Did you know that in one teaspoon of living soil there are 100 million to 1 billion bacteria,1 mile to 40 miles of fungal hyphae, and 1,000-100,000 protozoa? These organisms provide a variety of benefits for the plant. The bacteria eat the exudates (simple sugars, carbons, carbohydrates) that the plant puts out through its roots, who are then eaten by the protozoa, and what it excreted by the protozoa is plant available nutrients. Beneficial fungi protect the plant from pathogens and harmful microbes, as well as creating pathways in the soil that bring water and nutrients back to the plant from larger distances. Bacteria and fungi work together in decomposing organic material and making the nutrients plant available.
Certain strains of the soil bacteria Pseudomonas fluorescens have anti-fungal activity that inhibits some plant pathogens. P. fluorescens and other Pseudomonas and Xanthomonas species can increase plant growth in several ways. They may produce a compound that inhibits the growth of pathogens or reduces invasion of the plant by a pathogen. They may also produce growth factors that directly increase plant growth. These plant growth-enhancing bacteria occur naturally in soils, but not always in high enough numbers to have a dramatic effect. In the future, farmers may be able to inoculate seeds with anti-fungal bacteria, such as P. fluorescens, to ensure that the bacteria reduce pathogens around the seed and root of the crop.
These soil microorganisms do much more than nourish plants. Just as the microbes in the human body both aid digestion and maintain our immune system, soil microorganisms both digest nutrients and protect plants against pathogens and other threats. For over four hundred million years, plants have been forming a symbiotic association with fungi that colonize their roots, creating mycorrhizae, literally "fungus roots," which extend the reach of plant roots a hundred-fold. These fungal filaments not only channel nutrients and water back to the plant cells, they connect plants and actually enable them to communicate with one another and set up defense systems.
A recent experiment in the U.K. showed that mycorrhizal filaments act as a conduit for signaling between plants, strengthening their natural defenses against pests. When attacked by aphids, a broad bean plant transmitted a signal through the mycorrhizal filaments to other bean plants nearby, acting as an early warning system, enabling those plants to begin to produce their defensive chemical that repels aphids and attracts wasps, a natural aphid predator. Another study showed that diseased tomato plants also use the underground network of mycorrhizal filaments to warn healthy tomato plants, which then activate their defenses before being attacked themselves.
Bacteria need moisture and nutrients to multiply. Both are in short supply when gardening in sandy soil. Good garden compost is loaded with diverse populations of active and dormant bacteria and beneficial fungi, as well as residual bacterial and fungal “glues” that help bind sand particles together, while soaking up and holding moisture. Adding coir to sandy soil immediately increases it’s moisture-holding capacity. Populations of bacteria and beneficial fungi multiply, creating a virtuous cycle bacteria create microaggregates that increase their habitat, and beneficial fungi thread their way through and around these microaggregates, increasing the water-holding capacity of the soil.