Graphic of the soil food web.

Not All Nitrogen Is The Same

Excerpted from: Teaming With Microbes

  What is Nitrogen?

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The Nitrogen atom.

Nitrogen is a colorless, odorless, tasteless gas that is the most plentiful element in Earth’s atmosphere and is a constituent of all living matter. In combination, nitrogen is found in the rain and soil as ammonia and ammonium salts and in seawater as ammonium (NH4+), nitrite (NO2−), and nitrate (NO3−) ions. Nitrogen constitutes on the average about 16% by weight of the complex organic compounds known as proteins, present in all living organisms.

  Why Do Plants Need Nitrogen?

Healthy plants often contain 3-4% Nitrogen in their above-ground tissues. This is a much higher concentration compared to other nutrients. Carbon, hydrogen and oxygen, nutrients that don’t play a significant role in soil fertility.

Nitrogen is so vital because it is a major component of chlorophyll, the compound by which plants perform photosynthesis — using sunlight energy to produce sugars from water and carbon dioxide. It is also a major component of amino acids, the building blocks of proteins. Without proteins, plants wither and die.

  Nitrogen and the Soil Food Web

The most important soil nutrient is nitrogen — the basic building block of amino acids and, therefore, life. The biomass of fungi and bacteria determines the amount of nitrogen that is readily available for plant use.

From the plant’s perspective, the role of the Soil Food Web is to cycle down nutrients until they become temporarily immobilized in the bodies of bacteria and fungi and then mineralized.

Definition of Mineralization

Organic matter is present in soils. Nitrogen mineralization is the process by which inorganic nitrogen is obtained by decomposition of dead organisms such as bacteria, fungi and earthworms. This process releases ammonium which is also known as ammonification

In the 1980s, scientists began measuring the volume of bacteria and fungi in soils and found that the ratio of these 2 organisms differed by the type of soil. Undisturbed soils, such as old growth forests, had more fungi than bacteria, while disturbed soils, such as rototilled soils, had far more bacteria than fungi.

Further, plants appeared to have a preference for either fungal-dominated soil versus bacteria-dominated soil. In general, annual plants, grasses and vegetables prefer bacteria-dominated soils while trees, shrubs and perennials prefer fungal-dominated soils.

  Nitrogen From Fungi Versus Bacteria

For the gardener, the type of nitrogen releases is different between bacteria and fungi. When these organisms are eaten, some of the nitrogen is retained by the eater, but much of it is released as waste in the form of plant-available Ammonium (NH4). Depending on the soil environment, this can either remain as Ammonium or be converted into Nitrate (NO3) by special bacteria.

When does this conversion occur? When ammonium is released in soils that are dominated by bacteria. This is because such soils generally have an alkaline pH, which encourages the nitrogen-fixing bacteria to thrive.

The acids produced by fungi, as they begin to dominate, lower the pH and greatly reduce the amount of these bacteria. In fungal-dominated soil, much of the nitrogen remains in Ammonium form.

What Is the 'Soil Food Web'?

The soil food web is the community of organisms living all or part of their lives in the soil. It describes a complex living system in the soil and how it interacts with the environment, plants, and animals. Food webs describe the transfer of energy between species in an ecosystem.

  The Problem With Commercial Nitrogen Fertilizers

Hand holding small plant.

Commercial fertilizers provide plants with nitrogen but most do so in the form of Nitrates (NO3). An understanding of the Soil Food Web makes it clear that plants that prefer fungal-dominated soils won't flourish on a diet of nitrates.

Your trees, shrubs and native perennials do NOT prefer nitrogen in the form of nitrates.

In addition, chemical fertilizers are toxic to some soil organisms and ward of others. Important fungi and bacteria relationships don't form when a plant can't get free nutrients from these organisms.

When chemically fed, plants bypass the microbial-assisted method of obtaining nutrients, and microbial populations adjust accordingly altering the foundation of the Soil Food Web. It makes sense that once the bacteria, fungi, nematodes, and protozoa are gone, other members of the food web disappear as well. Earthworms, for example, lacking food and irritated by the synthetic nitrates in soluble nitrogen fertilizers, move out. Since they are major shredders of organic material, their absence is a great loss.

Without the activity and diversity of a healthy food web, you not only impact the nutrient system but all the other things a healthy soil food web brings. Soil structure deteriorates, watering can become problematic, pathogens and pests establish themselves and, worst of all, gardening becomes a lot more work than it needs to be.

Read more: Soil: An Overview
Read more: Soil Aggregates
Amino Acids A pigment that gives plants their green color, and it helps plants create their own food through photosynthesis.
Bacteria-dominated Soil Soils in which the soil biomass had a higher ratio of bacteria to fungi.
Biomass The total mass of organisms in a given area or volume.
Chlorophyll A green pigment, present in all green plants responsible for the absorption of light to provide energy for photosynthesis.
Fungal-dominated Soil Soils in which the soil biomass had a higher ratio of bacteria to fungi.
Nitrogen-fixing Bacteria Microorganisms capable of transforming atmospheric nitrogen into fixed nitrogen (inorganic compounds usable by plants).
pH Mitochondria are organelles within eukaryotic cells that produce the main energy molecule used by the cell.
Photosynthesis A quantitative measure of the acidity or basicity of a substance.
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