The Basics of the Soil Food Web

The soil food web is the foundation of healthy soil. As with most ecosystems, the diversity of the organisms in the soil is the key to its function. No one type of organism or group of organisms can make up for a lack of total diversity. In one gram of soil, there can be billions of different microorganisms and thousands of different species, and each one has a role to play. As you might imagine this can lead to dizzying levels of complexity and even with all of the tools and knowledge of modern science, we are just starting to scratch the surface of the depth of this topic. However, I will explain the major functional groups and their roles in the soil so we can understand how to care for them and let them thrive in our soils. If you recall from biology class, plants are usually at the bottom of any food chain (an outdated idea as things are actually much more connected which is where the idea of a “food web” comes from) and are called the primary producers. That name comes from the fact that they take energy from the sun and capture it (photosynthesis), this process produces a supply of energy for the consumers up the food chain. This energy gets diminished as it goes up the food chain. To explain this concept think of a simple chain like grass (primary producers), zebra (primary consumers), and lions (top of the food chain). There is a lot more grass than zebra because if they ate all the grasses they would starve, and a lot more zebra than lions for the same reason. In soil things are a little different, but much the same.

Primary Producers

In common ecosystems, energy is the currency that determines interactions. In soil, these interactions are guided by nutrients. The primary producers in the soil are recyclers and miners, who are responsible for decomposing old organic matter and extracting nutrients out of the minerals in the ground (sand, silt, clay, and rock). Most of the energy needed to free up the nutrients can be obtained by breaking down existing organic matter so energy is not as big a concern. The reason these producers are recycling and mining is simply for reproduction. Most of the organisms are very small and do not require anything structural, so by weight, they are almost completely made up of nutrients. The ratio of carbon to nitrogen is about 5:1 (nitrogen is a good indicator of protein content and proteins are where most nutrients are put to use). For reference, most green leaves on trees are about 20:1 carbon to nitrogen. However, for the primary producers to reproduce, and they reproduce quickly, they need a lot of nutrients so they have evolved to be very good at extracting them from whatever they can. There are two main types of producers in soils: bacteria, and fungi, each one has a slightly different role to play in how they extract nutrients and what they do to the soil. Both bacteria and fungi can grow rapidly and create large groups if conditions are right and that means extracting, and subsequently storing in their bodies, a large amount of all the nutrients that every living thing needs to live and grow. That will be important later, but for now, let's take a closer look at what bacteria and fungi do in the soil.

Bacteria

Bacteria are everywhere. They are an incredibly diverse group and can include some of the most resilient organisms that we know of. Some bacteria can even survive miles under the ground. In the soil, they are the first organisms to wake up, start growing, and extracting nutrients from their surroundings. One of the most important roles they play in the soil is that there are bacteria specialized in pulling nitrogen from the atmosphere (which is about 70% nitrogen) and making it available for use in proteins and other organic matter necessary for life this process is called “nitrogen-fixing”. In fact, different species of bacteria can specialize in gathering almost every nutrient needed for the healthy growth of plants. However, bacteria have an issue, they are very small. They are so small they can get moved around by water (or pretty much anything) very easily, so to sit in place so they can extract the nutrients they need, they produce glues to anchor themselves. The issue is that on their own bacteria do not create much structure in the soil, instead they have to glue it together. They tend to create what are called “microaggregates” which roughly translates to small (micro) clumps (aggregates). These small clumps don’t tend to help plants, or larger animals, get deeper in the soil. Bacteria dominated soil tends to be shallow soil. Generally, the plants that do best in shallow soil are only the most hardy of plants with the shallowest of roots, a.k.a. weeds. This trend is so strong in fact that you can infer a lot about the soil by what is growing in it. Do you have a lot of weeds growing in your garden? Are the weeds growing much better than whatever else you want to grow? You probably have a bacteria dominated soil. Now I want to emphasize here that having bacteria in your soil is a very very very good thing. The bacteria themselves do not cause the weeds to grow, the bacteria just need some help in creating the perfect soil conditions.

Fungi

To build structure in the soil it needs fungi. Fungi tend to grow in long strands kind of like a string. That property means that as they grow through the soil they tend to weave around and wrap up the microaggregates (small clumps) that the bacteria create and from what are called macroaggregates (big clumps). In really good soil (with lots of fungi) you can pick up a shovel full and it would look like cottage cheese, the curds being the macroaggregates. This added structure is what allows air into the soil for nitrogen-fixing and allows water down deep into the soil. The magic of fungus does not stop there, however. Fungi are incredible decomposers and scavengers of nutrients in the soil and can get access to many nutrients that bacteria cannot. This is mostly due to fungi being larger and having more complex and specialized proteins for breaking down really hard to break down or complex pieces of organic matter. Fungi also are incredible at storage, not just the nutrients they collect, but also water. One of the major reasons our wild areas can fare so much better during a heatwave or drought than our cultivated areas is due to fungi. The way to think about this property is to imagine the fungi as a straw, the organisms themselves only need a little bit of water to live, but they can fill up the entire straw full of water when it is available and hold onto it for long after the rains have stopped. Fungi even will share this stored water with the plants around them! The interactions of fungi and plants is a deep topic and many of our forests would not exist without fungi. They are also missing in the vast majority of our soils. Many of the land management practices we use are detrimental to fungi, the loss of fungi in our soils can be directly linked to the loss of fertility as well. However, with a little care and focus, fungi can be brought back and in doing so we can restore the water-holding capacity of our soils, fix the structure to allow deep root growth, and store massive amounts of carbon back in the ground. But alone fungi and bacteria and not enough to grow healthy plants.

