Water-Wise Gardening Science
|Particle type||Diameter of particles in mm||Number of particles per gm||Surface area in sq. cm.|
|Very coarse sand||2.00-1.00||90||11|
|Very fine sand||0.10-0.05||772,000||227|
|Soil Texture||Average Amount|
|Very coarse sand||0.5|
Source: Fundamentals of Soil Science.
Adhering water films can vary greatlyin thickness. But if the water molecules adhering to a soil particle become too thick,the force of adhesion becomes too weak to resist the force of gravity, and some waterflows deeper into the soil. When water films are relatively thick the soil feelswet and plant roots can easily absorb moisture. "Field capacity" is theterm describing soil particles holding all the water they can against the force ofgravity.
At the other extreme, the thinner thewater films become, the more tightly they adhere and the drier the earth feels. Atsome degree of desiccation, roots are no longer forceful enough to draw on soil moistureas fast as the plants are transpiring. This condition is called the "wiltingpoint." The term "available moisture" refers to the difference betweenfield capacity and the amount of moisture left after the plants have died.
Clayey soil can provide plants withthree times as much available water as sand, six times as much as a very coarse sandysoil. It might seem logical to conclude that a clayey garden would be the most droughtresistant. But there's more to it. For some crops, deep sandy loams can provide justabout as much usable moisture as clays. Sandy soils usually allow more extensiveroot development, so a plant with a naturally aggressive and deep root system maybe able to occupy a much larger volume of sandy loam, ultimately coming up with moremoisture than it could obtain from a heavy, airless clay. And sandy loams often havea clayey, moisture-rich subsoil.
Because of this interplay of factors,how much available water your own unique garden soil is actually capable of providingand how much you will have to supplement it with irrigation can only be discoveredby trial.
How Soil Loses Water
Suppose we tilled a plot about April1 and then measured soil moisture loss until October. Because plants growing aroundthe edge might extend roots into our test plot and extract moisture, we'll make ourtilled area 50 feet by 50 feet and make all our measurements in the center. And let'slocate this imaginary plot in full sun on flat, uniform soil. And let's plant absolutelynothing in this bare earth. And all season let's rigorously hoe out every weed whileit is still very tiny.
Let's also suppose it's been a typicalmaritime Northwest rainy winter, so on April 1 the soil is at field capacity, holdingall the moisture it can. From early April until well into September the hot sun willbeat down on this bare plot. Our summer rains generally come in insignificant installmentsand do not penetrate deeply; all of the rain quickly evaporates from the surfacefew inches without recharging deeper layers. Most readers would reason that a soilmoisture measurement taken 6 inches down on September 1, should show very littlewater left. One foot down seems like it should be just as dry, and in fact, mostgardeners would expect that there would be very little water found in the soil untilwe got down quite a few feet if there were several feet of soil.
But that is not what happens! The hotsun does dry out the surface inches, but if we dig down 6 inches or so there willbe almost as much water present in September as there was in April. Bare earth doesnot 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 dryout when bare are certain kinds of very heavy clays that form deep cracks. Theseever-deepening openings allow atmospheric air to freely evaporate additional moisture.But if the cracks are filled with dust by surface cultivation, even this soil typeceases to lose water.
Soil functions as our bank account,holding available water in storage. In our climate soil is inevitably charged tocapacity 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 causedby hot sun and wind working on plant leaves, making them transpire moisture drawnfrom the earth through their root systems. Plants desiccate soil to the ultimatedepth and lateral extent of their rooting ability, and then some. The size of vegetableroot systems is greater than most gardeners would think. The amount of moisture potentiallyavailable to sustain vegetable growth is also greater than most gardeners think.
Rain and irrigation are not the onlyways to replace soil moisture. If the soil body is deep, water will gradually comeup from below the root zone by capillarity. Capillarity works by the very same forceof adhesion that makes moisture stick to a soil particle. A column of water in avertical tube (like a thin straw) adheres to the tube's inner surfaces. This adhesiontends to lift the edges of the column of water. As the tube's diameter becomes smallerthe amount of lift becomes greater. Soil particles form interconnected pores thatallow an inefficient capillary flow, recharging dry soil above. However, the driersoil becomes, the less effective capillary flow becomes. That is why a thoroughlydesiccated surface layer only a few inches thick acts as a powerful mulch.
