Soil biota affect soil pH directly through excretion, and indirectly by acting on the physical environment. Many soil fungi, although not all of them, acidify the soil by excreting oxalic acid, a product of their respiratory metabolism. Oxalic acid precipitates calcium, forming insoluble crystals of calcium oxalate and thus depriving the soil solution from this necessary element. On the opposite side, earthworms exert a buffering effect on soil pH through their excretion of mucus, endowed with amphoteric properties.
By mixing organic matter with mineral matter, in particular clay particles, and by adding mucus as a glue for some of them, burrowing soil animals, e.g. fossorial rodents, moles, earthworms, termites, some millipedes and fly larvae, contribute to decrease the natural acidity of raw organic matter, as observed in mull humus forms.