Primary Consumers

Bacteria and Fungi are selfish, they work hard for the nutrients they gather and they don’t just give them away for free. For the nutrients to get to your plants, they need to be released. This stage is where the primary consumers come in. The consumers eat the producers, bacteria and fungi, and poop out whatever excess nutrients they take in. This has given rise to the nickname of nutrient cycling in soil: “the poop loop”. What is happening here is that by weight the consumer needs about half of the nutrients that the producer needs, but much more energy. The consumer has to eat a lot to get the energy but then ends up with toxic levels of almost any nutrient (too much of a good thing and all) so it has to get rid of it. Ever wonder why manure is such a good fertilizer? The consumer is trying to extract energy from what it eats and just so happens to get way more than it needs of those precious nutrients, so it gets rid of them and new organic matter ready to be recycled is produced (a.k.a. poop). This waste is extremely plant-available and the released nutrients are readily taken up by the roots. There are a ton of different types of primary consumers in the soil, but the two most prevalent by weight are also the smallest: the protozoa, and nematodes. These two groups are incredibly diverse and we have not even scratched the surface of uncovering all of the species. For now, all we need to know is that both have an important role in freeing up nutrients from the producers and making them available in the soil for plants.

Protozoa

This group is made up of several subgroups, they include ciliates, flagellates, and amoeba. You may recognize one or another of these groups, but the exact difference between them is not really important, they all play the same role in soil. The protozoa are bacteria feeders, perfectly equipped to roam around the soil in search of colines of their favorite flavor. They range in size dramatically, the smallest are flagellates which are about the size of a single cell (you need a microscope at about 400x to see them) the largest are easily visible to the naked eye. You may have seen the largest protozoa in your garden, they are called slime molds and look like they sound. Don’t worry though they are not bad for the soil or your plants at all. Having a lot of protozoa around is a very good thing in soil because bacteria grow fast. The protozoa are ensuring that the bacteria numbers don’t get out of hand and also that the nutrients the bacteria are extracting and holding onto get released back into the soil. However, protozoa are all singular-celled organisms, even the slime mold is a single cell, and that means they need easy access to oxygen to survive. This restriction means that in soil without good structure, where air can pass deeply into the soil, the protozoa cannot grow and thrive. If conditions get too bad they will curl into a ball and go dormant (these dormant stages are called cysts). Once conditions improve however they will wake up and can re-populate rapidly and get the nutrients cycling again.

Nematodes

Nematodes are one of the most diverse groups of organisms we know of. There are tens of thousands of species that have been defined and that is definitely not all of them. The diversity of the group also means that there is a huge range of what nematodes eat. There are three main groups important to a discussion on nutrient cycling and they are bacteria feeders, fungal feeders, and predatory nematodes. You might be able to guess what they eat from the names of the groups. Nematodes are incredibly important for releasing nutrients from almost every group in the soil. The predatory nematodes will even eat protozoa and other nematodes. This group is also the first multi-cellular consumer we are going to discuss and that means that generally, they are bigger than the rest. This size means increased oxygen requirements so nematodes need a soil with good structure to thrive, but as they move around in the soil they also make larger paths so they can help to build and improve soil structure over time. Nematodes also, being larger, need a lot of food (energy in particular) and they will go searching over a large area to get what they need. This means that they play a very important role in freeing up large amounts of nutrients from the producers, but also a role in spreading those nutrients around in the soil. The mobility of nematodes also means that they will leave a given area if the soil takes a turn for the worse, and then flock to an area where the living conditions are improving. In general, if you have a healthy and diverse nematode population in your soil, you have good soil.

Putting it together

The food web doesn't stop there, it continues all the way up to humans and bears or birds. Each has a role to play in the function of healthy soil and can solve most pest issues if allowed to. The key is diversity, in almost every healthy ecosystem that exists having a higher diversity of plants, insects, microorganisms, and other animals leads to more production and better outcomes. The soil microbes are the critical foundation for cycling nutrients, just as the plants are critical for producing sugars from the sunlight. Hopefully, now you have a solid understanding of the role each group plays in the soil and we can get to work improving and regenerating our soils.