Bigger Root Systems
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 the 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 soil air. Compacted soil also acts as a mechanical barrier to root system expansion. When gardening with 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.
Evaluating Potential Rooting Ability
The best practice a water-wise gardener can do is to rent or borrow a hand-operated fence post auger to bore a 3-foot-deep hole, in addition to purchasing a short section of ordinary water pipe to extend the auger's reach another 2 or 3 feet. In soil free of stones, using an auger is more instructive than using a conventional post-hole digger or shoveling out a small pit because where soil is loose, the hole deepens rapidly. Augers also lift the materials more or less as they are stratified. If your soil is somewhat stony, 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. 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, causing erosion.
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, it is best to move to another gardening location. Of course, you can still garden quite successfully on thin soil in the conventional, irrigated manner.
Eliminating Plow-pan
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 plow-pan may function as a barrier. Though moisture will rise slowly by capillarity and recharge soil above plow-pan, 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 drought gravel that effectively stops root growth. Some of these conditions are correctable. Plow-pan is commonly encountered by homesteaders on farm soils and may be found in suburbia too, fortunately, it is the easiest obstacle to remedy. Traditionally, American croplands have been tilled with the moldboard plow. 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 compacted layer develops. Once formed, plow-pan prevents the crop from rooting into the subsoil. Since winter rains leach nutrients from the topsoil and deposit them in the subsoil, plow-pan prevents access to these nutrients and effectively impoverish the field. So wise farmers periodically use a subsoil plow to fracture the pan. On a garden-sized plot, plow-pan 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 plow-pan loosened. Once this laborious chore is accomplished the first time, deep tillage will be far easier.
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 under laid with a dense, cement-like, clayey sand subsoil extending down several feet. A thick, dense deposition like this may be called hard pan. The spading fork cannot cure this condition as simply as it can eliminate thin plow-pan. Here is one situation to remedy this issue; 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 post-hole auger, spread green manure or compost an inch or two thick and bore numerous, almost adjoining holes. 4 feet deep all over the garden.
Clayey subsoil can supply surprisingly larger amounts of moisture than the granular sandy surface might imply but only if the earth is opened deeply and becomes more accessible to root growth. Fortunately, once root development increases at greater depths, the organic matter content and accessibility of this clayey layer can be maintained through intelligent green manuring, postponing for years the need to subsoil again. Green manuring is discussed in detail shortly.
Incorporation of extraordinarily large quantities of organic matter can turn the top few inches into something that behaves a little like loam, it is quite impractical to work in humus to a depth of 4 or 5 feet. Root development will still be limited to the surface layer. Very fine clay, do not produce dry gardens. Not all clay soils are "fine clay soils," totally compacted and airless. Jori clay can be 6 to 8 feet deep and are sufficiently porous and well drained which have been used for highly productive orchard crops. Water-wise gardeners can do wonders with Jori and other similar soils to grow the best root crops.
Spotting a Likely Site
Observing the condition of wild plants can reveal a good site to garden without much irrigation. Where Himalaya or Evergreen blackberries grow 2 feet tall and produce small, dull-tasting fruit, there is not much available soil moisture. Where they grow 6 feet tall and the berries are sweet and good sized, there is deep, open soil. When the berry vines are 8 or more feet tall and the fruits are especially huge, usually there is both deep, loose soil and a higher than usual amount of fertility. Other native vegetation can also reveal a lot about soil moisture reserves.
Using Humus to Increase Soil Moisture
Maintaining topsoil humus content in the 4 to 5 percent range is vital to plant health, vital to growing more nutritious food, essential to bringing the soil into that state of easy work-ability and cooperation known as good tilt. Humus is a spongy substance capable of holding several times more available moisture than clay. There are also new synthetic, long-lasting soil amendments that hold and release even more moisture than humus. Garden books frequently recommend tilling in extraordinarily large amounts of organic matter to increase a soil's water-holding capacity in the top few inches. Humus can improve many aspects of soil but will not reduce a garden's overall need for irrigation because it is simply not practical to maintain sufficient humus depth. Rotary tilling only blends amendments into the top 6 or 7 inches of soil. Rigorous double digging by actually trenching out 12 inches and then spading up the next foot, theoretically allows one to mix in significant amounts of organic matter to nearly 24 inches. But plants can use water from far deeper than that. Let's realistically consider how much soil moisture reserves might be increased by double digging and incorporating large quantities of organic matter.
