(introduction...)

NOTE: This chapter focuses on hand-watering vegetables during the dry season. For information on furrow irrigation of vegetables and field crops, refer to the bibliography in Appendix H.

It pays to use water wisely

In the tropics, especially in semi-arid regions, most vegetable production occurs in the dry season, meaning that virtually all the water needed must come from irrigation. Even during the rainy season, dry spells may be common and supplemental watering necessary.

Although water is usually in short supply during the dry season, it's often used inefficiently. This not only wastes water, but lowers crop yields and quality.

Some common watering mistakes and their effects

· Watering too shallowly: Roots won't grow downward into dry soil, so shallow watering leads to shallow rooting. Deep rooting not only enables plants to absorb more nutrients, but also permits longer intervals between waterings. It's important to understand that both frequent, light watering and less frequent, heavier watering can result in insufficient water penetration if the total amount applied per week is too small.

· Watering too deeply: This is very easy to do with shallow rooted crops or with sandy soils because of their low water-holding capacity. The excess water moves down beyond the root zone, not only wasting it but carrying away mobile nutrients like nitrate nitrogen and sulfur. (Such nutrient loss is called leaching.) On dense clayey soils, overwatering may cause poor drainage, depriving roots of needed oxygen and favoring soil-borne fungal and bacterial diseases. (To appreciate the difference in the depth of water penetration between a clayey and sandy soil, refer to Figure 2-2 in Chapter 2.)

· Inconsistent watering: "Feast or famine" watering not only stresses the plants and lowers yield, but may cause physiological problems like blossom end rot in tomatoes and peppers, fruit cracking in tomatoes, splitting heads in cabbage, and bitterness in squash.

· Watering too late in the day: This promotes development of damping-off fungal disease in young seedlings by keeping the soil surface moist overnight which helps the fungi proliferate. In older plants, it may favor leaf fungal diseases if the foliage remains wet overnight.

A note on watering in the heat of the day: Contrary to popular belief, hand watering in the full sun at mid-day will seldom injure plants, except in the case of some very tender ornamentals. There is no evidence that water droplets injure leaves by acting like tiny magnifying glasses. However, high soil moisture (after a heavy rain or watering) may sometimes cause a mild "burning" effect (known as sunscald) on the edges of leaves if it occurs in conjunction with high temperatures, intense sunlight, and wind. This is caused by guttation which is the evaporation of excess water from leaf cells, leaving salt deposits behind; these deposits draw water from nearby cells, causing "burning".

TAKE TIME TO LEARN THE BASIC PRINCIPLES OF WATERING

There are no quick and easy methods for determining how much water plants need and how often it should be applied. The so called "shiny layer" method popularized in some garden books isn't reliable enough. (With this method, the soil has supposedly received enough water when a shiny layer of water remains on the soil surface for a certain number of seconds after watering stops). You can greatly improve on such "ball park" methods and do much to help farmers, if you'll take the time to learn some fairly straightforward concepts and figures. So, here goes:

HOW WATER IS USED OR LOST

· Actual Plant Usage (called transpiration)) is very small when plants are tiny but rapidly increases with plant size and then becomes the main source of water loss. In fact, something like 220-660 liters of water are transpired into the air for every kilogram of dry matter produced.

· Evaporation from the soil surface is the main source of water loss when plants are small and much of the soil surface is unshaded. Still, evaporation losses are much less than transpiration losses of older plants and decrease as plant leaves shade the soil surface more completely.

· Downward drainage through the soil beyond the depth of rooting causes water loss when too much is applied at once. Remember that soils can hold water in their smaller pore spaces (micropores; see Chapter 2) without losing it to drainage (i.e. gravity). Only the water in the larger pore spaces (macropores) isn't held and keeps moving downward.

· Surface runoff usually doesn't amount to much except on sloping soils that lack conservation measures such as contouring.

Factors influencing plant water needs

There are 3 sets of factors that largely determine the frequency and amount of watering that plants need:

Soil Factors: Water-holding capacity and usable depth

Weather Factors: Temperature, wind, humidity, and rainfall

Crop Factors: Type, depth of roots, stage of growth

Let's look at these more closely:

Soil Factors Affecting Plant Water Needs

· A soil's water-holding capacity depends mainly on its texture (see Chapter 2). Compared with clayey soils, sandy soils have more macropores (large pore spaces) which don't retain water as well as micropores. In fact, sandy soils can hold only about half as much usable water per unit of depth as clayey soils which means that:

SANDY SOILS NEED MORE FREQUENT BUT LIGHTER WATERINGS THAN CLAYEY SOILS.

