Mycorrhizal association is another beneficial relationship that should exist between soil organisms and many higher plants. This symbiotic relationship involves fungi and plant roots. Fungi can be pathogenic, consuming living plants. But most of them are harmless and eat only dead, decaying organic matter. Most fungi are soil dwellers though some eat downed or even standing trees.
Most people do not realize that plant roots adsorb water and water-soluble nutrients only through the tiny hairs and actively growing tips near the very end of the root. The ability for any new root to absorb nutrition only lasts a short time, then the hairs slough off and the root develops a sort of hard bark. If root system growth slows or stops, the plant's ability to obtain nourishment is greatly reduced. Roots cannot make oxygen out of carbon dioxide as do the leaves. That's why it is so important to maintain a good supply of soil air and for the soil to remain loose enough to allow rapid root expansion.
When roots are cramped, top growth slows or ceases, health and disease resistance drops, and plants may become stressed despite applications of nutrients or watering. Other plants that do not seem to be competing for light above ground may have ramified (filled with roots) far wider expanses soil than a person might think. Once soil is saturated with the roots and the exudates from one plant, the same space may be closed off to the roots of another. Gardeners who use close plantings and intensive raised beds often unknowingly bump up against this limiting factor and are disappointed at the small size of their vegetables despite heavy fertilization, despite loosening the earth two feet deep with double digging, and despite regular watering. Thought about in this way, it should be obvious why double digging improves growth on crowded beds by increasing the depth to which plants can root.
The roots of plants have no way to aggressively breakdown rock particles or organic matter, nor to sort out one nutrient from another. They uptake everything that is in solution, no more, no less while replacing water evaporated from their leaves. However, soil fungi are able to aggressively attack organic matter and even mineral rock particles and extract the nutrition they want. Fungi live in soil as long, complexly interconnected hair-like threads usually only one cell thick. The threads are called "hyphae." Food circulates throughout the hyphae much like blood in a human body. Sometimes, individual fungi can grow to enormous sizes; there are mushroom circles hundreds of feet in diameter that essentially are one single very old organism. The mushrooms we think of when we think "fungus" are actually not the organism, but the transitory fruit of a large, below ground network.
Certain types of fungi are able to form a symbiosis with specific plant species. They insert a hyphae into the gap between individual plant cells in a root hair or just behind the growing root tip. Then the hyphae "drinks" from the vascular system of the plant, robbing it of a bit of its life's blood. However, this is not harmful predation because as the root grows, a bark develops around the hyphae. The bark pinches off the hyphae and it rapidly decays inside the plant, making a contribution of nutrients that the plant couldn't otherwise obtain. Hyphae breakdown products may be in the form of complex organic molecules that function as phytamins for the plant.
Not all plants are capable of forming mycorrhizal associations. Members of the cabbage family, for example, do not. However, if the species can benefit from such an association and does not have one, then despite fertilization the plant will not be as healthy as it could be, nor grow as well. This phenomenon is commonly seen in conifer tree nurseries where seedling beds are first completely sterilized with harsh chemicals and then tree seeds sown. Although thoroughly fertilized, the tiny trees grow slowly for a year or so. Then, as spores of mycorrhizal fungi begin falling on the bed and their hyphae become established, scattered trees begin to develop the necessary symbiosis and their growth takes off. On a bed of two-year-old seedlings, many individual trees are head and shoulders above the others. This is not due to superior genetics or erratic soil fertility. These are the individuals with a mycorrhizal association.
Like other beneficial microorganisms, micorrhizal fungi do not primarily eat plant vascular fluid, their food is decaying organic matter. Here's yet another reason to contend that soil productivity can be measured by humus content.
CHAPTER EIGHT
Maintaining Soil Humus
Organic matter benefits soil productivity not because it is present, but because all forms of organic matter in the soil, including its most stable form—humus—are disappearing. Mycorrhizal fungi and beneficial bacterial colonies around plant roots can exist only by consuming soil organic matter. The slimes and gums that cement soil particles into relatively stable aggregates are formed by microorganisms as they consume soil organic matter. Scats and casts that are soil crumbs form only because organic matter is being consumed. If humus declines, the entire soil ecology runs down and with it, soil tilth and the health and productivity of plants.