A healthy topsoil organic matter level in our climate is about 4 percent. This rapidly declines to less than 0.5 percent in the subsoil. Suppose a inch-thick layer of compost were spread and by double digging the organic matter into the content of a very sandy soil amended to 10 percent down to 2 feet. If that soil contained little clay, its water-holding ability in the top 2 feet could be doubled. By this amendment, we might add 1 inch of available moisture per foot of soil to the reserve which equals 2 extra inches of water, enough to increase the time between heavy irrigation by a week or 10 days. If the soil in question were a silty clay, it would naturally make 2 1/2 inches available per foot. A massive humus amendment would increase that to 3 1/2 inches in the top foot or two, relatively not as much benefit as in sandy soil. And I seriously doubt that many gardeners would be willing to thoroughly double dig 24 inches.
Trying to maintain organic matter levels above 10 percent is an almost self-defeating process. The higher the humus level gets, the more rapidly organic matter tends to decay. Finding or making enough well-finished compost to cover the garden several inches deep (what it takes to lift humus levels to 10 percent) is enough of a job. Double digging just as much more into the second foot is even more effort. But having to repeat that chore every year or two becomes downright discouraging. No, either your soil naturally holds enough moisture to permit dry gardening or it doesn't.
Keeping the Subsoil Open with Green Manuring
When roots decay, fresh organic matter and large long-lasting passageways can be left deep in the soil, allowing easier air movement and facilitating entry of other roots. But no cover crop that I am aware of will effectively penetrate firm plow-pan or other resistant physical obstacles. Such a barrier forces all plants to root almost exclusively in the topsoil. However, once the subsoil has been mechanically fractured the first time and if re-compaction is avoided by shunning heavy tractors and other machinery, green manure crops can maintain the openness of the subsoil. To accomplish this, correct green manure species selection is essential. Lawn grasses tend to be shallow rooting, while most regionally adapted pasture grasses can reach down about 3 feet at best. However, orchard grass (called colts foot in English farming books) will grow down 4 or more feet while leaving a massive amount of decaying organic matter in the subsoil after the sod is tilled. Sweet clover, a biennial legume, sprouts one spring then winters over to bloom the next summer, may penetrate 8 feet. Red clover, a perennial species, may thickly invade the top 5 feet. Other useful subsoil busters include densely sown Umbelliferae, such as carrots, parsley and parsnip. The chicory family produces a very large and penetrating taproots.
Though seed for wild chicory is hard to obtain, cheap varieties of endive (a semi-civilized relative) are easily available. And several pounds of your own excellent parsley or parsnip seed can be easily produced by letting about 10 row feet of overwintering roots form seed. Orchard grass and red clover can be had quite inexpensively at many farm supply stores. Sweet clover is not currently grown by our region's farmers and so can only be found by mail. Poppy seed used for cooking will often sprout. Sown densely in October, it forms a thick carpet of frilly spring greens under laid with countless massive taproots that decompose rapidly if the plants are tilled in in April before flower stalks begin to appear.
For country gardeners, the best rotations include several years of perennial grass-legume-herb mixtures to maintain the openness of the subsoil followed by a few years of vegetables and then back. Early in the spring, I broadcast a concoction I call "complete organic fertilizer", till again after the soil dries down a bit and then use a spading fork to open the subsoil before making a seedbed. The first time around, I had to break the century old plow-pan; forking compacted earth a foot deep is a lot of work. In subsequent rotations this practice becomes much easier.
