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The rhizosphere is the absorbing root-soil interface. It is the zone, about one millimeter in width, surrounding the epidermis of living root hairs and the boundary cells of mycorrhizae as well as hyphae growing out from some mycorrhizae.
The rhizoplane is the boundary where soil elements in water are absorbed into the tree. Under an electron microscope, the rhizoplane appears as a jelly where microorganisms and tree cells mix, making it impossible to tell which side is tree and which is soil.
A constantly changing mix of organisms inhabit the rhizosphere and surrounding soil. Bacteria, actinomycetes, fungi, protozoa, slime molds, algae, nematodes, enchytraeid worms, earthworms, millipedes, centipedes, insects, mites, snails, small animals and soil viruses compete constantly for water, food, and space.
The rhizosphere is a battleground and the wars are continuous. Amoebae are eating bacteria. Some bacteria are poisoning other bacteria. Fungi are killing other fungi. Nematodes are spearing roots. Fungi are trapping nematodes. Earthworms are eating anything they can find. Sometimes the victors benefit the tree and sometimes they do not.
Every tree treatment affects the rhizosphere in some way. The more you know about the rhizosphere, the better the chances are that your treatments will lead to benefits rather than harm.
Declines and the Starving Rhizosphere
Go anywhere in the world and you will learn that some local trees have a "new" decline problem. Declines usually mean the trees are sick because there is a problem in the rhizosphere.
Trees die, as all organisms do, in three basic ways: depletion, dysfunction and disruption. Disruption means wounding, severe mechanical impacts and fracturing. Dysfunction means some parts and processes of the living system have developed problems that retard or prevent their functioning and growth. Depletion means that the basic substances for life begin to decrease to the point where injury and death are certain. One of the ways depletion injures organisms is by starvation.
Soils and wood share a common problem: They are thought of as dead substances. This has come about because wood-products research gained an early lead over research on wood in living trees. With soils, many texts still define soils as "loose material of weathered rock and other minerals, and also partly decayed organic matter that covers large parts of the land surface on Earth."
Sapwood in living trees has many more living cells than dead cells. In upper layers where most absorbing roots of plants grow, soils have more soil organisms than grains of weathered rock. In great disrespect, most people still refer to soil as dirt! When researchers first discovered the great value of soil microorganisms for human antibiotics and profit, the living nature of the soil began to emerge.
A more correct definition of soil should be that it is a substance made up of sands, silts, clays, decaying organic matter, air, water and an enormous number of living organisms. Survival of all living systems depends greatly on synergy and efficiency to optimize the functioning of all processes and to keep waste as low as possible. When synergy and efficiency begin to wane, declines follow.
Trees are dependent on the light energy from the sun for their energy, water and 14 elements from the soil for their building blocks of life. Some trees decline when incorrect treatments or abiotic injuries lead to starvation of organisms in the rhizosphere. When there are troubles in the rhizosphere, there will be troubles with the tree.