· A soil's humus content also affects its water-holding capacity, but only on sandier soils. Adding compost or manure to clayey soils won't improve their already high water-holding ability. (Humus is partly decomposed organic matter that has become dark and crumbly.)

· Usable soil depth is another important factor. Shallow soils or those with hardpans or very compacted subsoils that restrict root depth will require lighter and more frequent waterings than usual. Very acid subsoils (below a pH of 5.0-5.5.) can also restrict normal rooting depth.

Weather Factors Affecting Plant Water Needs

An easy way of understanding this is to realize that any weather condition that speeds up drying your clothes on the line also increases plant water usage. For example:

· Temperature: Plants use more water on hot days, plus evaporation losses from the soil surface are also higher.

· Sunlight: Plants use more water on sunny days than cloudy days. Unshaded soil will lose more water on sunny days too.

· Relative humidity: Water use by plants increases as humidity decreases; the same is true with evaporation.

· Wind: It increases plant water usage as well as evaporation losses.

NOTE: In some areas such as the Sahel region of Africa, a combination of low humidity, high temperatures and persistent wind is common during much of the the dry season and can dramatically increase water needs.

Crop Factors Affecting Plant Water Needs

· Type of Crop: Among field crops, the millets are the most drought-tolerant, followed by grain sorghum and peanuts. Cowpeas, while not as drought-tolerant as sorghum and peanuts, do better than common beans and especially maize when moisture is low.

Among the root crops, manioc (cassava) is very drought-hardy, and sweet potatoes have some resistance. Tropical yams (Dioscorea sup.) can tolerate short dry spells, but most types of true taro (Colocasia spp.) require high soil moisture. However, the taro-like Xanthosoma sagittifolium (tannia, yautia) tolerates drier conditions.

NOTE: In all cases, even drought-resistant crops like millet will yield much less under low moisture. However, non-tolerant crops will often fail.

In general, vegetable crops don't have good drought tolerance, and both yield and quality can be severely affected by moisture stress. However, watermelon and other deeperrooted veggies like okra, eggplant, and tomatoes are more resistant to dry spells than the shallow-rooted veggies like lettuce, onion, and the Crucifer family (cabbage, cauliflower, collards, broccoli, radish, turnip, Brussels sprouts). These shallow-rooted crops need more frequent and lighter waterings than deeper-rooted ones. (See Table 5-1).

· Depth of roots: Aside from differences in final root depth just discussed, all plants will need more frequent and lighter waterings when young. As their roots grow deeper, watering intervals can be spread out and larger amounts applied per application.

· Stage of growth: Plant water needs increase with growth and reach a peak around flowering, fruiting, or heading time. With most vegies, this peak use period continues until harvest time. However, for field crops like maize, sorghum, and dry beans that are harvested at the fully mature, dry stage, water needs taper off as maturity nears.

NOTE: While young plants can often fully recover from a period of moisture stress, a water shortage during flowering, fruiting, or heading can severely affect yield and quality.

TABLE 5-1 Rooting Depth of Crops When There's no Barrier to Penetration

Ok, so get to the point! how much water do plants need and how often?

Well, as you can see by all the variables above, there's no quick answer. BUT, we can give you some definite parameters; you'll need to adjust the figures to suit conditions. First, let's see how much water plants need per week and then deal with how often Per week.

Amount Needed per Week

Crop watering recommendations are often given in terms of inches or millimeters (mm) of water per week. One inch (or one mm) of water is equal to filling a flat-bottom tub with 1 inch (or 1 mm) of water. Note that these measurements refer only to the actual thickness of the water layer and say nothing about the size of the tub (or field), nor how deep the water will penetrate in a soil. In terms of actual water volume needed per area, here are some very useful conversions:

1 INCH OF WATER = 7 GALLONS (25 liters) PER SQUARE METER

1 MILLIMETER OF WATER = 1 LITER PER SQUARE METER

TABLE 5-2 TOTAL WEEKLY WATER NEEDS (1)
(Includes both plant usage and evaporation from the soil)

*1. If the root zone is very dry, it should be watered before planting to "recharge" it. (See the section on pre-irrigation further along in this chapter.)

