The Greening of Brown Thumbs

Some people seem to have an inborn ability to grow plants, but I am beginning to suspect their green thumbs come from knowledge rather than luck. Having been born with ten thumbs, all brown, I figure it is time to try to learn some of those secrets the expert gardeners appear to have absorbed automatically.

Among the most important, obviously, is an understanding of what constitutes a good growing medium for plants. A reading of some texts on soil and plant science indicates that the ideal growing soil for most garden plants is one that is roughly 45% mineral soil, 5% organic soil, 20 to 30% water and 20 to 30% air.

One can substitute for the solid soil particles but not for the water or the air. That, in fact, is the idea behind hydroponic gardening. Even there, it is common to let the plant roots be supported by a matrix of gravel or other inert material. At frequent intervals the matrix is flooded with a water bath containing minerals and fertilizer, or else a stream of gas is bubbled through the solution continuously.

A plant begins to wilt as soon as there is insufficient water around the small root hairs that take in the water needed by the plant. Root hairs are numerous and tiny; typically they are less than a half inch long, and there may be as many as one-hundred-thousand of them per square inch of root. If these hairs are exposed to dry air for even a few minutes they usually die.

Nor can most plant roots withstand being totally immersed in water for too many hours since that too will kill the root. The key to root survival and a good rate of plant growth is for the root hairs to be exposed continuously to an environment that contains both liquid water and air that is rich in oxygen and saturated with water vapor.

This environment is provided by the pore spaces between the solid particles in soil. Ideally, the walls of a pore space are coated with a thick layer of liquid water; thick in this case means only a tiny fraction of an inch. At least some of the pore spaces must be large enough to contain an atmosphere of water-saturated air in contact with the hair root as well as the layer of liquid water.

A soil with adequate particle size distribution will take up a goodly supply of liquid water in the layers lining the pore spaces and still allow excess water to drain away. Then during the hours and days ahead, the water layers will keep the air in the pore spaces saturated by giving up water molecules to the air and directly to the root hairs. Eventually the water layers become too thin to easily give up water; that is when it is time to add water again.

Remember when watering a garden that the water needs to get down where the root hairs are, perhaps even a few inches deeper so that there is a better reserve for the plants to draw from using capillary action. Remember also, that the water filters down slowly, often taking several hours to get to a depth of one foot. This is why slow watering of a garden or lawn for several hours at a time is recommended.