For a couple of years, vegetables will grow vigorously on this new ground supported only with a complete organic fertilizer. But vegetable gardening makes humus levels decline rapidly. So every 2 years, I start a new garden on another plot and replant the old garden to green manures. I never remove vegetation during the long rebuilding under green manures, but merely till it once or twice a year and allow the organic matter content of the soil to redevelop. Consider growing vegetables in the front yard for a few years and then switching to the back yard. Having lots of space, as I do now, I keep three or four garden plots available, one in vegetables and the others restoring their organic matter content under grass.
Mulching
Gardening under a permanent thick mulch of crude organic matter is recommended and disciples a surefire way to drought-proof gardens, while eliminating virtually any need for tillage, weeding, and fertilizing. What follows in this section is addressed to gardeners who have already read glowing reports about mulching. Permanent mulching with vegetation actually does not reduce summertime moisture loss any better than mulching with dry soil, sometimes called "dust mulching." True, while the surface layer stays moist, water will steadily be wicked up by capillarity and be evaporated from the soil's surface. If frequent light sprinkling keeps the surface perpetually moist, subsoil moisture loss can occur all summer, so nu-mulched soil could eventually become desiccated many feet deep. However, capillary movement only happens when soil is damp. Once even a thin layer of soil has become quite dry, it almost completely prevents any further movement.
Unfortunately, by the time a dusty layer forms, quite a bit of soil water may have risen from the depths and obsolete. The gardener can significantly reduce spring moisture loss by frequently hoeing weeds until the top inch or two of earth is dry and powdery. This effort will probably be necessary in any case, because weeds will germinate prolifically until the surface layer is sufficiently desiccated. If it should rain hard during summer, it is wise to hoe a few times to rapidly restore the dust mulch. If hand cultivation seems very hard work, I suggest you learn to sharpen your hoe. A mulch of dry hay, grass clippings, leaves and the like, will also retard rapid surface evaporation. Gardeners think mulching prevents moisture loss better than bare earth because under mulch the soil stays damp right to the surface. However, dig down 4 to 6 inches under a dust mulch and the earth is just as damp as under hay. Soil moisture studies have proved that overall moisture loss using vegetation mulch slightly exceeds loss under a dust mulch.
By using a permanent thick mulch, without a long winter freeze, quickly breeds many slugs, earwigs and sow-bugs that cannot be maintained for more than one year and vegetable gardening becomes very difficult. Laying down a fairly thin mulch in June after the soil has warmed up and raking up what remains of the mulch early the next spring and composting it, prevents destructive insect population levels from developing. This will simultaneously reduce surface compaction by winter rains and beneficially enhance the survival and multiplication of earthworms. A thin mulch also enhances the summer germination of weed seeds without being thick enough to suppress their emergence and any mulch even a thin one, makes hoeing virtually impossible and hand weeding tedious.
Mulch has some unqualified pluses in hotter climates. Most of the organic matter in soil and consequently most of the available nitrogen is found in the surface few inches. Levels of other mineral nutrients are usually two or three times as high in the topsoil as well. However, if the surface few inches of soil becomes completely desiccated, no root activity will occur and the plants are forced to feed deeper in soil which is less fertile. Keeping the topsoil damp greatly improves the growth of some shallow-feeding species such as, lettuce and radishes. But with our climate's cool nights, most vegetables need the soil as warm as possible and the cooling effect of mulch can be as much a hindrance as a help. I've tried mulching quite a few species while dry gardening and found little or no improvement in plant growth with most. Probably, the enhancement of nutrition compensates for the harm from lowering soil temperature. Fertigation is better all around.
Windbreaks
Plants transpire more moisture when the sun shines, when temperatures are high and when the wind blows; it is just like drying laundry. Windbreaks also help the garden grow in winter by increasing temperature.
Fertilizing, Fertigating and Foliage Spraying
In our heavily leached regions, almost no soil is naturally rich, while fertilizers, manures and potent composts mainly improve the topsoil. But the water-wise gardener must get nutrition down deep, where the soil stays damp through the summer. If plants with enough remaining elbow room stop growing in summer and begin to appear gnarly, it is just as likely due to lack of nutrition as lack of water. Several things can be done to limit or prevent midsummer stunting. First, before sowing or transplanting large species like tomato, squash or big brassicas, dig out a small pit about 12 inches deep and below that blend in a handful or two of organic fertilizer. Then fill the hole back in. This double-digging process places concentrated fertility mixed 18 to 24 inches below the seeds or seedlings.
