Water Wise Gardening
Plants Are Water
Like all other carbon-based life forms on earth, plants conduct their chemical processes in a water solution. Every substance that plants transport is dissolved in water. When insoluble starches and oils are required for plant energy, enzymes reverts them into water-soluble sugars for movement to other locations. Even cellulose and lignin, insoluble structural materials that plants cannot convert back into soluble materials are made from molecules that once were in solution. Water is so essential that when a plant can no longer absorb as much water due to dehydration, it wilts in self-defense. The drooping leaves transpire (evaporate) less moisture because the sun glances off them. Some weeds can wilt temporarily and resume vigorous growth as soon as their water balance is restored.
Most vegetable species aren't as tough because moisture stressed vegetables may survive, but once stressed, the quality of their yield usually declines. Yet in deep, open soil, most vegetable species may be grown quite successfully with very little or no supplementary irrigation and without mulching because they're capable of being supplied the water stored in the soil.
Soil's Water-Holding Capacity
Soil is capable of holding on to water, mostly by adhesion. For example, I'm sure that at one time or another you have picked up a wet stone from a river or by the sea. A thin film of water clings to its surface. This is adhesion. The more surface area there is, the greater the amount of moisture that can be held by adhesion. If we crushed that stone into dust, we would greatly increase the amount of water that could adhere to the original material. Clay particles are so small that it's ability to hold water is not as great as its surface area would indicate. This direct relationship between particle size, surface area and water-holding capacity is so essential to understanding plant growth, that the surface areas presented by various sizes of soil particles have been mathematically calculated. Soils are not composed of a single size of particle. If the mix is primarily sand, we call it a sandy soil. If the mix is primarily clay, we call it a clay soil. If the soil is a relatively equal mix of all three, containing no more than 35 percent clay, we call it loam.
Adhering water films can vary greatly in 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 water flows deeper into the soil. When water films are relatively thick, the soil feels wet and plant roots can easily absorb moisture. "Field capacity" is the term describing soil particles holding water against the force of gravity. The other extreme, the thinner the water films, the tightly they adhere and the drier the earth feels. At some degree of desiccation, roots are no longer forceful enough to draw on soil moisture as fast as the plants transpire. This condition is called the "wilting point." The term "available moisture" refers to the difference between field capacity and the amount of moisture left after plants have died.
Clayey soil can provide plants with three times as much available water as sand, six times as much as a very coarse sandy soil. It might seem logical to conclude that a clayey garden would be the most drought resistant, but there's more... For some crops, deep sandy loams can provide just about as much usable moisture as clays. Sandy soils usually allows extensive root development, so a plant with a naturally aggressive and deep root system may be able to occupy a much larger volume of sandy loam, ultimately coming up with more moisture than it could obtain from a heavy, airless clay. 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 irrigation. This can only be discovered by trial.
How Soil Loses Water
From early April, well into September, the hot sun will beat down on this bare plot. Our summer rains generally come in insignificant installments, do not penetrate deeply and the rain quickly evaporates from the surface a few inches without recharging deeper layers. Most readers would reason that a soil moisture measurement taken 6 inches down on September1, should show very little water left. 12 inches below seems like it should be just as dry in fact, most gardeners would expect there would be very little water found in the soil until we reached down quite a few feet, but that is not what happens! The hot sun does dry out the surface inches, but if we dig below 6 inches, 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 soil that continues to dehydrates when bare, are the heavy clays that form deep cracks. These ever-deepening openings allow atmospheric air to freely evaporate additional moisture. If these cracks are filled with dust by surface cultivation, even this soil type ceases to lose water. Soil functions by holding available water in storage. In our climate, soil is inevitably charged to capacity by winter rains, but the hot sun and wind working directly on soil, do not remove much water. Plants desiccate soil to the ultimate depth and lateral extent of their rooting ability. The size of vegetable root systems is greater and the amount of moisture potentially available to sustain vegetable growth is greater than most gardeners think.
Rain and irrigation are not the only way 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 surface. This adhesion tends to lift the edges of the column of water. As the tube's diameter become smaller, the amount of lift becomes greater and soil particles form interconnected pores, allow an inefficient capillary flow recharging the 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 needed by crops. But conventional agriculture focuses on maximized yields, through high plant densities. Capillarity is too slow to support dense crop where numerous root systems are competing, but when a single plant can without any competition occupy a large enough area, then moisture replacement by capillarity becomes significant.
