Plants and water
You don’t need to be a gardener to understand that water is the life blood of the garden (or the world for that matter). With water you have a luxuriance of green growth all summer, without it you have a brown desert. In dry areas, making sure that your plants have all the water they need is one of your primary summer tasks. In such places it determines how much you can grow.
Why plants need water
Plants are mostly composed of water (80 – 95%) and it is an essential part of every aspect of the plants life. It is needed for photosynthesis, growth, keeping cells (and the plant itself) rigid and the transport and absorption of nutrients. Less than 5% of the water taken up by the roots is actually incorporated into the plant, which is why it takes 200 – 500 units of water to produce one unit of dry plant material. The rest of the water is transpired out into the air. It’s been estimated that an acre of corn may use 3-4000 gallons of water per day, which helps to explain how 10% of the water in the atmosphere comes from plants.
This “lost” water actually has an essential role in plant growth. It is a part of the transpiration process, whereby the plant moves water, nutrients, wastes and manufactured foods around. It also helps to keep it cool and turgid (rigid). See Plant growth for more on this.
Plants also need the water that remains in the soil and makes up the soil solution. This is the route by which nutrient ions are transported around the soil and how they end up in plant roots. Water is also essential for the soil organisms that feed plants and is a factor in both mechanical and chemical weathering of soil, which liberates plant nutrients (see Soil for more on this).
Effects of too little water
In many areas water is the most common factor limiting plant growth (even in relatively humid climates). If a plant can’t get enough water it closes its stomata to prevent loss through transpiration. This prevents the plant taking in carbon dioxide and without this photosynthesis can’t take place. When this happens the plant must live off of its stored food reserves, so actually consumes food, rather than producing it. Consequently growth slows down, or stops completely. Of course this then affects food storage, flowering and subsequent fruiting (flowers and fruits often drop).
Water stress doesn’t only slow growth and reduce yield. It may also cause plants to bolt, or affect the quality of the edible part, making them fibrous (their cells get thicker and their stems tougher), pungent or bitter. One of the reasons the French market gardeners grew such large quantities of high quality vegetables was that they ensured their crops got all the water they needed at all times.
Effects of too much water
Too much water can delay flowering and fruiting and causes plants to make succulent growth that is attractive to disease and insect pests. This succulent growth contains a lot of water, so is less nutritious and less flavorful (the latter may not be a bad thing with strongly flavored crops). If the soil becomes waterlogged there isn’t enough air in the soil and roots may die.
The spacing of plants in the bed has an effect on irrigation practices. Obviously the more plants in a given area, the more water they will extract from the soil. If you fill a bed with tomato plants 18˝ apart you may have to water every few days, if you plant them 6 feet apart you may never have to water them at all (there may already be enough water stored in the soil).
Individual plant requirements
Crop plants vary greatly in their need for water. Celery originally grew in marshes and like lots of water, while Watermelon comes from a semi- desert and can make do with very little. Deep-rooted crops generally need irrigating less frequently than shallow rooted crops. Some plants need water constantly, others have a critical stage of growth when extra water can greatly increase the final harvest (in some cases this can double the final yield).
The stage of growth affects how often you need to water. Root growth is rapid in young vigorously growing plants (as much as an inch a day), but it slows considerably as a plant matures and starts to flower and set fruit.
Young plants: These use less water than larger plants,but need it more frequently because their roots are small and their sparse foliage doesn’t shade the soil very well (so evaporation is greater).
Older plants: Use more water, but have deeper roots. They also have larger leaves that form their own moist shady microclimate which reduces evaporation.
Leaf crops: These need a steady supply of moistureif they are to produce heavy yields of tasty succulent leaves. Water stress can have a dramatic effect on quality, plants may get fibrous, develop unpleasant flavors, or bolt prematurely. Keep the soil constantly moist for these crops.
Root crops: These have their greatest need for water when the roots are sizing up, but should always receive a steady supply. Water stress may cause them to bolt or develop unpleasant flavors. Irregular watering, alternately too much and too little, can stimulate them to put on little spurts of growth, which can cause them to grow unevenly and split.
Fruit and seed bearing crops: Fruit crops also respond to regular watering with heavy production of lush foliage, but this isn’t necessarily a good thing, as it often delays flowering. Conversely the “stress” of infrequent watering can actually encourage flowering.
