white, cloudy layer of microbes covering the rice. Some blue or red mold spots are normal in small amounts, but if those colors dominate, the batch has gone wrong and should be discarded.
IMO-2: Stabilization
Transfer the colonized rice to a mixing container. Add brown sugar at a 1:1 ratio by weight. Mix thoroughly for about five minutes until the sugar is fully incorporated. Pack the mixture into a glass, ceramic, or clay jar. Cover the opening with a piece of cloth or paper towel and secure it with twine. Store the jar in a cool, shaded area out of direct sunlight for seven days.
The resulting product is your IMO foundation stock. It can be stored for months and used as a starter culture. To apply it to your garden, dilute small amounts into water or mix it into a soil amendment like compost or rice bran before spreading it around your plants.
Making Bokashi Bran
Bokashi bran is a dry, shelf-stable carrier for beneficial microbes that you can mix into compost bins or apply directly to soil. The main ingredients are wheat or rice bran, water, molasses, and a microbial starter. You can use the Lactobacillus serum from the rice wash method above as your starter, or purchase a commercial effective microorganisms (EM) solution.
Mix roughly 10 pounds of bran with a solution of water, a few tablespoons of molasses, and a few tablespoons of your microbial starter. The bran should feel damp when you squeeze it (a few drops of water should emerge) but not dripping wet. Pack the moistened bran tightly into a bucket or heavy-duty bag, pressing out as much air as possible, and seal it. This creates the oxygen-free environment that Lactobacillus and other anaerobic fermenters prefer.
Let the bran ferment for at least two weeks. When finished, it should smell sweet and pickled, like vinegar or fermented grain. Spread it out to dry, then store it in a sealed container. Dried bokashi bran stays viable for months.
How to Tell if a Batch Has Gone Wrong
Fermentation always carries the risk of growing the wrong organisms. The signs of contamination are straightforward: a strong, foul, or putrid smell (as opposed to the pleasant sour or yeasty smell of healthy fermentation) is the clearest warning. Black, green, or heavily pink mold growth also indicates problems. Healthy batches may develop white or light gray fuzz, which is typically harmless.
Research on anaerobic fermentation has found that feedstock materials can harbor pathogens like Salmonella and Listeria, and these organisms sometimes survive the fermentation process, particularly at warmer temperatures. To reduce this risk, use clean ingredients, avoid adding animal waste to your ferments unless you’re experienced, and don’t apply homemade microbial products to the edible portions of plants close to harvest. Applying them to the soil around the base of plants is the safest approach.
If a batch smells rotten or looks visually off, discard it entirely. The materials are inexpensive enough that starting over is always the better choice.
Storage and Shelf Life
Sugar-stabilized liquid cultures (like your Lactobacillus serum mixed with brown sugar or molasses) generally remain viable for three to six months at room temperature when stored in a sealed jar away from direct sunlight. Refrigeration extends this further. The sugar acts as both a preservative and a food source for the dormant bacteria.
Dried products like bokashi bran last longer because the low moisture content keeps microbes in a dormant state. Stored in a sealed, airtight container in a cool location, dried bran can remain effective for six months or more. Keep all products away from high humidity and temperature swings, which accelerate microbial die-off or encourage unwanted organisms to take over.
Application Tips for Best Results
Timing and consistency matter more than concentration. Apply your microbial solutions in the early morning or late evening, when UV light from the sun is weakest. Ultraviolet radiation kills many beneficial bacteria on contact, so spraying at midday wastes most of your product.
For soil drenching, water the area first so the soil is already moist. Microbes need moisture to move through soil pores and colonize the root zone. For foliar sprays, a fine mist applied to the undersides of leaves is more effective than heavy spraying on top, since the undersides have more of the tiny openings (stomata) where microbial interactions occur.
Apply consistently every one to two weeks during the growing season rather than applying heavily once or twice. You’re building and maintaining a living population in your soil, not adding a one-time fertilizer dose. The microbes need repeated reinforcement to establish colonies that outcompete harmful organisms and form lasting relationships with plant roots.