Foliage feeding is another water-wise technique that keeps plants growing through the summer. Soluble nutrients sprayed on plant leaves are rapidly taken into the vascular system. Unfortunately, dilute nutrient solutions that won't burn leaves only provoke a strong growth response for 3 to 5 days. Optimally, foliage nutrition must be applied weekly or frequently. To efficiently spray a garden larger than a few hundred square feet, I suggest buying an industrial-grade, 3-gallon backpack sprayer with a side-handle pump. The store that sells it (probably a farm supply store) will also support you with a complete assortment of inexpensive nozzles that can vary the rate of emission and the spray pattern. High-quality equipment like this outlasts many least expensive smaller sprayers designed for the consumer market and replacement parts are also available. Keep in mind that consumer merchandise is designed to be consumed; stuff made for farming is built to last.
Increasing Soil Fertility Saves Water
Does crop growth equal water use? Most people would say this statement seems likely to be true. Actually, faster-growing crops use much less soil moisture than slower-growing ones. As early as 1882, it was determined that less water is required to produce a pound of plant material when soil is fertilized than when it is not fertilized. One experiment required 1,100 pounds of water to grow 1 pound of dry matter on infertile soil, but only 575 pounds of water to produce a pound of dry matter on rich land. Perhaps the single most important thing a water-wise gardener can do is to increase the fertility of the soil, especially the subsoil. Poor plant nutrition increases the water cost of every pound of dry matter produced.
Using foliar fertilizers requires a little caution and forethought. Spinach, beet and chard leaves seem particularly sensitive to foliar spraying
(even to organic insecticides) and may be damaged by even half-strength applications. And the cabbage family coats its leaf surfaces with a waxy, moisture-retentive sealant that makes sprays bead up and run off rather than stick and be absorbed. Mixing foliar feed solutions with a little spreader/sticker, Safer's Soap, or if bugs are a problem, with a liquid organic insecticide, eliminates surface tension and allows the fertilizer to have an effect on brassicas. Sadly, in terms of nutrient balance, the poorest foliage sprays are organic. That's because it is nearly impossible to get significant quantities of phosphorus or calcium into solution using any combination of fish emulsion and seaweed or liquid kelp. The most useful possible organic foliage is 1/2 to 1 tablespoon each of fish emulsion and liquid seaweed concentrate per gallon of water.
One thing fertilizer makers find expensive to accomplish is concocting a mixture of soluble nutrients that also contains calcium, a vital plant food. If you dissolve calcium nitrate into a solution containing other soluble plant nutrients, many of them will precipitate out because few calcium compounds are soluble. Fertilize every two to four weeks is the best technique for maximizing yield while minimizing water use. I usually make my first fertilization late in June and continue periodically through early September. I use six or seven plastic 5-gallon "drip system" buckets, (see below) set one by each plant, and fill them all with a hose each time I work in the garden. Doing 12 or 14 plants each time I'm in the garden, takes no special effort to rotate through them.
To make a drip bucket, drill a 3/16-inch hole through the side of a 4-to-6-gallon plastic bucket about 1/4-inch up from the bottom or in the bottom at the edge. The empty bucket is placed so that the fertilized water drains out close to the stem of a plant. It is then filled with liquid fertilizer solution. It takes 5 to 10 minutes for 5 gallons to pass through a small opening and because of the slow flow rate, water penetrates deeply into the subsoil without wetting much of the surface. Each fertilization makes the plant grow rapidly for two to three weeks, I suspect as a result of improved nutrition than from added moisture, the strength of the fertilizer you need, depends on how much and how deep you place nutrition in the subsoil.