When the fruit is setting and sizing up, they need extra water, so make sure you give them enough when the flowers and fruits appear (or they may drop off). They are vulnerable to water stress at this time, not only because water is needed to size up the fruit, but also because root growth slows down at this stage. Water stress at this time can reduce the final yield considerably (though it may make the fruit sweeter).
Bulb crops: These need the most water when young, less when they are maturing and none at all when they are curing.
Soil and water
Water enters the soil as rainfall and leaves via plant roots, evaporation or gravity (it drains away). Water isn’t static in the soil, it can move downward by gravity and upwards and sideways by capillary action (which is actually the attraction of molecules to each other). About 25% of the volume of the average soil is composed of water, but this fluctuates with the weather.
The rate at which water enters the soil is known as its infiltration capacity and is largely dependent on the soil structure and texture (especially of the surface layer). If the infiltration capacity is low, water tends to puddle on the surface, rather than soaking in. This may then run off downhill (potentially causing erosion) and disappear. This wastes water that could have helped us to grow plants, so we must do everything we can to ensure that the soil absorbs all the rainfall that lands upon it.
How water enters the soil
When a drop of water enters the soil it connects with the nearest soil particles and is incorporated into the film of water that surrounds them. This water is held on to the particles by the force of adhesion, which allows the film to build up around the particle (to a thickness of around 0.06 mm, to get specific). When each particle has as much water as it can hold, the excess water fills up the spaces between the particles (the micropores). All of this water is known as capillary water, because it can move around the soil by capillary action.
Any water entering the soil after the micropores are full, goes in to the spaces between the aggregates (the macropores). This water is known as gravitational water, for the obvious reason that it moves by the force of gravity and isn’t physically held by the soil at all.
If more water enters the soil it penetrates deeper, by force of gravity, connecting with all available soil particles and filling up all available pore spaces, until it hits the water table (where the soil is permanently saturated).
If so much water enters the soil that all available pore spaces are filled right down to the water table, the soil is said to be at saturation point. Any more water falling on the soil after this point can’t soak into the saturated soil and so either stands on the surface or runs off.
When water stops entering the soil, the gravitational water in the macropores slowly drains away and is replaced by air. When all of this water has drained off (which may take days), the soil particles and micropores are left holding as much capillary water as they are physically able (the 0.06 mm thick film) and the soil is said to be at field capacity. This is the ideal water level for most plants, with the water forming a continuous film from surface to water table, but with the larger macropores still filled with air. The amount of water the soil can hold in this state is known as its moisture holding capacity.
How water moves in soil
The soil near the surface dries out most rapidly, as its moisture is removed by evaporation and by plant roots. This is replaced by capillary water, which moves through the soil pores from wetter areas. This happens because the film around a soil particle gets thinner as it dries out, causing its surface tension to increase, which pulls water from wetter particles. In this way water may move in any direction, from wetter areas to drier ones, until all are equally moist. In practice they never become equally moist, because water continues to be lost (by evaporation, or to plant roots) as fast as it can be replaced.
If water is continually removed from the soil without replenishment, it is eventually depleted to the point where more water can’t move through the soil quickly enough to replace that consumed. Capillary movement is related to tension and decreases as the soil film dries out, until it essentially ceases as the soil gets close to wilting point. Once the continuous film of soil moisture is broken, the plant can no longer get enough water to replace that transpired, so it shuts down its stomata, wilts and stops growing. If there is still moisture deep in the soil this reduction in demand may enable more water to move up into the soil around the root zone. If this doesn’t happen the plant may eventually die from lack of water.
The capillary movement of water is too slow and limited to provide plants with sufficient water for good growth, so in dry soil their roots must search out water continuously. This is actually the main incentive for roots to grow into new soil (they can only grow into moist soil). This root growth, combined with capillary movement, enables plants to get most of the available water in the soil.
All of the water in the soil isn’t of equal value for growing crops. As plants suck up water, the film of moisture around each soil particle gets thinner and it takes more energy to get it. By the time the wilting point is reached, this film is so thin that plants can’t extract any more water from it.
