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Plant Exudates: How Plants Control the Soil Ecosystem

Excerpted from: Teaming With Microbes

  Plants Are In Control

Most gardeners think of plants as only taking up nutrients through root systems and feeding the leaves. Few realize that a great deal of the energy that results from photosynthesis in the leaves is actually used by plants to produce chemicals they secrete through their roots. These secretions are known as exudates.

A good analogy is perspiration, a human’s exudate.

  What Are Plant Exudates?

ex·u·date

Botany: substance secreted by a plant or insect.

Root exudates are in the form of carbohydrates, including sugars, and proteins. Amazingly, their presence wakes up, attracts, and grows specific beneficial bacteria and fungi living in the soil that subsist on these exudates and the cellular material sloughed off as the plant’s root tips grow.

All this secretion of exudates and sloughing-off of cells takes place in the rhizosphere, a zone immediately around the roots, extending out about a tenth of an inch, or a couple of millimeters (1 millimeter = 1/25 inch). The rhizosphere, which can look like a jelly or jam under the electron microscope, contains a constantly changing mix of soil organisms, including bacteria, fungi, nematodes, protozoa, and even larger organisms.

All this “life” competes for the exudates in the rhizosphere, or its water or mineral content. At the bottom of the soil food web are bacteria and fungi, which are attracted to and consume plant root exudates. In turn, they attract and are eaten by bigger microbes, specifically nematodes and protozoa (amoebae, paramecia, flagellates, and ciliates). The eat bacteria and fungi primarily for carbon to fuel their metabolic functions. Anything they don’t need is excreted as wastes, which plant roots are readily able to absorb as nutrients.

What Is the 'Rhizosphere'?

The rhizosphere is the narrow region of soil or substrate that is directly influenced by root secretions and associated soil microorganisms known as the root microbiome.

  Plants Are At The Center Of The 'Soil Food Web'

Graphic showing the structures of a root

At the center of any viable soil food web are plants. Plants control the food web for their own benefit, an amazing fact that is too little understood and surely not appreciated by gardeners who are constantly interfering with nature’s system.

Studies indicate that individual plants can control the numbers and the different kinds of fungi and bacteria attracted to the rhizosphere by the exudates they produce. During different times of the growing season, populations of rhizosphere bacteria and fungi wax and wane, depending on the nutrient needs of the plant and the exudates it produces.

Soil bacteria and fungi are like small bags of fertilizer, retaining in their bodies nitrogen and other nutrients they gain from root exudates and other organic matter. Carrying on the analogy, soil protozoa and nematodes act as fertilizer spreaders by releasing the nutrients locked up in the bacteria and fungi fertilizer bags.

The nematodes and protozoa in the soil come along and eat the bacteria and fungi in the rhizosphere. They digest what they need to survive and excrete excess carbon and other nutrients as waste.

Left to their own devices, then, plants produce exudates that attract fungi and bacteria and, ultimately, nematodes and protozoa. Their survival depends on the interplay between these microbes.

Soil life provides the nutrients needed for plant life, and plants initiate and fuel the cycle by producing exudates.

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.

  Plant Root Symbiotic Relationships

Special soil fungi, called mycorrhizal fungi, establish themselves in a symbiotic relationship with roots, providing them not only with physical protection but with nutrient delivery as well. In return for exudates, these fungi provide water, phosphorus, and other necessary plant nutrients. Soil food web populations must be in balance, or these fungi are eaten and the plant suffers.

Bacteria produce exudates of their own, and the slime they use to attach to surfaces traps pathogens. Sometimes, bacteria work in conjunction with fungi to form protective layers, not only around roots in the rhizosphere but on an equivalent area around leaf surfaces, the phyllosphere. Leaves produce exudates that attract microorganisms in exactly the same way roots do; these act as a barrier to invasion, preventing disease-causing organisms from entering the plant’s system.

  Soil Life Creates Soil Structure

Representative figure of major rhizosphere processes in the soil

The protozoa and nematodes that feasted on the fungi and bacteria attracted by plant exudates are in turn eaten by arthropods. Insects, spiders, even shrimp and lobsters are arthropods. Soil arthropods eat each other and themselves are the food of snakes, birds, moles, and other animals. Simply put, the soil is one big fast-food restaurant. In the course of all this eating, members of a soil food web move about in search of prey or protection, and while they do, they have an impact on the soil.

Bacteria are so small they need to stick to things, or they will wash away; to attach themselves, they produce a slime, the secondary result of which is that individual soil particles are bound together. Fungal hyphae, too, travel through soil particles, sticking to them and binding them together, thread-like, into aggregates.

Worms, together with insect larvae and moles and other burrowing animals, move through the soil in search of food and protection, creating pathways that allow air and water to enter and leave the soil. Even microscopic fungi can help in this regard.

The soil food web, then, in addition to providing nutrients to roots in the rhizosphere, also helps create soil structure: the activities of its members bind soil particles together even as they provide for the passage of air and water through the soil.

Read More: Soil Aggregates and Soil Structure

Further Information

 Improving Soil Health
 Using Soil Inoculants
 The Wood Wide Web
 Soil In Winter
 Benefits of Mycorrhizae Fungi

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