Resource
Water-wise vegetables / Steve Solomon / ISBN 0-912365-75-7
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 the 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 soil air. Compacted soil also acts as a mechanical barrier to root system expansion. When gardening with 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.
Evaluating Potential Rooting Ability
The best practice a water-wise gardener can do is to rent or borrow a hand-operated fence post auger to bore a 3-foot-deep hole, in addition to purchasing a short section of ordinary water pipe to extend the auger's reach another 2 or 3 feet. In soil free of stones, using an auger is more instructive than using a conventional post-hole digger or shoveling out a small pit because where soil is loose, the hole deepens rapidly. Augers also lift the materials more or less as they are stratified. If your soil is somewhat stony, 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. 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, causing erosion.
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, it is best to move to another gardening location. Of course, you can still garden quite successfully on thin soil in the conventional, irrigated manner.
Eliminating Plow-pan
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 plow-pan may function as a barrier. Though moisture will rise slowly by capillarity and recharge soil above plow-pan, 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 drought gravel that effectively stops root growth. Some of these conditions are correctable. Plow-pan is commonly encountered by homesteaders on farm soils and may be found in suburbia too, fortunately, it is the easiest obstacle to remedy. Traditionally, American croplands have been tilled with the moldboard plow. 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 compacted layer develops. Once formed, plow-pan prevents the crop from rooting into the subsoil. Since winter rains leach nutrients from the topsoil and deposit them in the subsoil, plow-pan prevents access to these nutrients and effectively impoverish the field. So wise farmers periodically use a subsoil plow to fracture the pan. On a garden-sized plot, plow-pan 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 plow-pan loosened. Once this laborious chore is accomplished the first time, deep tillage will be far easier.
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 under laid with a dense, cement-like, clayey sand subsoil extending down several feet. A thick, dense deposition like this may be called hard pan. The spading fork cannot cure this condition as simply as it can eliminate thin plow-pan. Here is one situation to remedy this issue; 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 post-hole auger, spread green manure or compost an inch or two thick and bore numerous, almost adjoining holes. 4 feet deep all over the garden.
Clayey subsoil can supply surprisingly larger amounts of moisture than the granular sandy surface might imply but only if the earth is opened deeply and becomes more accessible to root growth. Fortunately, once root development increases at greater depths, the organic matter content and accessibility of this clayey layer can be maintained through intelligent green manuring, postponing for years the need to subsoil again. Green manuring is discussed in detail shortly.
Incorporation of extraordinarily large quantities of organic matter can turn the top few inches into something that behaves a little like loam, it is quite impractical to work in humus to a depth of 4 or 5 feet. Root development will still be limited to the surface layer. Very fine clay, do not produce dry gardens. Not all clay soils are "fine clay soils," totally compacted and airless. Jori clay can be 6 to 8 feet deep and are sufficiently porous and well drained which have been used for highly productive orchard crops. Water-wise gardeners can do wonders with Jori and other similar soils to grow the best root crops.
Spotting a Likely Site
Observing the condition of wild plants can reveal a good site to garden without much irrigation. Where Himalaya or Evergreen blackberries grow 2 feet tall and produce small, dull-tasting fruit, there is not much available soil moisture. Where they grow 6 feet tall and the berries are sweet and good sized, there is deep, open soil. When the berry vines are 8 or more feet tall and the fruits are especially huge, usually there is both deep, loose soil and a higher than usual amount of fertility. Other native vegetation can also reveal a lot about soil moisture reserves.
Using Humus to Increase Soil Moisture
Maintaining topsoil humus content in the 4 to 5 percent range is vital to plant health, vital to growing more nutritious food, essential to bringing the soil into that state of easy work-ability and cooperation known as good tilt. Humus is a spongy substance capable of holding several times more available moisture than clay. There are also new synthetic, long-lasting soil amendments that hold and release even more moisture than humus. Garden books frequently recommend tilling in extraordinarily large amounts of organic matter to increase a soil's water-holding capacity in the top few inches. Humus can improve many aspects of soil but will not reduce a garden's overall need for irrigation because it is simply not practical to maintain sufficient humus depth. Rotary tilling only blends amendments into the top 6 or 7 inches of soil. Rigorous double digging by actually trenching out 12 inches and then spading up the next foot, theoretically allows one to mix in significant amounts of organic matter to nearly 24 inches. But plants can use water from far deeper than that. Let's realistically consider how much soil moisture reserves might be increased by double digging and incorporating large quantities of organic matter.