The remaining water is held tightly to the soil particles by electrical forces (at 15 times atmospheric pressure) and is known as “hygroscopic water”. Even though this water can’t be used by plants, it is still important because water loving soil organisms live in it and can survive there even when the soil is very dry By determining the amount of soil suction with a tensiometer, we can determine how much water is available for plants.
The soil water that plants can use is called “available water”, or the soil reservoir. That which can’t be used is called (appropriately enough) “unavailable water” and includes gravitational water and hygroscopic water.
It is not good to let the soil lose more than half of its available water, as the plants then have to work harder to meet their needs. When a plant has to expend energy just to obtain water, it has less left over for growth, which then slows down. It is your job as a gardener to give your crops all the water they need, so they can grow as efficiently as possible.
Factors affecting the loss of water from the soil
Evaporation: On a hot day a bare soil can lose a lot of water through evaporation. Cultivation increases this loss by increasing the surface area of the soil and so exposing more soil moisture to the air. Sandy soils tend to lose less water from evaporation than clay soils, because they tend to dry out on the surface creating a dust mulch effect (and there is less capillary action to bring more moisture to the surface).
Wind increases evaporation and is another reason why the garden should be sheltered from strong winds.
A layer of mulch stops water loss by evaporation almost completely. Not only does it prevent sunlight hitting the soil, it also creates a humid layer under the mulch.
Transpiration: The amount of water a plant losesthrough transpiration is determined by temperature, humidity and sun intensity. On a hot sunny summer day a plant may use eight times as much water as it does on a cool cloudy spring day. Strong wind can increase transpiration losses even further.
Soil texture and water
The ability of the soil to handle water is largely determined by its texture, structure and organic matter content.
Sand: These soils may hold about 10% water at field capacity. The large pore spaces enable water to enter and leave easily, so it fills up or dries out fairly quickly. Such soils don’t have a large reservoir of water, so must be watered frequently to keep crops supplied in dry weather. Offsetting this to some extent is the fact that roots can penetrate more easily and deeply into sandy soils in search of water. Some sandy soils are hard to re-wet if they dry out, water simply rolls off them.
Clay: These soils can hold as much as 40% water atfield capacity, but a much greater proportion of this is unavailable when compared to a sandy soil. When a clay soil dries out it shrinks and cracks and these cracks can damage roots and make it hard to re-wet the soil (water drains down the cracks before it can soak into the soil). The infiltration rate in a poor clay soil is low, because water enters the small pores so slowly, it often puddles on the surface and runs off. Water also drains slowly, which can be a problem in wet climates. Clay soils have a large soil reservoir, so don’t need watering as frequently as sandy ones.
Silt: These soils have characteristics somewherebetween those of sand and clay.
A well-structured soil has a large volume of pore space, with lots of macropores that allow water to percolate freely and micropores that hold water.
Soil organic matter
Organic matter actually acts like a sponge, absorbing and holding excess water and only releasing it when the film of moisture around the soil particles is depleted. This greatly increases the water holding capacity of light sandy soils.
Organic matter also improves the crumb structure of a soil, which increases the amount of pore space. This increases aeration and improves drainage, making it easier for roots to penetrate deeply in search of water. This is particularly valuable for heavy clay soil.
A slope drains better than flat land and so dries out faster (especially in light soils). South and west facing slopes dry out most rapidly because of their increased solar gain. Land at the bottom of a slope gets extra water as it drains down from above.
Climate affects how much water plants use and determines whether you can get away with occasional hand watering, or whether you must irrigate your plants from seedling to harvest. In hot arid areas you would expect to have to irrigate plants regularly, but even where summer rainfall is regular, loss by evaporation and transpiration often exceeds rainfall. In such cases some irrigation will be necessary to achieve maximum yields. This is especially true in intensive beds, as the fast growing, closely spaced, plants need a lot of water.
Climate and soil water
There is a considerable seasonal variation in the amount of water in the soil.
The most obvious reason for such fluctuation is rainfall. In many areas this is seasonal, some seasons being much wetter than others. Where I live on the West coast, rain is abundant in winter, but pretty much non-existent in summer.
Another reason is temperature. In the cool weather of spring and autumn there may be very little evaporation from the soil and less transpiration from plants. This means that the soil dries out much more slowly than in summer. Often there is no need to water at all at these times, especially in spring when the soil is full of water from winter rains.
