(Perma)Culture and Sanity
In Nature, the soil's surface is rarely smooth and flat. Instead, a varied surface of hills and valleys, high spots and drainages gives plants a wide range of soil and moisture conditions to adapt to. In this way, many different plants (each with very different requirements for moisture, sun, wind, and nutrients) can find their needs and grow within a short distance of each other. The result is the great diversity of plants (and animals) which we find flourishing in natural environments.
We can use these same ideas in our own gardens to allow us to grow a wider variety of plants than we otherwise could, and with less trouble since each individual plant can more easily find exactly the microclimate it prefers. All that is required of the gardener is that she shape the surface of her garden beds to follow the same patterns which can be observed in Nature.
In order to create a more diverse environment in the garden, the gardener must first decide where the high and low spots will be placed, and what will be grown in those spots. Many plants from arid lands, such as rosemary and fava beans, will grow in humid, wetter climates if they are placed in a bed of porous soil (such as a sandy loam) and raised a foot or two above the surrounding area.
Other plants, such as canna lilies and irises, prefer to have their feet wet and will grow well in drainage areas or spots where the drainage is naturally poor.
Most vegetables prefer conditions somewhere in between wet and dry, and do very well in slightly raised beds placed close to wet areas where their roots can find sub-soil water during drier periods. Each of these conditions is easily established in the home garden with a little planning when the beds are being installed.
If the gardener wishes to grow only plants which prefer arid conditions, shaping beds may be as simple as using imported sandy soil to create raised planting areas. If only moisture-loving plants will be grown, the garden might consist of a large, shallow low spot which floods a few inches deep when it rains.
Most of the time, however, a gardener will want to grow a variety of plants and should install a wider variety of topological conditions in order to create wet, dry, and in-between spots. In this case, the garden might be shaped so as to include tall and short raised beds decoratively winding their way through lower areas where water collects during rains.
In poorly drained soils, low areas can be dug like a creek system formed of linked pools that spill into each other and out of the garden in heavy rains. Low areas constructed this way will provide drainage for the raised beds during wet periods, while at the same time allowing water to collect in pools below the beds during wet periods for infiltration into the soil.
In a conventional garden, the soil's surface is levelled prior to final preparation and mulching. Plants which need more water must have water supplied often, somtimes creating problems for nearby plants with a need for drier conditions, and vice versa. In the multi-level garden, each plant can be placed so as to meet its own water needs most of the itme, creating healthier plants, avoiding conflicts and saving time for the gardener. Water infiltrated into the soil from low areas spreads sideways to provide sub-surface irrigation for all the beds without adversely affecting surface drainage conditions, which means that the gardener can water less often.
In areas where the water quality is poor, as in regions where the water contains salt (which collects in the soil and causes a loss of the soil's fluffiness or "crumb structure"), watering more heavily but less often can be very beneficial for plant and soil health. Shallow watering concentrates salts near the soil's surface where they can do the most harm—deeper watering pushes these same salts farther down into the soil, out of the reach of evaporation and plants' shallow 'feeder roots.'
When water is allowed to soak into the ground for long periods of time without a means of using the water which is infiltrated, the soil can become waterlogged. Even more serious is the possibility of raising the water table over a period of time until it reaches the surface, which causes soil salinization because of the salt content of most water tables. Both these problems can be avoided easily, however (and a further yield gotten from the garden), by planting trees in the garden every forty or fifty feet in each direction. Trees, which use large amounts of water, can reach down and tap into water tables deep in the ground, keeping them from rising too close to the surface. They provide shade for human enjoyment, and can also be grown during the late spring and early summer. Furthermore, trees provide frost protection for tender plants, and can extend the growing season for tender heat-loving plants as well as for the cool-season ones.
In regions with heavy clay soils, water may at first infiltrate only slowly into drainage bottoms. It is wise to arrange the drainages so that all standing water soaks into the soil in less than two or three days. The rate at which water soaks into the ground can be speeded up if the bottom of the drainages are broken up a little, and some gypsum (calcium sulfate) added under a layer of leaves. The leaves and gypsum will work together to loosen up even the stiffest of clays, and water will soon soak into the clay at an acceptable rate.
Reprinted from (Perma) Culture and Sanity Website