A healthy topsoil organic matter level in our climate is about 4 percent. This rapidly declines to less than 0.5 percent in the subsoil. Suppose a inch-thick layer of compost were spread and by double digging the organic matter into the content of a very sandy soil amended to 10 percent down to 2 feet. If that soil contained little clay, its water-holding ability in the top 2 feet could be doubled. By this amendment, we might add 1 inch of available moisture per foot of soil to the reserve which equals 2 extra inches of water, enough to increase the time between heavy irrigation by a week or 10 days. If the soil in question were a silty clay, it would naturally make 2 1/2 inches available per foot. A massive humus amendment would increase that to 3 1/2 inches in the top foot or two, relatively not as much benefit as in sandy soil. And I seriously doubt that many gardeners would be willing to thoroughly double dig 24 inches.
Trying to maintain organic matter levels above 10 percent is an almost self-defeating process. The higher the humus level gets, the more rapidly organic matter tends to decay. Finding or making enough well-finished compost to cover the garden several inches deep (what it takes to lift humus levels to 10 percent) is enough of a job. Double digging just as much more into the second foot is even more effort. But having to repeat that chore every year or two becomes downright discouraging. No, either your soil naturally holds enough moisture to permit dry gardening or it doesn't.
Keeping the Subsoil Open with Green Manuring
When roots decay, fresh organic matter and large long-lasting passageways can be left deep in the soil, allowing easier air movement and facilitating entry of other roots. But no cover crop that I am aware of will effectively penetrate firm plow-pan or other resistant physical obstacles. Such a barrier forces all plants to root almost exclusively in the topsoil. However, once the subsoil has been mechanically fractured the first time and if re-compaction is avoided by shunning heavy tractors and other machinery, green manure crops can maintain the openness of the subsoil. To accomplish this, correct green manure species selection is essential. Lawn grasses tend to be shallow rooting, while most regionally adapted pasture grasses can reach down about 3 feet at best. However, orchard grass (called colts foot in English farming books) will grow down 4 or more feet while leaving a massive amount of decaying organic matter in the subsoil after the sod is tilled. Sweet clover, a biennial legume, sprouts one spring then winters over to bloom the next summer, may penetrate 8 feet. Red clover, a perennial species, may thickly invade the top 5 feet. Other useful subsoil busters include densely sown Umbelliferae, such as carrots, parsley and parsnip. The chicory family produces a very large and penetrating taproots.
Though seed for wild chicory is hard to obtain, cheap varieties of endive (a semi-civilized relative) are easily available. And several pounds of your own excellent parsley or parsnip seed can be easily produced by letting about 10 row feet of overwintering roots form seed. Orchard grass and red clover can be had quite inexpensively at many farm supply stores. Sweet clover is not currently grown by our region's farmers and so can only be found by mail. Poppy seed used for cooking will often sprout. Sown densely in October, it forms a thick carpet of frilly spring greens under laid with countless massive taproots that decompose rapidly if the plants are tilled in in April before flower stalks begin to appear.
For country gardeners, the best rotations include several years of perennial grass-legume-herb mixtures to maintain the openness of the subsoil followed by a few years of vegetables and then back. Early in the spring, I broadcast a concoction I call "complete organic fertilizer", till again after the soil dries down a bit and then use a spading fork to open the subsoil before making a seedbed. The first time around, I had to break the century old plow-pan; forking compacted earth a foot deep is a lot of work. In subsequent rotations this practice becomes much easier.