Always consider the weather before watering, never water routinely. If possible don’t waste water by irrigating before rain (somehow a particularly conscientious watering always seems to bring rain.) A rain gauge can help you to determine the quantity of both natural rainfall and overhead irrigation.
Water holding capacity of soils
The amount of water a soil can hold is determined by its texture and organic matter content. Soil depth is also significant, in that a deep soil has a much greater storage volume than a shallow one.
|Water in the top 12˝ of soil
|Permanent wilting point
Water sources and quality
It’s worth thinking about the purity of your water source. In arid areas dissolved salts can build up in the soil and become quite toxic. Hard water contains calcium and magnesium and may raise the pH. In some areas surface water is quite acid (from acid rain) and will lower the soil pH. If you have any suspicions you can have your water tested (it may be worth having an agricultural water test done when having your soil tested).
The best water for plants is rainwater, as it contains small quantities of nitrogen, sulfur and other nutrients. On average half of all rainfall falling on the ground runs off and is lost. You should do everything you can to ensure that the soil retains as much rainfall as possible.
If you have rain barrels underneath all of your downspouts, you can collect a surprising amount of water from your roof. See below for more on Rainwater collection.
Probably the commonest source of water for gardeners is the utility company. City water doesn’t contain many nutrients and often contains a lot of chlorine (and sometimes fluoride) which plants and soil organisms don’t like (I once tried to sprout Alfalfa seed in heavily chlorinated water and it kept rotting). If you are watering seedlings you could leave chlorinated water in open barrels for a day or so, to allow the chlorine to dissipate. Don’t use water that has been through a chemical water softener, as it contains a lot of salt.
City water is becoming expensive in many areas and increasingly unreliable due to droughts and hose pipe bans. One advantage is that it has good pressure for overhead irrigation.
Many suburban and rural gardeners get their water from wells. This is often superior to city water, but sometimes contains large amounts of dissolved salts. An increasing number of wells are contaminated from various industrial and farm sources, or from lawn chemicals.
In some situations you may be able to get water from clean rivers, lakes, or streams. This is usually of good quality, though you should be sure of your legal position before using it.
Gray water is any water that has been used in the house, but doesn’t contain sewage (which is known as black water). Actually it may contain very dilute sewage, as well as bacteria, viruses and a variety of chemicals so it isn’t completely harmless. The average house uses 100 – 200 gallons of water a day, which could be all of the water you need for your garden. This water is already piped, and just requires a little modification of your plumbing (and some of your more wasteful habits) Logic says that if you can re-use some of this water you should, though it may be illegal to do so in many places, due to health code restrictions.
In wet climates using untreated gray water isn’t really worthwhile (except in times of drought). In dry climates it doesn’t make sense not to use it. In an ideal world you would use the gray water for flushing toilets and would use the fresh water thus saved for irrigation. This isn’t always practical though
Gray water should be used carefully as it does contain some things plants don’t like. Use on perennials and shrubs rather than annuals, keep it off foliage and rotate its use around the garden (don’t put it all in one place). Use fresh water in between the gray water irrigations.
The cleanest gray water is that wasted while waiting for the hot water tap to get hot. The next cleanest comes from the shower and bathtub, then the washing machine and finally from the dishwasher and kitchen sink (the dirtiest). The latter is rich in plant nutrients, but also contains grease and oils and lots of food particles, so potentially causes a lot more problems. If you are adverse to disgusting things you should forget about using kitchen sink water. Instead try washing your vegetables outdoors in the garden sink. This water can go directly onto the garden and is very clean.
If you intend to use gray water for irrigation you should alter your washing habits to reduce its toxicity. Be careful which soaps, shampoos and detergents you use, no boron, high sodium, bleach (especially), water softeners, perfumes or lanolin.
It is important that gray water is used immediately and isn’t stored for any length of time. Bacteria will start to grow after a few hours and it will soon start to stink.
A good way to use gray water is to irrigate compost crops, such as reeds or comfrey, grown in their own special bed. It can also be used for willows or bamboo. If you get really ambitious you could create your own grey water treatment marsh to purify the gray water before use.
Where I live we get 3 – 8 feet of rain in winter and none at all in summer. In an ideal world we could store some of that winter rain for use in summer.
