GARDENING WITHOUT IRRIGATION TEXT

And sandy loams often have a clayey, moisture-rich subsoil. _Because of this interplay of factors, how much available water your own unique garden soil is actually capable of providing and how much you will have to supplement it with irrigation can only be discovered by trial._ How Soil Loses Water 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. 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. How Plants Obtain Water 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. Lowered Plant Density: The Key to Water-Wise Gardening I always think my latest try at writing a near-perfect garden book is quite a bit better than the last. _Growing Vegetables West of the Cascades_, recommended somewhat wider spacings on raised beds than I did in 1980 because I'd repeatedly noticed that once a leaf canopy forms, plant growth 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 assumed crowded 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 if moisture 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 readily available to the roots of another. So allocating more elbow room allows vegetables to get larger and yield longer and allows the gardener to reduce the frequency of irrigations. Though hot, baking sun and wind can desiccate the few inches of surface soil, withdrawals of moisture from greater depths are made by growing plants transpiring moisture through their leaf surfaces. The amount of water a growing crop will transpire is determined first by the nature of the 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 lost through transpiration. Dealing with a Surprise Water Shortage 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? First, if at all possible before the restrictions take effect, water very heavily and long to ensure there is maximum subsoil moisture. Then eliminate all newly started interplantings and ruthlessly hoe out at least 75 percent of the remaining immature plants and about half of those about two weeks away from harvest. For example, suppose you've got a a 4-foot-wide intensive bed holding seven rows of broccoli on 12 inch centers, or about 21 plants. Remove at least every other row and every other plant in the three or four remaining rows. Try to bring plant density down to those described in Chapter 5, "How to Grow It: A-Z" Then shallowly hoe the soil every day or two to encourage the surface inches to dry out and form a dust mulch. You water-wise person--you're already dry gardening--now start fertigating. How long available soil water will sustain a crop is determined by how many plants are drawing on the reserve, how extensively their root systems develop, and how many leaves are transpiring the moisture. If there are no plants, most of the water will stay unused in the barren soil through the entire growing season. If a crop canopy is established midway through the growing season, the rate of water loss will approximate that listed in the table in Chapter 1 "Estimated Irrigation Requirement." If by very close planting the crop canopy is established as early as possible and maintained by successive interplantings, as is recommended by most advocates of raised-bed gardening, water losses will greatly exceed this rate. Many vegetable species become mildly stressed when soil moisture has dropped about half the way from capacity to the wilting point. On very closely planted beds a crop can get in serious trouble without irrigation in a matter of days. But if that same crop were planted less densely, it might grow a few weeks without irrigation. And if that crop were planted even farther apart so that no crop canopy ever developed and a considerable amount of bare, dry earth were showing, this apparent waste of growing space would result in an even slower rate of soil moisture depletion. On deep, open soil the crop might yield a respectable amount without needing any irrigation at all. West of the Cascades we expect a rainless summer; the surprise comes that rare rainy year when the soil stays moist and we gather bucketfuls of chanterelle mushrooms in early October. Though the majority of maritime Northwest gardeners do not enjoy deep, open, moisture-retentive soils, all except those with the shallowest soil can increase their use of the free moisture nature provides and lengthen the time between irrigations. The next chapter discusses making the most of whatever soil depth you have. Most of our region's gardens can yield abundantly without any rain at all if only we reduce competition for available soil moisture, judiciously fertigate some vegetable species, and practice a few other water-wise tricks. _Would lowering plant density as much as this book suggests equally lower the yield of the plot? Surprisingly, the amount harvested does not drop proportionately. In most cases having a plant density one-eighth of that recommended by intensive gardening advocates will result in a yield about half as great as on closely planted raised beds._ 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. Chapter 3 Helping Plants to Need Less Irrigation Dry though the maritime Northwest summer is, we enter the growing season with our full depth of soil at field capacity. Except on clayey soils in extraordinarily frosty, high-elevation locations, we usually can till and plant before the soil has had a chance to lose much moisture. There are a number of things we can do to make soil moisture more available to our summer vegetables. The most obvious step is thorough weeding. Next, we can keep the surface fluffed up with a rotary tiller or hoe during April and May, to break its capillary connection with deeper soil and accelerate the formation of a dry dust mulch. Usually, weeding forces us to do this anyway. Also, if it should rain during summer, we can hoe or rotary till a day or two later and again help a new dust mulch to develop. Building Bigger Root Systems Without irrigation, most of the plant's water supply is obtained by expansion into new earth that hasn't been desiccated by other competing roots. Eliminating any obstacles to rapid growth of root systems is the key to success. So, keep in mind a few facts about how roots grow and prosper. The air supply in soil limits or allows root growth. Unlike the leaves, roots do not perform photosynthesis, breaking down carbon dioxide gas into atmospheric oxygen and carbon. Yet root cells must breathe oxygen. This is obtained from the air held in spaces between soil particles. Many other soil-dwelling life forms from bacteria to moles compete for this same oxygen. Consequently, soil oxygen levels are lower than in the atmosphere. A