For a couple of years, vegetables will grow vigorously on this new ground supported only with a complete organic fertilizer. But vegetable gardening makes humus levels decline rapidly. So every 2 years, I start a new garden on another plot and replant the old garden to green manures. I never remove vegetation during the long rebuilding under green manures, but merely till it once or twice a year and allow the organic matter content of the soil to redevelop. Consider growing vegetables in the front yard for a few years and then switching to the back yard. Having lots of space, as I do now, I keep three or four garden plots available, one in vegetables and the others restoring their organic matter content under grass.
Mulching
Gardening under a permanent thick mulch of crude organic matter is recommended and disciples a surefire way to drought-proof gardens, while eliminating virtually any need for tillage, weeding, and fertilizing. What follows in this section is addressed to gardeners who have already read glowing reports about mulching. Permanent mulching with vegetation actually does not reduce summertime moisture loss any better than mulching with dry soil, sometimes called "dust mulching." True, while the surface layer stays moist, water will steadily be wicked up by capillarity and be evaporated from the soil's surface. If frequent light sprinkling keeps the surface perpetually moist, subsoil moisture loss can occur all summer, so nu-mulched soil could eventually become desiccated many feet deep. However, capillary movement only happens when soil is damp. Once even a thin layer of soil has become quite dry, it almost completely prevents any further movement.
Unfortunately, by the time a dusty layer forms, quite a bit of soil water may have risen from the depths and obsolete. The gardener can significantly reduce spring moisture loss by frequently hoeing weeds until the top inch or two of earth is dry and powdery. This effort will probably be necessary in any case, because weeds will germinate prolifically until the surface layer is sufficiently desiccated. If it should rain hard during summer, it is wise to hoe a few times to rapidly restore the dust mulch. If hand cultivation seems very hard work, I suggest you learn to sharpen your hoe. A mulch of dry hay, grass clippings, leaves and the like, will also retard rapid surface evaporation. Gardeners think mulching prevents moisture loss better than bare earth because under mulch the soil stays damp right to the surface. However, dig down 4 to 6 inches under a dust mulch and the earth is just as damp as under hay. Soil moisture studies have proved that overall moisture loss using vegetation mulch slightly exceeds loss under a dust mulch.
By using a permanent thick mulch, without a long winter freeze, quickly breeds many slugs, earwigs and sow-bugs that cannot be maintained for more than one year and vegetable gardening becomes very difficult. Laying down a fairly thin mulch in June after the soil has warmed up and raking up what remains of the mulch early the next spring and composting it, prevents destructive insect population levels from developing. This will simultaneously reduce surface compaction by winter rains and beneficially enhance the survival and multiplication of earthworms. A thin mulch also enhances the summer germination of weed seeds without being thick enough to suppress their emergence and any mulch even a thin one, makes hoeing virtually impossible and hand weeding tedious.
Mulch has some unqualified pluses in hotter climates. Most of the organic matter in soil and consequently most of the available nitrogen is found in the surface few inches. Levels of other mineral nutrients are usually two or three times as high in the topsoil as well. However, if the surface few inches of soil becomes completely desiccated, no root activity will occur and the plants are forced to feed deeper in soil which is less fertile. Keeping the topsoil damp greatly improves the growth of some shallow-feeding species such as, lettuce and radishes. But with our climate's cool nights, most vegetables need the soil as warm as possible and the cooling effect of mulch can be as much a hindrance as a help. I've tried mulching quite a few species while dry gardening and found little or no improvement in plant growth with most. Probably, the enhancement of nutrition compensates for the harm from lowering soil temperature. Fertigation is better all around.
Windbreaks
Plants transpire more moisture when the sun shines, when temperatures are high and when the wind blows; it is just like drying laundry. Windbreaks also help the garden grow in winter by increasing temperature.
Fertilizing, Fertigating and Foliage Spraying
In our heavily leached regions, almost no soil is naturally rich, while fertilizers, manures and potent composts mainly improve the topsoil. But the water-wise gardener must get nutrition down deep, where the soil stays damp through the summer. If plants with enough remaining elbow room stop growing in summer and begin to appear gnarly, it is just as likely due to lack of nutrition as lack of water. Several things can be done to limit or prevent midsummer stunting. First, before sowing or transplanting large species like tomato, squash or big brassicas, dig out a small pit about 12 inches deep and below that blend in a handful or two of organic fertilizer. Then fill the hole back in. This double-digging process places concentrated fertility mixed 18 to 24 inches below the seeds or seedlings.