Infiltration: In-soil storage is the cheapest and simplest way to store significant amounts of water. Your first priority should be to ensure that any rain that falls on your soil soaks in, rather than running off into drains. The simplest way to do this is by ensuring that the ground is covered with vegetation, so water slowly soaks into the ground. On steeper slopes you may have to create ditches, or swales, to intercept fast flowing runoff and hold it up long enough so that it can soak in. You could also terrace the land so water doesn’t run away.
Of course water that is stored in the soil isn’t as convenient or versatile as water stored in a tank or pond (from which it can be directed to where you need it).
Probably the cheapest way to store water is in a pond, Runoff from roads, paths and roofs could be channeled into ponds (lined with a rubber pond liner), for later use for irrigation.
Most houses already come with a ready made rainwater collection system, the roof. If you have gutters (and you should) all you have to do is direct the water from the downspouts to some kind of storage container. The water can even go uphill somewhat, so long as the tank is lower than the gutters themselves.
To estimate how much rain you are likely to collect from your roof, you must first determine the square footage of the footprint of your roof (looking down directly from above, not its actual surface area). This is usually a little more than the square footage of the upper floor of your house. Multiply this number by the average yearly rainfall in inches. Divide this number by 12 – get the number of cubic feet of water. Then multiply this by 7.5 – get the number of gallons.
For example, my roof is approximately 800 square feet and we normally get about 60 inches of rain per year. This works out to 4800, which divided by 12 equals 4000. Multiply this by 7.5 and you get 30,000 gallons of water. If only storing it were as easy as calculating it!
In areas with year round rainfall, storage doesn’t present much of a problem. All you need to do is store enough water from recent rains to see you through the rainless stretches. A couple of 50 gallon barrels might be enough, or you might use an above-ground pool if you have lots of plants, or long dry spells.
In areas with no rain for long periods you need large capacity storage. The cheapest would be an above-ground pool or a hole in the ground lined with plastic (or pond liner). A much more expensive (but durable) option would be a plastic water tank (or several).
Observation of soil and plants
The essence of good watering is making sure the plants get all the water they need, exactly when they need it and no more. If you irrigate a plant that already has an adequate supply, it won’t have any beneficial effect and you won’t be using your water supply or time efficiently. Conversely if you don’t water until plants wilt, their growth will be slowed (or they may even bolt), so again you won’t be using your water supply efficiently.
The ideal time to water is just as the easily available water starts to run low, but before the plant has to start working harder to meet its needs (plants start to wilt much more readily at this time). Generally when half of the available water is gone from the soil you should irrigate. You have to watch the soil and plants carefully to know exactly when this stage is reached.
Signs of water stress
You will soon learn how to tell when plants are stressed merely by their appearance. Serious water stress is pretty obvious because the plants wilt, but plants begin to show subtle signs of stress long before lack of water forces them to shut down completely. As water gets harder to obtain, plants begin to lose the sheen on their leaves. As their stress increases they may sag slightly instead of standing rigidly upright and their leaf edges may start to curl. In extreme cases the growing tips and leaves go completely limp and if this continues for long they die and go dry and crisp.
Wilting isn’t always a sign that the soil is dry, in extremely hot and sunny weather plants sometimes wilt intentionally to reduce water loss. However they recover as soon as the temperature drops. Some plants are much more prone to wilting than others, especially those with thin, or big leaves, such as Rhubarb and the Cucurbits. Check your plants for water stress in the cool of early morning or evening. If they are wilting at this time, they are seriously stressed and aren’t getting enough water (either the soil is dry, they are diseased, or they have root damage).
Checking soil moisture
Check the moisture level of the soil by digging down to the depth of your index finger (about 4˝) and picking up a small amount of soil. If the soil is so dry it won’t squeeze into a ball, it probably needs water (remember that clay forms a ball more easily than sand). Don’t forget to fill up the hole again afterward.
|Canary plants A canary plant is a plant that is particularly sensitive to lack of moisture and can warn you when the soil in your garden is getting dry (if you use your imagination this is somewhat like the canary in the coal mine warning of poisonous gases – hence the name). The sunflower is ideal for this, as it is one of the first plants to show signs of water stress. If you plant some sunflowers around your garden, you will have a handy indicator of when your soil is getting low on moisture. Whenever your sunflowers start to wilt, it is time to think about watering.