Industrial farming and modern gardeningtend to discount the replacement of surface moisture by capillarity, consideringthis flow an insignificant factor compared with the moisture needs of crops. Butconventional agriculture focuses on maximized yields through high plant densities.Capillarity is too slow to support dense crop stands where numerous root systemsare competing, but when a single plant can, without any competition, occupy a largeenough area, moisture replacement by capillarity becomes significant.
How Plants Obtain Water
Most gardeners know that plants acquirewater and minerals through their root systems, and leave it at that. But the processis not quite that simple. The actively growing, tender root tips and almost microscopicroot hairs close to the tip absorb most of the plant's moisture as they occupy newterritory. As the root continues to extend, parts behind the tip cease to be effectivebecause, as soil particles in direct contact with these tips and hairs dry out, theolder roots thicken and develop a bark, while most of the absorbent hairs sloughoff. This rotation from being actively foraging tissue to becoming more passive conductiveand supportive tissue is probably a survival adaptation, because the slow capillarymovement of soil moisture fails to replace what the plant used as fast as the plantmight like. The plant is far better off to aggressively seek new water in unoccupiedsoil than to wait for the soil its roots already occupy to be recharged.
A simple bit of old research magnificentlyillustrated the significance of this. A scientist named Dittmer observed in 1937that a single potted ryegrass plant allocated only 1 cubic foot of soil to grow inmade about 3 miles of new roots and root hairs every day. (Ryegrasses are known tomake more roots than most plants.) I calculate that a cubic foot of silty soil offersabout 30,000 square feet of surface area to plant roots. If 3 miles of microscopicroot tips and hairs (roughly 16,000 lineal feet) draws water only from a few millimetersof surrounding soil, then that single rye plant should be able to continue ramifyinginto a cubic foot of silty soil and find enough water for quite a few days beforewilting. These arithmetical estimates agree with my observations in the garden, andwith my experiences raising transplants in pots.
Lowered Plant Density: The Key to Water-Wise Gardening
I always think my latest try at writinga near-perfect garden book is quite a bit better than the last. Growing VegetablesWest of the Cascades, recommended somewhat wider spacings on raised beds thanI did in 1980 because I'd repeatedly noticed that once a leaf canopy forms, plantgrowth slows markedly. Adding a little more fertilizer helps after plants "bump,"but still the rate of growth never equals that of younger plants. For years I assumedcrowded plants stopped producing as much because competition developed for light.But now I see that unseen competition for root room also slows them down. Even ifmoisture is regularly recharged by irrigation, and although nutrients are replaced,once a bit of earth has been occupied by the roots of one plant it is not so readilyavailable to the roots of another. So allocating more elbow room allows vegetablesto get larger and yield longer and allows the gardener to reduce the frequency ofirrigations.
Though hot, baking sun and wind candesiccate the few inches of surface soil, withdrawals of moisture from greater depthsare made by growing plants transpiring moisture through their leaf surfaces. Theamount of water a growing crop will transpire is determined first by the nature ofthe species itself, then by the amount of leaf exposed to sun, air temperature, humidity,and wind. In these respects, the crop is like an automobile radiator. With cars,the more metal surfaces, the colder the ambient air, and the higher the wind speed,the better the radiator can cool; in the garden, the more leaf surfaces, the faster,warmer, and drier the wind, and the brighter the sunlight, the more water is lostthrough transpiration.
Suppose you are growing a conventional, irrigated garden and something unanticipated interrupts your ability to water. Perhaps you are homesteading and your well begins to dry up. Perhaps you're a backyard gardener and the municipality temporarily restricts usage. What to do?
Internet Readers: In the print copy of this book are color pictures of my own "irrigationless" garden. Looking at them about here in the book would add reality to these ideas. I suggest you look at "Carrots," "Endive," and "Kale" now.