At first glance, soil looks rather uninteresting. It might be brown, red, gray, white, or other colors depending on organic matter content and mineral characteristics, pieces of crop residue may be present, and if you are lucky, you might also see an earthworm. Have you considered, however, that each teaspoon of soil contains billions of bacteria and countless other microscopic organisms? This world, hidden in plain sight, influences many aspects of soil productivity, nutrient cycling, and crop yields. Let’s dive in and take a look!
Farmers and gardeners have long recognized that healthy, productive soil is the foundation of growing a bumper crop. Nowadays, topics like soil health, regenerative agriculture, sustainability, carbon capture, and water quality are growing in importance, not just for farmers, but for society as a whole. While the goals and approaches might be different for each of these initiatives, they all have one common denominator – understanding the biology of soil and managing it to its fullest potential. Soil science has been around for a long time, and the basics of soil chemistry like pH, cation exchange capacity, and phosphorus and potassium fertility have been well defined. The newest frontier in soil science is to identify, quantify, and understand the functions of microorganisms in the soil. Throughout my remaining blog posts, I am going to explore a few basics of soil biology. Let’s get started by meeting the cast of characters.
In terms of population size, bacteria make up most of the soil biology. It may not come as a surprise, but bacteria are small – between 250,000 and 500,000 bacterial cells can fit within the area of the period at the end of this sentence. What do bacteria do in the soil? Just about everything: decomposing dead plant and animal material, cycling nutrients like nitrogen, suppressing disease-causing organisms, and directly stimulating plant growth. Many soil bacteria live in close association with plant roots in what is known as the rhizosphere. Plants and bacteria communicate with each other, sort of like sending text messages back and forth. This dialogue results in plants exuding sugars and other nutrients to feed the bacteria in exchange for the protection or nutrition that the bacteria provide. The exact number of bacterial species in the soil is unknown as we are just beginning to identify the full roster of soil life through the tools of modern genetics, but a couple of bacteria that you may have heard of include Nitrosomonas and Nitrobacter. These are the bacteria responsible for nitrification, the conversion of ammonium to nitrate.
You may associate fungi with mushrooms, and it is true that mushrooms are the reproductive structures of a specific group of fungi referred to as basidiomycetes. Fungi are a diverse group of organisms, at least 100,000 species are known to scientists, that inhabit soil and many other environments. Within the soil, the role of fungi is decomposers. In comparison to bacteria, which live, reproduce, and die quickly, fungi are much more resilient organisms that gradually increase in numbers and size all while slowly nibbling away at hard to digest organic matter like cellulose and lignin. The body of a fungus is referred to as a mycelium. This is the thread like material that you might have noticed in a pile of wood chips that has been undisturbed. In addition to decomposing organic materials, some specialized fungi known as mycorrhizae form symbiotic relationships with plant roots. Through this association, the fungus receives sugars from the plant, and the fungus, through its extensive network of hyphae (microscopic branches), increases the surface area available for plant uptake of water and nutrients. Finally, just like the bacterial clan, not all fungi are friendly. Some are plant disease causing organisms, sudden death syndrome (Fusarium virgiliforme), as an example.
As a soybean grower, the mention of a nematode may strike fear in your heart. Soybean cyst nematode (SCN) is one of the most yield-limiting pests of soybeans and it is true that there are many types of nematodes that are plant parasitic. That’s only part of the story as each teaspoon of soil contains 40 to 50 nematodes, and many of these have beneficial roles in soil biology. In addition to the plant parasitic nematodes like SCN, other nematodes consume bacteria, some munch on fungi, while others hunt and prey on protozoa (another microscopic inhabitant of the soil). As nematodes feed on other soil life and produce, well let’s call it excrement, they play a vital role in cycling key nutrients like nitrogen and making them available for use by plants.
Other citizens of the soil
In addition to bacteria, fungi, and nematodes, there are many other microscopic lifeforms in the soil including algae, slime molds, protozoa, and archaea (similar to bacteria but more unique) as well as organisms that you can see like earthworms, insects, millipedes, and others. In a healthy soil, all of these organisms live, eat, fight with each other, collaborate, and die. Along the way, they cycle nutrients, store carbon, and improve soil quality that benefits our crops and society. In my next blog, I will take a deeper dive into a few ways in which soil microorganisms impact nitrogen cycling. Stay tuned to learn more about the hidden world beneath your feet!