*2. Refers to sq. meters of actual planted area. Where the bed-and-alley system is used, only the bed area itself should be watered at these rates; don't water the alleyways, because little or no root growth occurs there.

*3. Severe weather conditions (high heat + hot/dry winds) can increase these rates up to 20% above the maximums given.

How to Use Table 5-2: You'll need to consider weather conditions and crop stage of growth. As a crop grows larger, you'll want to gradually increase the weekly total of water (barring any sudden change in the weather), rather than suddenly increasing it from 25 liters to 45 liters per sq. meter.

Note also that weekly water needs are the same whether a crop is grown on a sandy or a clayey soil. The difference is that clayey soils can tolerate longer intervals between waterings than sandy soils. An exception might be those clayey soils prone to severe cracking when they begin drying out; in this case, considerable extra water could be lost by evaporation from the cracks, unless the soil were mulched or heavily shaded by the crop's leaves. (Heavy additions of sand or organic matter will lessen cracking.)

How Often to Water

There are basically two approaches you can take:

· Lighter, but more frequent waterings.
· Heavier, but less frequent waterings.

Both approaches will satisfy crop water needs, as long as the total amount applied per week is adequate. (Refer to the water dosage table above.) Both methods will achieve the same depth of water penetration, given equal amounts of water per week. It's possible that frequent watering may result in somewhat higher evaporation losses on unmulched (or unshaded) soils; on the other hand, frequent watering may help prevent soil cracking on certain clayey soils, thus reducing evaporation losses.

In deciding which of the approaches to take, you'll need to consider 4 factors: soil waterholding capacity, root depth, water supply, and labor considerations.

· Soil water-holding capacity: Sandy soils need more frequent (about twice as often) but lighter waterings than clayey soils, because they can hold only about half as much usable water per unit of depth.

· Root depth: The shallower the root system, the more often watering is needed. Young plants need more frequent watering, because their roots are shallow and the water around them more quickly exhausted. Naturally shallow-rooted crops like lettuce and cabbage need more frequent watering than deeper-rooted crops like eggplant and tomato.

NOTE: During the first few days following transplanting, seedlings will often need more frequent watering than their size would indicate. In hot weather, twice-daily watering may be needed for up to a week after setting. Seedlings that have been container-grown suffer less root damage during transplanting and are less susceptible to drying out. Likewise, proper hardening by restricting watering for 7-10 days prior to setting out the seedlings will lessen initial water needs.

· Water supply: If you're hand-watering from a well that has a limited daily output, it may be necessary to make light applications once or twice a day instead of heavier, less frequent ones which might exhaust a hand-dug well's daily capacity. (An alternative would be to water only a portion of the garden each day.). Farmers using furrow-irrigation from a cooperative system may receive water only once every several days.

· Labor considerations: In some cases, farmers/gardeners will prefer to even out the watering labor by watering as often as once or twice a day (using light applications), even though crop/soil factors might allow one heavier watering every 2-4 days of more.

How often to water before seedling emergence: Most seeds must be surrounded by constantly moist soil to be able to sprout. Large seeds such as maize and beans can usually be planted deep enough so that they will require no additional water after planting in order to sprout. However, most smaller seeds, especially the tiny ones like such as lettuce and amaranth, need to be planted very shallow (5-15 mm). In this case, the soil surface should be kept continually moist until seedling emergence. Unless a pre-emergence mulch is used, this may require watering up to 3 times a day on sandy soils in hot, sunny weather. (Pre-emergence mulching is explained in Chapter 8).

Some Practical Examples of Watering Frequency

NOTE: These examples are designed to calculate the minimum allowable frequency per week; it's OK to water once or twice a day, as long as the liters per application are reduced proportionally so the same total is applied per week.

EXAMPLE 1: Suppose you're growing cabbages on a clayey soil in warm weather, and they're at the heading stage. You figure that 40 liters per sq. meter are needed weekly. Clayey soil has a good water-holding capacity, but cabbage is shallow-rooted, so you'll probably need 2 waterings a week of 20 liters per sq. meter each.

