Lactic Acid Bacteria in the Soil: How LAB Serum Transforms Your Rhizosphere

Lactic acid bacteria are among the most important and most overlooked inputs in organic growing. They are naturally present in healthy soil, on plant surfaces and in fermented foods. In Korean Natural Farming, LAB serum concentrates and delivers these bacteria directly to the root zone, where their activity improves mineral availability, competes with pathogens and supports the microbial ecosystem that drives plant quality.

FFJ made through proper fermentation already contains LAB as a byproduct of the fermentation process. Understanding what LAB do explains why a well-fermented biological input is more useful than a simple fruit extract.

What lactic acid bacteria are

Lactic acid bacteria (LAB) are gram-positive anaerobic and facultatively anaerobic bacteria. The primary genus in KNF and fermented plant inputs is Lactobacillus, though related genera like Leuconostoc and Pediococcus are also present in wild ferments. These are the same bacteria responsible for yogurt, sauerkraut, kimchi and lacto-fermented vegetables.

They metabolize sugars through lactic acid fermentation, producing lactic acid as the primary metabolic output. Some strains also produce acetic acid, ethanol and carbon dioxide as secondary byproducts depending on conditions. This fermentation is what drives the pH down in a healthy FFJ or LAB ferment.

LAB are competitive. In a sugar-rich, slightly anaerobic environment, they outcompete most other bacterial populations. This competitive dominance is why fermented products with an established LAB population resist spoilage and why LAB serum applied to soil can shift the microbial balance.

How LAB serum is made in KNF

The traditional method captures wild LAB from the environment. Rice wash water, the cloudy liquid from rinsing uncooked rice, contains the carbohydrates and sugars that LAB feed on. Left out at room temperature for 3-5 days in a loosely covered container, wild Lactobacillus from the air and the rice surface colonize the water and begin fermenting it.

The fermented rice water is then mixed with 10 parts whole milk. Lactobacillus produces enough lactic acid to precipitate the casein protein in the milk, separating it into solid curds and liquid whey. After 12-24 hours, the curds sink or float and the clear yellowish whey is strained off. This is the LAB serum, rich in Lactobacillus populations, lactic acid, and metabolic byproducts including B vitamins and free amino acids.

The serum can be preserved by adding an equal volume of raw sugar or by refrigeration.

What LAB do in soil

Rhizosphere acidification. LAB lower the localized pH around roots through lactic acid production. This modest pH shift improves the solubility of phosphorus, iron, manganese and several trace minerals that become locked up in neutral to alkaline soil. A slightly acidic rhizosphere is where most garden plants perform best.

Competition with pathogens. LAB produce bacteriocins, antimicrobial peptides that inhibit competing bacterial species. In a root zone with strong LAB populations, many pathogenic soil bacteria find it harder to establish. This is not a fungicide or a pesticide — it is biological competitive exclusion.

Organic matter decomposition. LAB contribute to the breakdown of complex organic matter in the root zone, releasing nutrients in plant-available forms. They work alongside fungi and other bacteria in the decomposition chain.

Metabolic byproducts. The lactic acid and other organic acids LAB produce function as chelating agents, binding mineral ions in soluble complexes that are more easily absorbed by roots. B vitamins produced during fermentation are also present in LAB serum, though their direct benefit to plants through root uptake is less well-established.

Applying LAB serum

Dilute at 1:500 to 1:1000 for soil drench. Apply once or twice during veg and once per week during early flower. Avoid applying immediately before or after heavy doses of other amendments to prevent competition effects.

LAB serum can also be applied as a foliar at 1:1000 to reduce pathogen pressure on leaf surfaces and support plant surface microbiology.

Because FFJ fermentation naturally produces LAB as part of the process, growers applying FFJ regularly are already getting LAB delivery with each application. The LAB serum step is most relevant for growers building a KNF program who are not yet using FFJ, or for those who want to specifically boost rhizosphere LAB populations during the vegetative stage.

For more on the rhizosphere ecosystem and what drives plant quality, see our soil microbiology guide.