Suppose we tilled a plot about April 1 and then measured soil moisture loss until October. Because plants growing around the edge might extend roots into our test plot and extract moisture, we'll make our tilled area 50 feet by 50 feet and make all our measurements in the center. And let's locate this imaginary plot in full sun on flat, uniform soil. And let's plant absolutely nothing in this bare earth. And all season let's rigorously hoe out every weed while it is still very tiny.
Let's also suppose it's been a typical maritime Northwest rainy winter, so on April 1 the soil is at field capacity, holding all the moisture it can. From early April until well into September the hot sun will beat down on this bare plot. Our summer rains generally come in insignificant installments and do not penetrate deeply; all of the rain quickly evaporates from the surface few inches without recharging deeper layers. Most readers would reason that a soil moisture measurement taken 6 inches down on September 1, should show very little water left. One foot down seems like it should be just as dry, and in fact, most gardeners would expect that there would be very little water found in the soil until we got down quite a few feet if there were several feet of soil.
But that is not what happens! The hot sun does dry out the surface inches, but if we dig down 6 inches or so there will be almost as much water present in September as there was in April. Bare earth does not lose much water at all. Once a thin surface layer is completely desiccated, be it loose or compacted, virtually no further loss of moisture can occur.
The only soils that continue to dry out when bare are certain kinds of very heavy clays that form deep cracks. These ever-deepening openings allow atmospheric air to freely evaporate additional moisture. But if the cracks are filled with dust by surface cultivation, even this soil type ceases to lose water.
Soil functions as our bank account, holding available water in storage. In our climate soil is inevitably charged to capacity by winter rains, and then all summer growing plants make heavy withdrawals. But hot sun and wind working directly on soil don't remove much water; that is caused by hot sun and wind working on plant leaves, making them transpire moisture drawn from the earth through their root systems. Plants desiccate soil to the ultimate depth and lateral extent of their rooting ability, and then some. The size of vegetable root systems is greater than most gardeners would think. The amount of moisture potentially available to sustain vegetable growth is also greater than most gardeners think.
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Rain and irrigation are not the only ways to replace soil moisture. If the soil body is deep, water will gradually come up from below the root zone by capillarity. Capillarity works by the very same force of adhesion that makes moisture stick to a soil particle. A column of water in a vertical tube (like a thin straw) adheres to the tube's inner surfaces. This adhesion tends to lift the edges of the column of water. As the tube's diameter becomes smaller the amount of lift becomes greater. Soil particles form interconnected pores that allow an inefficient capillary flow, recharging dry soil above. However, the drier soil becomes, the less effective capillary flow becomes. That is why a thoroughly desiccated surface layer only a few inches thick acts as a powerful mulch.
Industrial farming and modern gardening tend to discount the replacement of surface moisture by capillarity, considering this flow an insignificant factor compared with the moisture needs of crops. But conventional agriculture focuses on maximized yields through high plant densities. Capillarity is too slow to support dense crop stands where numerous root systems are competing, but when a single plant can, without any competition, occupy a large enough area, moisture replacement by capillarity becomes significant.
Most gardeners know that plants acquire water and minerals through their root systems, and leave it at that. But the process is not quite that simple. The actively growing, tender root tips and almost microscopic root hairs close to the tip absorb most of the plant's moisture as they occupy new territory. As the root continues to extend, parts behind the tip cease to be effective because, as soil particles in direct contact with these tips and hairs dry out, the older roots thicken and develop a bark, while most of the absorbent hairs slough off. This rotation from being actively foraging tissue to becoming more passive conductive and supportive tissue is probably a survival adaptation, because the slow capillary movement of soil moisture fails to replace what the plant used as fast as the plant might like. The plant is far better off to aggressively seek new water in unoccupied soil than to wait for the soil its roots already occupy to be recharged.
A simple bit of old research magnificently illustrated the significance of this. A scientist named Dittmer observed in 1937 that a single potted ryegrass plant allocated only 1 cubic foot of soil to grow in made about 3 miles of new roots and root hairs every day. (Ryegrasses are known to make more roots than most plants.) I calculate that a cubic foot of silty soil offers about 30,000 square feet of surface area to plant roots. If 3 miles of microscopic root tips and hairs (roughly 16,000 lineal feet) draws water only from a few millimeters of surrounding soil, then that single rye plant should be able to continue ramifying into a cubic foot of silty soil and find enough water for quite a few days before wilting. These arithmetical estimates agree with my observations in the garden, and with my experiences raising transplants in pots.
~ Andrew Weil
Whether you are an avid gardener or just starting out, the idea of creating a garden using organic methods can seem overwhelming at first. But organic gardening is less daunting than you may think if you understand some basic principles and can be done right in your own backyard.
There are many reasons why today, more than ever before, every owner of a small place should have his, or her, own garden. The old days of home meat producing, cheese-making, and traditional cooking are gone. The growing of healthy organic vegetables and fruits that taste great cannot be so easy disposed of.
Most things, in the course of development, change from the simple to the complex. The art of gardening has in many ways been an exception to the rule. The methods of culture used for many crops are more simple than those in vogue a generation ago. The last twenty years has seen also a tremendous...