EXAMPLE 2: Now let's substitute tomatoes at the flowering/fruiting stage in the above example. Unlike cabbages, they're a deep-rooted crop (barring no barriers to root penetration). Supposing that 50 liters/sq. meter are needed weekly, you could probably water them once every 5 days. Here's how you'd calculate the amount needed per watering:

50 liters/sq. meter per week needed

5/7ths x 50 liters = about 36 liters/sq. meter every 5 days

EXAMPLE 3: Now let's take a nursery seedbed with young tomatoes, peppers, and cabbage on a very sandy soil in warm weather. Weekly water needs will be about 20-25 liters/sq. m. You'd probably have to water once or twice a day during the first week after seedling emergence, applying 3-3.5 liters/sq. m daily. Obviously, if you put on the entire 2025 liters/sq. m all at once, most of the water would end up beyond the root zone and be wasted. By the time the plants are 1-2 weeks old, you could probably reduce the frequency to once every 2 days.

All the above examples are only approximations; you'll have to decide the amount and frequency that bests suits the situation.

How to Tell When Plants Need Water

Initial signs of moisture stress: Wilting, leaf curling (or rolling), and, in some cases, color changes (maize and other plants will often turn bluish-green).

Advanced symptoms of moisture stress Yellowing and eventual browning ("firing") of the leaves, starting at the tips.

NOTE: Most of these symptoms can also be caused by anything else that interferes with water uptake or transport such as nematodes, soil insects, stem borers, fungal and bacterial wilts, fertilizer burn, and even high temperatures. N deficiency can cause yellowing too.

Ideally, plants should never be allowed to reach the advanced stages of moisture stress between waterings. A little wilting won't affect young plants, but even a day of it can lower yields and quality of crops at the flowering, fruiting, and heading stage.

Two Tests to Determine if Watering is Needed

· The "Scratch" Test can be used on young seedlings when their roots are shallow. Scratch into the soil with your finger and see how far down you have to go to reach moist soil. If the soil is dry more than 2-3 cm down, it may be time to water shallow rooted seedlings.

· The "Squeeze" Test: Using the table in Appendix B, you can estimate the percentage of available water left in the root zone. Water should usually be applied before half of the root zone's available water has been used up. Plants take up about 40 percent of their water needs from the the top quarter of the root zone. Once this top top quarter gets down to 0 percent available moisture, it's time to apply more.

Measuring how deep water has penetrated: Use a 10-12 mm (about 0.5") diameter steel rod slightly tapered at end. Wait about 12-24 hours after watering and then push the tapered end into the ground. It should penetrate fairly easily until it strikes dry soil. (Hardpans may affect the accuracy of this method.)

Figuring in Rainfall

Since rainfall will affect the need for watering, it's important to record it. Buy a rain gauge or make one out of a tin can with straight sides. Amounts below 6 mm (1/4 inch) aren't much use to plants, because a lot may be lost to evaporation. Likewise, heavy downpours may result in much wasted runoff or loss from downward drainage beyond the root zone. For example, a 100 mm (4") rainfall may only add the useful equivalent of 20-30 liters/sq. meter for a shallow-rooted crop like cabbage on a sandy soil. (Remember that each millimeter of rainfall is equal to 1 liter of water per sq. meter.)

What about Pre-Irrigation?

It's often advisable to pre-irrigate the soil down to eventual rooting depth before planting, especially if the root zone is very dry. Use the chart in Appendix B to determine the moisture status of the soil. Then use the water-holding capacity chart in Chapter 2 to determine how much pre-irrigation is needed. If the proper amount of water is applied, little will be lost except the small amount that will evaporate from the soil surface and the first few centimeters. The rest will be safely held in the micropores for future use. (Clayey soils prone to cracking while drying out may have have higher evaporation losses). Pre-irrigation also has some other possible benefits:

· Improving filth (workability) of very clayey soils: Very hard, dry, clayey soils can be much more easily worked if given a good soaking (at least 25 liters/sq. meter 1-2 days before land preparation).

· Pre-planting weed control in vegetable plots: Watering a prepared vegetable bed 7-10 days before planting will encourage many weed seeds to sprout. A very shallow weeding with a sharp hoe blade (using a scraping action) will kilt these weeds without moving more weed seeds closer to the soil surface where they can more easily germinate. This can substantially reduce weed problems on soils with high populations of annual weeds (i.e. those that reproduce by seed).

Some methods for improving water use efficiency

In much of the Third World, water is scarce during the dry season, yet irrigated crop production requires tremendous quantities. Below are some suggestions for helping farmers use water more efficiently. While each of them will help conserve water, using several of these methods together will be the most effective.

· Draw up a watering quantity and frequency schedule tailored to the specific soil, weather, and crop. Remember to consider the water supply and labor factors mentioned above. For example, it might be possible to water older tomatoes just once very 4-5 days on a clayey soil, yet the well may not be able to supply the large quantity needed all at once.

