Microbial biomass is the total mass of living microorganisms in a given habitat, measured as weight, volume, or energy content. In food and agricultural science it is used to quantify the microbial load of soils, composts, and fermentation systems, where it reflects biological activity and nutrient cycling. Methods include direct cell counting under the microscope, ATP measurement, phospholipid fatty acid analysis, and substrate-induced respiration. Tracking biomass helps compost producers judge maturity, helps fermenters confirm that starter cultures are active, and helps wastewater treaters balance their biological treatment stages. Because it responds quickly to changes in feed, temperature, and toxins, it is a sensitive process indicator.