Time of day to water
There is some controversy as to the best time of day to irrigate a garden. Some gardeners believe early morning is best, because the water has a chance to soak into the soil before the sun gets warm and is immediately available to the plants when they need it most (which might have some relevance in sandy soils). They say evening watering encourages pests and disease by leaving the soil moist all night. If you have serious disease problems you may be best sticking to morning watering. Morning watering is also best in cool climates, as the soil can warm up quickly in the morning sun.
Other gardeners prefer to water in the early evening, after the hottest part of the day, but early enough to allow the plants and soil surface to dry out before dark. At this time the soil is warm, so the water warms up quicker. Also the water has all night to soak into the soil, so there is less loss from evaporation than with morning watering. For this reason evening watering particularly lends itself to arid areas. Advocates of evening watering remind us that plants don’t shut down completely at night, they still use water.
Most gardeners avoid watering in the middle of the day in hot weather, because any water on leaves, or the soil surface, will evaporate before it can do any good. This is particularly true when using overhead watering. It is often said that in strong sunlight water droplets can burn the leaves by magnifying the suns rays, but I have never seen this actually happen.
Unless there is some overriding reason for using only evening or morning watering most people water when it is convenient, rather than to some schedule. Drip irrigation can be used at any time of day or night, but I would always avoid the hottest part of the day.
Any soil crust should be broken up before watering, as it can impede infiltration (see Soil Capping below). If your soil doesn’t absorb water easily, you can increase absorption by putting shallow holes in the bed with a fork, or dibbing larger holes between plants with a dibber (these tricks are particularly useful when hand watering). In some cases shallow furrows made along the contour with a rake will also work. You could also put a lip around the edge of the bed to increase absorption (or simply have a flat bed).
How much water is enough?
How much water to apply depends on many factors, climate, plants, soil type, plant spacing. One rule of thumb says you should give your plants 1˝ of water per week in summer and about ½˝ in spring and fall (but of course it all really depends on temperature and humidity). This works out to roughly ⅔ gallon per square foot, 66 gallons per 100 square feet, or 28,000 gallons per acre and should penetrate 6 – 12˝ into the ground. By observing the soil and plants you will soon know if it is too much or too little.
Intensively planted beds use a lot of water because they are producing a lot of biomass in a short time; there is a lot of growth going on. Fortunately these beds can store a lot of water because of their high organic matter content.
In rainy climates a rain gauge (or other container) is useful to monitor how much rain has fallen, so you can decide when your crops need additional irrigation.
Soil has only a limited storage capacity, so you should only apply as much water as is needed to get it back up to field capacity. If the soil is very dry this can be quite a lot. I have watered a clay soil for eight hours and only got about four or five inches of penetration. If you add more than this, not only will it be wasted, but the excess may actually damage the soil.
Over-watering: This encourages shallow rooting, making plants more vulnerable to drought and nutrient deficiencies. It may also leach away soluble nutrients and damage soil structure (especially on poor soils).
Of course over-watering wastes water and in dry areas you simply can’t afford this. Even in humid areas water shortages are getting more frequent and water is getting more expensive. Over-watering also involves unnecessary work, as it takes time and effort to put water into the soil.
Irregular watering: Too much water following too little water may cause fruit such as tomatoes to split. It can also caused splitting in iceberg lettuce, cabbage and Chinese cabbage .
|Deep watering or shallow watering There are two schools of thought on watering, the deep infrequent waterers and the frequent shallow waterers. Initially your soil may determine which technique you use, but as the soil improves you can choose which one you prefer (climate and crop may also affect your choice.) Deep watering When deep watering you fill the soil with as much water as it can hold (no more or you will waste water) and don’t water again until the top few inches of soil have dried out. This forces your crops to send down deep roots to search for water and such plants are less susceptible to drought. Deep watering has the advantage that it is less time consuming than shallow watering, but you have to careful to not let the soil dry out too much, It works best on deep, moisture retentive soils, . Shallow watering Shallow watering means you water every day, so the soil is always at field capacity. This is probably the best strategy for watering light soils that can’t hold a lot of water, though it will work on any soil. You might think this method contradicts the advice I gave above about watering deeply, but it really doesn’t because you never allow the soil to dry out. This method may be better for the beginner, as there is more margin for error – if you forget to water for a day or two there is no harm done. If you are going to be hand watering, then you are probably going to be shallow watering simply because of the time it takes to water deeply. Shallow watering may be particularly appealing to obsessive compulsive people who like doing stuff, as it gives them something to do every day. A variation on shallow watering in hot dry climates is to allow the subsoil to dry out. So long as you water regularly and don’t allow the topsoil to dry out your plants will do okay.
