|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?|
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.