Foliage feeding is another water-wise technique that keeps plants growing through the summer. Soluble nutrients sprayed on plant leaves are rapidly taken into the vascular system. Unfortunately, dilute nutrient solutions that won't burn leaves only provoke a strong growth response for 3 to 5 days. Optimally, foliage nutrition must be applied weekly or frequently. To efficiently spray a garden larger than a few hundred square feet, I suggest buying an industrial-grade, 3-gallon backpack sprayer with a side-handle pump. The store that sells it (probably a farm supply store) will also support you with a complete assortment of inexpensive nozzles that can vary the rate of emission and the spray pattern. High-quality equipment like this outlasts many least expensive smaller sprayers designed for the consumer market and replacement parts are also available. Keep in mind that consumer merchandise is designed to be consumed; stuff made for farming is built to last.
Increasing Soil Fertility Saves Water
Does crop growth equal water use? Most people would say this statement seems likely to be true. Actually, faster-growing crops use much less soil moisture than slower-growing ones. As early as 1882, it was determined that less water is required to produce a pound of plant material when soil is fertilized than when it is not fertilized. One experiment required 1,100 pounds of water to grow 1 pound of dry matter on infertile soil, but only 575 pounds of water to produce a pound of dry matter on rich land. Perhaps the single most important thing a water-wise gardener can do is to increase the fertility of the soil, especially the subsoil. Poor plant nutrition increases the water cost of every pound of dry matter produced.
Using foliar fertilizers requires a little caution and forethought. Spinach, beet and chard leaves seem particularly sensitive to foliar spraying
(even to organic insecticides) and may be damaged by even half-strength applications. And the cabbage family coats its leaf surfaces with a waxy, moisture-retentive sealant that makes sprays bead up and run off rather than stick and be absorbed. Mixing foliar feed solutions with a little spreader/sticker, Safer's Soap, or if bugs are a problem, with a liquid organic insecticide, eliminates surface tension and allows the fertilizer to have an effect on brassicas. Sadly, in terms of nutrient balance, the poorest foliage sprays are organic. That's because it is nearly impossible to get significant quantities of phosphorus or calcium into solution using any combination of fish emulsion and seaweed or liquid kelp. The most useful possible organic foliage is 1/2 to 1 tablespoon each of fish emulsion and liquid seaweed concentrate per gallon of water.
One thing fertilizer makers find expensive to accomplish is concocting a mixture of soluble nutrients that also contains calcium, a vital plant food. If you dissolve calcium nitrate into a solution containing other soluble plant nutrients, many of them will precipitate out because few calcium compounds are soluble. Fertilize every two to four weeks is the best technique for maximizing yield while minimizing water use. I usually make my first fertilization late in June and continue periodically through early September. I use six or seven plastic 5-gallon "drip system" buckets, (see below) set one by each plant, and fill them all with a hose each time I work in the garden. Doing 12 or 14 plants each time I'm in the garden, takes no special effort to rotate through them.
To make a drip bucket, drill a 3/16-inch hole through the side of a 4-to-6-gallon plastic bucket about 1/4-inch up from the bottom or in the bottom at the edge. The empty bucket is placed so that the fertilized water drains out close to the stem of a plant. It is then filled with liquid fertilizer solution. It takes 5 to 10 minutes for 5 gallons to pass through a small opening and because of the slow flow rate, water penetrates deeply into the subsoil without wetting much of the surface. Each fertilization makes the plant grow rapidly for two to three weeks, I suspect as a result of improved nutrition than from added moisture, the strength of the fertilizer you need, depends on how much and how deep you place nutrition in the subsoil.
Resource
Water-wise vegetables / Steve Solomon / ISBN 0-912365-75-7