How to apply water
Water should be applied to the soil only as fast as it can soak in. This means it should fall on the bed gently, approximating a light rain shower, but not so fine that water is lost in clouds of mist. If you apply water faster than it can be absorbed it will puddle and the surface structure may break down. When this dries out a crust will form, which further impedes water (and air) infiltration (this is known as capping). Of course if water runs off of the beds it will be wasted (it may also take soil with it).
When the soil surface reaches saturation point it starts to glisten and as soon as this happens (or just before) you should move the water source momentarily to allow it to soak in. Overhead irrigation does this automatically of course.
One of the commonest mistakes of novices (particularly when hand watering) is to water until the soil surface looks nice and dark and wet and then move on. Appearances are deceptive though, the surface may be thoroughly moist, but only an inch or so down it may still be bone dry. If the soil is regularly watered in this way, plant roots will stay near the surface where the water is and the plants will be very vulnerable to fluctuations in moisture content. If you are unable to water such shallow rooted plants for a few days they will suffer.
When watering you must pay particular attention to the edges and ends, of the beds, as they dry out faster than the middle. Also watch any slopes facing away from the water source, it will get less water than slopes facing towards the water source.
Some crops like to have water applied to their leaves, but you should do it when they will dry out quickly (morning or early evening). Others (especially fuzzy ones) are very prone to disease if their leaves get wet, so you must avoid this if possible.
If you are using sprinklers or oscillators you might want to put out a jar or rain gauge, to determine exactly how much water they are putting out.
Always be careful of hoses when watering. It’s easy to inadvertently drag the hose over the corner of the bed and wipe out plants.
If the top of a bed isn’t totally level, a lot of the water landing on it may run off before it has a chance to soak into the soil. Avoid this by making make sure the surface is level, or even slightly concave.
In my garden I have an area of soil that is pretty much hydrophobic (water repellent) and it gives me the same problem. Water simply pools up and runs off rather than soaking in. Adding organic matter can help to cure this in the long term. In the short term you can poke a multitude of holes into the surface with a fork.
Whether you water shallowly or deeply the soil should be very close to field capacity when you finish. Examine the soil carefully after irrigation (dig down again) to make sure it is thoroughly moist with no dry areas. Take note of how the soil and plants look after watering and compare the bed to one which still needs irrigating.
If water is applied in large droplets, or faster than it can be absorbed into the soil, the surface aggregates break down and the smaller particles wash down between the larger ones. When the surface dries out this forms a dense crust known as a cap. This most often happens on poorly structured soils, especially silt or fine sands, but heavy watering can cause it to happen even on fairly good ones. This crust not only reduces water penetration, but also hinders seedling emergence and may inhibit the exchange of air in the soil (which can cause toxic gases to build up and inhibit root growth).
Small areas of crust can be broken up with the fingers. Use a hoe for larger areas. A thin layer of mulch will help to prevent most surface crusting.
Disease and watering
In some climates careless watering can lead to fungus disease problems in vulnerable crops. If you must use overhead watering do it in the morning or early evening so the leaves and soil surface can dry out quickly. Handling wet foliage can also spread some diseases, so don’t work near newly watered plants until their leaves have dried off. Wet foliage is also heavy, so wet plants may be easily damaged or knocked over.
If your soil gets covered in a coat of green algae it is usually because it is staying wet all of the time. This means you are watering too much for the prevailing conditions. While not really serious, this can be detrimental in that it tends to seal the surface of the soil, preventing good exchange of air (as with Soil capping).
If this happens allow the soil surface to dry out and cultivate lightly to get air into the soil. In future allow the soil surface to dry out between irrigations.