The world looks very different under a microscope. Healthy soil is the most diverse and prolific ecosystem on the planet and with the aid of a little technology we can see the living microscopic world at work in our own gardens. It is important to understand what life looks like below the surface and how it affects soil and plant health.
Identifying Soil Microbes
Bacteria are the most abundant of the soil organisms (>100 million per gram or teaspoon of soil) and the most important within the top 6 inches of soil. Bacteria are very small single-celled microorganisms that reproduce by cell division. Great diversity exists among bacteria: some species are aerobic, some anaerobic, some autotrophs and some heterotrophs.
Bacteria are responsible for many key biological reactions such as nitrogen fixation and sulfur oxidation, which make these nutrients available to plants. Also, bacteria are instrumental in the breakdown of root cellulose.
Algae are multicellular organisms with chlorophyll and, therefore, are autotrophic in their nutritional requirements. Thus, they must reside near the soil surface where light for photosynthesis is abundant. Examples include cyanobacteria, green algae and diatoms.
Algae reproduce by simple cell division and have cell walls similar to higher plants. Considered “early colonizing” organisms in natural habitats, algae are important in initial stages of soil formation.
Many bugs, known as arthropods, make their home in the soil. They get their name from their jointed (arthros) legs (podos). Arthropods are invertebrates, that is, they have no backbone, and rely instead on an external covering called an exoskeleton. Arthropods range in size from microscopic to several inches in length. They include insects, such as springtails, beetles, and ants; crustaceans such as sowbugs; arachnids such as spiders and mites; myriapods, such as centipedes and millipedes; and scorpions.
Arthropods can be grouped as shredders, predators, herbivores, and fungal-feeders, based on their functions in soil. Most soil-dwelling arthropods eat fungi, worms, or other arthropods. Root-feeders and dead-plant shredders are less abundant. As they feed, arthropods aerate and mix the soil, regulate the population size of other soil organisms, and shred organic material.
Fungi are not the most abundant of the soil organisms, but they account for the greatest amount of living mass in soil. Scientists now believe the largest living things are soil-inhabiting fungi. There are reports of individual fungi that extend through several acres of soil. Fungi are multicellular and do not contain chlorophyll; therefore, they cannot manufacture their own food. Familiar examples include mildews, molds, rusts and mushrooms.
A better-understood symbiotic relationship occurs between plants and fungi known as endophytes. Without doing harm, endophytes colonize the interior of roots. Like mycorrhizal fungi, endophytes obtain nutrients from the plants and, in return, produce toxic chemicals called alkaloids that protect the plant from certain insects, pathogenic fungi and grazing mammals.
Nematodes are microscopic roundworms that can comprise as much as 90 percent of the multicellular invertebrates in soil. Millions of nematodes can inhabit a few square feet of soil. Nematodes reproduce by eggs and are most prevalent in warm, moist, sandy soils. They are essential in the soil food web, especially in the recycling of soil nutrients.
Generally found in the upper 6 inches of soil, protozoa are the most abundant of the soil-inhabiting organisms that can be considered “animal” life. Protozoans include amoeba and paramecium, which feed on organic matter and other soil microbes. They are thought to regulate bacterial populations through predation and competition.
Most people become familiar with these soft, slimy, invertebrates at a young age. Earthworms are hermaphrodites, meaning that they exhibit both male and female characteristics. They are major decomposers of dead and decomposing organic matter, and derive their nutrition from the bacteria and fungi that grow upon these materials. They fragment organic matter and make major contributions to recycling the nutrients it contains.
Earthworms occur in most temperate soils and many tropical soils. They are divided into 23 families, more than 700 genera, and more than 7,000 species. They range from an inch to two yards in length and are found seasonally at all depths in the soil. In terms of biomass and overall activity, earthworms dominate the world of soil invertebrates, including arthropods.
Click here for your downloadable PDF for the Soil Microbe Identification activity associated with this lesson.