How Plants Obtain Water
Most gardeners know that plants acquire water and minerals through their root systems, 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. The soil particles become a direct contact with these tips and the hairs dry out while the older roots thicken. These roots develop a bark and most of the absorbent hairs slough off. This rotation creates a passive conductive and supportive tissue, a survival adaptation, because the slow capillary movement of soil moisture fails to replace what the plant effectively used. The plant is better off to aggressively seek new water in unoccupied soil, than to wait for the 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 rye grass plant allocated only 1 cubic foot of soil, produced nearly 3 miles of new roots and root hairs every day. (Rye grasses 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 mathematical estimates agree with my observations in the garden and with my experiences raising transplants in liners and pots.
Lowered Plant Density: The Key to Water-Wise Gardening
Adding a little fertilizer (compost), 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 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 irrigation.
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're a backyard gardener and the municipality temporarily restricts usage. What to do? First, if at all possible before the restrictions take effect, water heavily and long to ensure there is maximum subsoil moisture. Then eliminate all newly started inter-plantings 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 have a 4-foot-wide intensive bed holding seven rows of broccoli on 12 inch centers. Remove at least every other row and every other plant in the three or four remaining rows. 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 (the application of soil amendments or other water soluble products through an irrigation system).
How long 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. As recommended by most advocates of raised-bed gardening in close planting, the crop canopy is established early and is maintained by successive inter-plantings and water losses will greatly exceed this rate. Many vegetable species become mildly stressed when soil moisture has dropped by half from capacity to the wilting point. In closely planted beds, a crop can wilt and dry out without irrigation in a matter of days, but if that same crop were densely planted, it might grow a few weeks without irrigation. If that crop is planted 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 a slower rate of soil moisture depletion. On deep open soil, the crop might yield a respectable amount without needing any irrigation at all.
"Would lowering plant density as much as this book suggests equally lower the yield of the plot? Surprisingly, the amount harvested does not drop by proportion. In most cases having a plant density one-eighth of that recommended by intensive gardening, will result in a yield about half as great as the closely planted raised beds."
Resource
Water-wise vegetables / Steve Solomon / ISBN 0-912365-75-7
Like all other carbon-based life forms on earth, plants conduct their chemical processes in a water solution. Every substance that plants transport is dissolved in water. When insoluble starches and oils are required for plant energy, enzymes reverts them into water-soluble sugars for movement to other locations. Even cellulose and lignin, insoluble structural materials that plants cannot convert back into soluble materials are made from molecules that once were in solution. Water is so essential that when a plant can no longer absorb as much water due to dehydration, it wilts in self-defense. The drooping leaves transpire (evaporate) less moisture because the sun glances off them. Some weeds can wilt temporarily and resume vigorous growth as soon as their water balance is restored.
Most vegetable species aren't as tough because moisture stressed vegetables may survive, but once stressed, the quality of their yield usually declines. Yet in deep, open soil, most vegetable species may be grown quite successfully with very little or no supplementary irrigation and without mulching because they're capable of being supplied the water stored in the soil.
Soil's Water-Holding Capacity
Soil is capable of holding on to water, mostly by adhesion. For example, I'm sure that at one time or another you have picked up a wet stone from a river or by the sea. A thin film of water clings to its surface. This is adhesion. The more surface area there is, the greater the amount of moisture that can be held by adhesion. If we crushed that stone into dust, we would greatly increase the amount of water that could adhere to the original material. Clay particles are so small that it's ability to hold water is not as great as its surface area would indicate. This direct relationship between particle size, surface area and water-holding capacity is so essential to understanding plant growth, that the surface areas presented by various sizes of soil particles have been mathematically calculated. Soils are not composed of a single size of particle. If the mix is primarily sand, we call it a sandy soil. If the mix is primarily clay, we call it a clay soil. If the soil is a relatively equal mix of all three, containing no more than 35 percent clay, we call it loam.
Adhering water films can vary greatly in 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 water flows deeper into the soil. When water films are relatively thick, the soil feels wet and plant roots can easily absorb moisture. "Field capacity" is the term describing soil particles holding water against the force of gravity. The other extreme, the thinner the water films, the tightly they adhere and the drier the earth feels. At some degree of desiccation, roots are no longer forceful enough to draw on soil moisture as fast as the plants transpire. This condition is called the "wilting point." The term "available moisture" refers to the difference between field capacity and the amount of moisture left after plants have died.
