|Soils, Crops and Fertilizer Use: A Field Manual for Development Workers (Peace Corps, 1986, 338 p.)|
|Chapter 5: Watering vegetables: When? How Often? How Much?|
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)
Inches of Water
Millimeters of Water
Liters Needed (2) per Sq. Meter
VERY YOUNG PLANTS IN WARM WEATHER
PEAK USE RATES FOR VEGETABLES IN WARM WEATHER (during flowering, fruiting, or heading)
35-50 liters 3
PEAK USE RATES FOR FIELD CROPS (from pollination through first 3-4 weeks of grain fill)
*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
· 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).