Use the bed-and-alley system for small scale vegetable production (see Chapter 4). By allowing much closer spacing of plants and reducing soil compaction, it usually results in higher yields than using a conventional wide-row spacing system. The higher plant populations and yields per sq. meter of bed actually require little or no extra water, because the closely-spaced plants shade the soil surface more effectively, thus acting as a living mulch. Due to higher yields, water needs per kg of production are usually much less.

Use windbreaks where hot, drying winds are a problem. They'll cut down water losses from evapo-transpiration as well as protect from wind damage itself. Staggered rows of fast-growing trees around the windward portion of a garden can be very effective and also provide fruit, animal feed, and firewood. (Refer to Chapter 3 for further details on windbreaks.)

Mulching the soil surface with peanut hulls, rice hulls, straw, or dried grass will greatly reduce water evaporation losses. Mulching has several other benefits (see Chapter 8) but may attract crickets, ants, termites, slugs, and other harmful insects. Consider both preemergence and post-emergence mulching.

Choice of seedbed: Raised beds (if not mulched) dry out more quickly than sunken or flat beds. Under very dry conditions, sunken beds are usually the best choice, especially since they prevent water from running off. (See Chapter 4 for seedbed styles).

Microcatchments are shallow "bowls" from about 30 cm to several meters in diameter made in the soil with a plant or tree placed in the center. They are very effective at collecting water from the surrounding area and concentrating it around the roots. In rainy conditions, however, they may promote poor drainage and encourage soil-borne diseases.

Various kinds of macrocatchments can be used which collect water from larger areas that have moderate slopes. One technique is contour damming where soil barriers are run across the slope to collect water runoff.

Soil conservation measures like contouring and terracing will markedly improve water retention by lessening runoff from rainfall or watering. See Chapter 3.

Rainwater can be collected off roofs and used for gardening. Each sq. meter of roof will provide 1 liter of water per I mm of rainfall.

Good weed control will also reduce water losses, because weeds can use up considerable amounts of water.

Where soils are very clayey and compacted, double-digging the plots and adding organic matter to the subsoil will improve rooting depth and allow greater intervals between watering. (See Chapter 4).

Adding compost or manure to sandy soils will increase their water-holding capacity.

Use partial shade on less heat tolerant crops like lettuce and the Crucifer family (cabbage, broccoli, cauliflower, etc.).

The "double transplant" ("double nursery") method of raising vegie transplants is worth trying. When transplants are big enough to be set in the field, they are instead transplanted to a larger nursery seedbed where they can be spaced about 15 cm apart. This allows them to be kept in a confined area much longer, thus saving water as well as labor. This method works well with tomatoes, since they have good tolerance to delayed field-setting. (They should be "deep set" when transplanted so that most of the stem is buried underground where it will generate new roots; most other vegies lack this ability.)

Choose drought-resistant varieties: This applies more to field crops like maize where more work has been done on selection and breeding for drought resistance. For example, even though maize isn't very drought tolerant, there is considerable variation among varieties. However, choosing drought tolerance at the expense of needed disease resistance or other desirable traits may not be a good tradeoff.

Balanced fertilization, whether chemical or organic, helps improve water use efficiency by stimulating more extensive root growth. Note, however, that crops can't utilize as much fertilizer when moisture is limiting.

Soil nematode control: When present, root-feeding nematodes can seriously inhibit water (and fertilizer) uptake by plant roots.

Drip irrigation is a method of supplying water to plants by running small diameter plastic tubing down the row. Porous tubing (trickle tubing) is used for closely-spaced crops like carrots; for crops wider-spaced crops such as tomatoes, small individual emitters are used to concentrate the water around the plants. Water is provided at low volume and pressure, and plants are watered about once every day or two.

Drip irrigation can save water by delivering it close to the plants and by reducing water runoff and the amount of wetted soil surface prone to evaporation. However, water use efficiency is highest when the system is used with widely-spaced plants like tomatoes and squash since less area needs to be wetted. Orip irrigation is relatively inexpensive and, except for the micro-tubing, most components can be fabricated from in-country materials. Fertilizer can also be distributed through the system, though special soluble types are needed. The main problem with drip irrigation is its tendency to clog, even when fiItering is used. Improved tubing and emitter designs have overcome this problem somewhat. With care, the microtubing may be used for several seasons.