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