slow exchange of gases does occur between soil air and free atmosphere, but deeper in the soil there will inevitably be less oxygen. Different plant species have varying degrees of root tolerance for lack of oxygen, but they all stop growing at some depth. Moisture reserves below the roots' maximum depth become relatively inaccessible. Soil compaction reduces the overall supply and exchange of soil air. Compacted soil also acts as a mechanical barrier to root system expansion. When gardening with unlimited irrigation or where rain falls frequently, it is quite possible to have satisfactory growth when only the surface 6 or 7 inches of soil facilitates root development. When gardening with limited water, China's the limit, because if soil conditions permit, many vegetable species are capable of reaching 4, 5, and 8 eight feet down to find moisture and nutrition. Evaluating Potential Rooting Ability One of the most instructive things a water-wise gardener can do is to rent or borrow a hand-operated fence post auger and bore a 3-foot-deep hole. It can be even more educational to buy a short section of ordinary water pipe to extend the auger's reach another 2 or 3 feet down. In soil free of stones, using an auger is more instructive than using a conventional posthole digger or shoveling out a small pit, because where soil is loose, the hole deepens rapidly. Where any layer is even slightly compacted, one turns and turns the bit without much effect. Augers also lift the materials more or less as they are stratified. If your soil is somewhat stony (like much upland soil north of Centralia left by the Vashon Glacier), the more usual fence-post digger or common shovel works better. If you find more than 4 feet of soil, the site holds a dry-gardening potential that increases with the additional depth. Some soils along the floodplains of rivers or in broad valleys like the Willamette or Skagit can be over 20 feet deep, and hold far more water than the deepest roots could draw or capillary flow could raise during an entire growing season. Gently sloping land can often carry 5 to 7 feet of open, usable soil. However, soils on steep hillsides become increasingly thin and fragile with increasing slope. Whether an urban, suburban, or rural gardener, you should make no assumptions about the depth and openness of the soil at your disposal. Dig a test hole. If you find less than 2 unfortunate feet of open earth before hitting an impermeable obstacle such as rock or gravel, not much water storage can occur and the only use this book will hold for you is to guide your move to a more likely gardening location or encourage the house hunter to seek further. Of course, you can still garden quite successfully on thin soil in the conventional, irrigated manner. _Growing Vegetables West of the Cascades_ will be an excellent guide for this type of situation. Eliminating Plowpan Deep though the soil may be, any restriction of root expansion greatly limits the ability of plants to aggressively find water. A compacted subsoil or even a thin compressed layer such as plowpan may function as such a barrier. Though moisture will still rise slowly by capillarity and recharge soil above plowpan, plants obtain much more water by rooting into unoccupied, damp soil. Soils close to rivers or on floodplains may appear loose and infinitely deep but may hide subsoil streaks of droughty gravel that effectively stops root growth. Some of these conditions are correctable and some are not. Plowpan is very commonly encountered by homesteaders on farm soils and may be found in suburbia too, but fortunately it is the easiest obstacle to remedy. Traditionally, American croplands have been tilled with the moldboard plow. As this implement first cuts and then flips a 6-or 7-inch-deep slice of soil over, the sole--the part supporting the plow's weight--presses heavily on the earth about 7 inches below the surface. With each subsequent plowing the plow sole rides at the same 7-inch depth and an even more compacted layer develops. Once formed plowpan prevents the crop from rooting into the subsoil. Since winter rains leach nutrients from the topsoil and deposit them in the subsoil, plowpan prevents access to these nutrients and effectively impoverishes the field. So wise farmers periodically use a subsoil plow to fracture the pan. Plowpan can seem as firm as a rammed-earth house; once established, it can last a long, long time. My own garden land is part of what was once an old wheat farm, one of the first homesteads of the Oregon Territory. From about 1860 through the 1930s, the field produced small grains. After wheat became unprofitable, probably because of changing market conditions and soil exhaustion, the field became an unplowed pasture. Then in the 1970s it grew daffodil bulbs, occasioning more plowing. All through the '80s my soil again rested under grass. In 1987, when I began using the land, there was still a 2-inch-thick, very hard layer starting about 7 inches down. Below 9 inches the open earth is soft as butter as far as I've ever dug. On a garden-sized plot, plowpan or compacted subsoil is easily opened with a spading fork or a very sharp common shovel. After normal rotary tilling, either tool can fairly easily be wiggled 12 inches into the earth and small bites of plowpan loosened. Once this laborious chore is accomplished the first time, deep tillage will be far easier. In fact, it becomes so easy that I've been looking for a custom-made fork with longer tines. Curing Clayey Soils In humid climates like ours, sandy soils may seem very open and friable on the surface but frequently hold some unpleasant subsoil surprises. Over geologic time spans, mineral grains are slowly destroyed by weak soil acids and clay is formed from the breakdown products. Then heavy winter rainfall transports these minuscule clay particles deeper into the earth, where they concentrate. It is not unusual to find a sandy topsoil underlaid with a dense, cement-like, clayey sand subsoil extending down several feet. If very impervious, a thick, dense deposition like this may be called hardpan. The spading fork cannot cure this condition as simply as it can eliminate thin plowpan. Here is one situation where, if I had a neighbor with a large tractor and subsoil plow, I'd hire him to fracture my land 3 or 4 feet deep. Painstakingly double or even triple digging will also loosen this layer. Another possible strategy for a smaller garden would be to rent a gasoline-powered posthole auger, spread manure or compost an inch or two thick, and then bore numerous, almost adjoining holes 4 feet deep all over the garden.