Clayey soil can provide plants with three times as much available water as sand, six times as much as a very coarse sandy soil. It might seem logical to conclude that a clayey garden would be the most drought resistant, but there's more... For some crops, deep sandy loams can provide just about as much usable moisture as clays. Sandy soils usually allows extensive root development, so a plant with a naturally aggressive and deep root system may be able to occupy a much larger volume of sandy loam, ultimately coming up with more moisture than it could obtain from a heavy, airless clay. 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 irrigation. This can only be discovered by trial.
How Soil Loses Water
From early April, well into September, the hot sun will beat down on this bare plot. Our summer rains generally come in insignificant installments, do not penetrate deeply and the rain quickly evaporates from the surface a few inches without recharging deeper layers. Most readers would reason that a soil moisture measurement taken 6 inches down on September1, should show very little water left. 12 inches below seems like it should be just as dry in fact, most gardeners would expect there would be very little water found in the soil until we reached down quite a few feet, but that is not what happens! The hot sun does dry out the surface inches, but if we dig below 6 inches, 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 soil that continues to dehydrates when bare, are the heavy clays that form deep cracks. These ever-deepening openings allow atmospheric air to freely evaporate additional moisture. If these cracks are filled with dust by surface cultivation, even this soil type ceases to lose water. Soil functions by holding available water in storage. In our climate, soil is inevitably charged to capacity by winter rains, but the hot sun and wind working directly on soil, do not remove much water. Plants desiccate soil to the ultimate depth and lateral extent of their rooting ability. The size of vegetable root systems is greater and the amount of moisture potentially available to sustain vegetable growth is greater than most gardeners think.
Rain and irrigation are not the only way 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 surface. This adhesion tends to lift the edges of the column of water. As the tube's diameter become smaller, the amount of lift becomes greater and soil particles form interconnected pores, allow an inefficient capillary flow recharging the 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 needed by crops. But conventional agriculture focuses on maximized yields, through high plant densities. Capillarity is too slow to support dense crop where numerous root systems are competing, but when a single plant can without any competition occupy a large enough area, then moisture replacement by capillarity becomes significant.
How Plants Obtain Water
Most gardeners know that plants acquire water and minerals through their root systems, 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. The soil particles become a direct contact with these tips and the hairs dry out while the older roots thicken. These roots develop a bark and most of the absorbent hairs slough off. This rotation creates a passive conductive and supportive tissue, a survival adaptation, because the slow capillary movement of soil moisture fails to replace what the plant effectively used. The plant is better off to aggressively seek new water in unoccupied soil, than to wait for the 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 rye grass plant allocated only 1 cubic foot of soil, produced nearly 3 miles of new roots and root hairs every day. (Rye grasses 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 mathematical estimates agree with my observations in the garden and with my experiences raising transplants in liners and pots.
Lowered Plant Density: The Key to Water-Wise Gardening
Adding a little fertilizer (compost), 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 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 irrigation.
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're a backyard gardener and the municipality temporarily restricts usage. What to do? First, if at all possible before the restrictions take effect, water heavily and long to ensure there is maximum subsoil moisture. Then eliminate all newly started inter-plantings 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 have a 4-foot-wide intensive bed holding seven rows of broccoli on 12 inch centers. Remove at least every other row and every other plant in the three or four remaining rows. 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 (the application of soil amendments or other water soluble products through an irrigation system).
How long 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. As recommended by most advocates of raised-bed gardening in close planting, the crop canopy is established early and is maintained by successive inter-plantings and water losses will greatly exceed this rate. Many vegetable species become mildly stressed when soil moisture has dropped by half from capacity to the wilting point. In closely planted beds, a crop can wilt and dry out without irrigation in a matter of days, but if that same crop were densely planted, it might grow a few weeks without irrigation. If that crop is planted 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 a slower rate of soil moisture depletion. On deep open soil, the crop might yield a respectable amount without needing any irrigation at all.
"Would lowering plant density as much as this book suggests equally lower the yield of the plot? Surprisingly, the amount harvested does not drop by proportion. In most cases having a plant density one-eighth of that recommended by intensive gardening, will result in a yield about half as great as the closely planted raised beds."
Resource
Water-wise vegetables / Steve Solomon / ISBN 0-912365-75-7
