Soil Food-Web

“Feed the soil, not the plants” is a common phrase heard at all levels of agricultural practice, from the largest corporate farms to the smallest community garden plots. One significant way that organic cultivation differs from conventional chemical-based methods is that emphasis is placed on understanding what it means to “feed the soil.” Rather than an inert sponge that you “input” with nutrients, soil is a complex ecosystem home to billions of organisms of wildly varying size. These lifeforms are what you feed when you fertilize soil and they are all connected in a series of interrelated food chains known as the soil food web.

The three primary nutrients required for plant growth are nitrogen (N), phosphorous (P), and potassium (K). Conventional fertilizers are chock full of these nutrients but deliver them to the soil through water-soluble salt molecules. This is not only inefficient since plants require NPK in their non-soluble forms as well, but also dangerous to the organisms in the soil food web since salt kills them. The soil food web is what naturally produces and retains a lot of these nutrients, so when you kill these organisms you “kill” the soil. This is how land becomes dependent on chemical fertilizer, requiring more and more input each season. This is good for chemical fertilizer companies but bad for everyone else since it ultimately results in barren land.

A healthy soil food web is also resistant to most opportunistic organisms, reducing the need for the nerve gas variants commonly known as “pesticides.” Reliance on chemical pesticides not only results in food that is unsafe for people to eat but also contributes to the development of “superbugs”---opportunistic organisms that are resistant to the very chemicals designed to control them. This is another vicious cycle that is good for agricultural industry but bad for pretty much everybody else.

So what organisms constitute a healthy soil food web, exactly? What follows is a list of some of the key players.

Plants not only absorb nutrient through the soil but release them through their leaves and roots. These excess nutrients are called exudates, which are what attract and feed the microorganisms that constitute most of the soil food web.

Bacteria is the simplest and most common of these organisms. They tend to concentrate in the area surrounding plant roots (known as the rhizosphere). Bacteria decompose organic matter and convert nitrogen into forms consumable by plants. There are forms of bacteria hostile to vegetables but an intact soil food web keeps this type of disease in check. Bacteria also serve as a food source for more complex microorganisms.

Fungi is another major decomposer of organic matter but has a reach that bacteria lacks. Through regular and prodigious growth fungi transport nutrients throughout the soil. They also contribute to a healthy soil structure, both aerating it and forming mycelium networks that assist water and nutrient retention.

Protozoa, whether in the form of paramecia, flagellates, or amoebas, help keep the bacteria population in check. Some protozoa form symbiotic relationships with particular species of bacteria, but most just eat it. They themselves serve as a food source for other higher orders of soil life.

Nematodes are microscopic flat worms---the very largest that you are likely to encounter is the size of an eyelash. There are thousands of different species, and they feed upon just about everything: bacteria, protozoa, small arthropods, other nematodes. Certain types of nematodes attack plant roots, but these are kept under control by fungal hyphae that are common in a healthy soil food web. Nematodes also deposit nutrients and contribute to soil aeration.

Arthropods, bugs in their many splendid forms, (spiders, mites, lice, scorpions, centipedes, millipedes, etc.) seem to love fungi and bacteria---and not just to eat. In addition to shredding organic matter and creating soil structures ideal for microbial life to move around in, they “taxi” that life around on their exteriors. They also totally gross one of us out.

Earthworms are easily the most famous members of the soil food web, but perhaps the most misunderstood. They don't “eat dirt” because they like it, but for the bacteria that lives there. Regardless, they are fabulous natural aerators whose “castings” (poop) is pure gold as far as plants are concerned.

Higher order animals should not be neglected a worthy mention. Birds, rodents, reptiles, and mammals such as human beings all play a significant role in the soil food web, whether inadvertently taxiing the smaller constituents around or volitionally modifying the environment.

This is just an overview of the major players in the soil food web. Not only is each of these the subject of countless books and academic studies, we have neglected a few of the minor players---slugs anyone?

Safe, organic methods by which we can maintain and, if necessary, restore a healthy soil food web will be the subject of upcoming.

For more information, we recommend visiting the Soil FoodWeb online, or check out The Soul of Soil for a boring, technical (but classic and thorough) read or Teaming with Microbes for an interesting, informative and practical read.