Vegetables need a regular supply of water from planting until harvest, and can’t be grown successfully under rain-fed conditions in the drier parts of southern Africa.
Here is some advice for saving water when growing veggies.
The secret to using water wisely is to water slowly, deeply, and less often to avoid water loss through drainage and evaporation. Slow, deep watering encourages deep root growth which enables plants to reach moisture and nutrients more easily than plants that develop shallow root systems because they have been shallow-watered.
The following techniques will help you cut down on the water you need per unit area and reduce evaporation from the
soil surface.
Use drip or trickle irrigation. This method wets the soil slowly, allowing for slow, deep penetration. Drip irrigation uses 60% less water than conventional sprinkler systems.
Water at low application rates. If water runs off the soil surface or forms puddles, the water application rate is too high and water is wasted.
Irrigate at night or in early morning when temperatures are cooler and humidity is higher. This will result in fewer losses due to evaporation.
Plant vegetables at optimal spacing to achieve high yield and good quality. Optimal spacing will ensure the optimal coverage of the area, maximise yields and minimise production costs.
Use drought-tolerant (water-conserving) plants or cultivars. Separate water-conserving plants from water-demanding plants.
Group together plants that have similar water needs. This will help to prevent over- or under-irrigation.
MULCHES
A thick layer (5cm to 15cm deep) of organic material (also known as a mulch) can be spread over otherwise bare soil. Using mulches offers a number of advantages. They can help to:
- regulate soil moisture and temperature
- enhance water infiltration
- prevent erosion
- suppress weeds
- prevent crop damage.
Don’t mulch wet, low-lying soil and remember that mulch (especially dry grass) may attract termites and also has limitations, such as the lack of durability and the possibility of being a fire hazard.
Mulch can be used after sowing (and must be removed just after crop emergence to avoid the development of leggy plantlets) or placed around the stems of growing plants. Plantlets could also be placed in holes in mulch (such as plastic sheeting).
Mulches can be made from several sources including:
Lawn mowings – Mowings from most lawns contain more than 1% nitrogen and 2% potash. Lawn mowings are also useful as a green manure to be worked into the soil a few weeks before planting and for making compost.
Straw – This is a clean source that rarely contains weed seeds. It is inexpensive if bought directly from farms and is quick and easy to lay down. During autumn, dig it in, and by spring it will have become an indistinguishable part of the soil.
Wood shavings/sawdust – Generally speaking, sawdust and wood shavings are safe and effective soil improvers and won’t acidify soils. They help to aerate the soil and increase its water-holding capacity. Sawdust should be well broken down (decomposed) before use.
Newspaper – This has excellent moisture-holding qualities. A layer of hay, straw, or wood shavings over the paper will improve the appearance and keep the wind from blowing it away. Paper is efficient at preventing weed growth. Because it is dense enough to keep sunlight from passing readily through to the soil, it is best applied after irrigation or rain. Paper does not decay quickly, but eventually breaks down and adds humus to the soil. Empty paper bags could also be used.
Plastic mulches – These are inorganic and very effective, but compatible mainly with drip irrigation. They are also useful for raising the temperature when planting early in spring.
Another application of plastic is constructing small tunnels to protect crops against cold early in the season in areas where frost occurs. The covers are removed during the day.
CROP REQUIREMENTS
Water requirements vary with the type of crop as well as its age. The following general watering guidelines may be adapted to suit the conditions in your vegetable garden.
Seedlings – The critical time is between sowing and the emergence of the seedling. The soil in contact with the seed should be kept moist at all times.
Transplants – Watering before and after transplanting is essential, particularly in hot weather. Water should be applied twice per day during the first week and once a day during the second week.
Leafy crops – Leafy crops and brassicas generally have high water requirements and need about 25ℓ/m2 of water a week. These vegetables should be actively growing from the time they are sown or transplanted. Any lack in water supply can have a negative impact on yield and quality. During summer, it may be necessary to irrigate thoroughly for up to three times a week.
Roots and tuber crops – These crops need 10ℓ/m2 to 15ℓ/m2 water per week during the first month after planting. From one month after planting until plants approach maturity, 30ℓ/m2 per week should be adequate. Water regularly if there isn’t any rain. Farmers should ensure that water penetrates deeply into the soil. Shallow watering discourages deep root development.
Potatoes should be watered twice a week, but need additional water when tubers start forming. Carrots should be watered three times a week from three weeks after planting.
Other crops – Green beans and other legumes, cucurbits, solanaceous crops, such as tomatoes and peppers need about 25ℓ/m2 of water per week.
Water requirements vary according to the stage of development. During week one after planting or sowing, water twice a day; during week two, water once a day; from week three onwards, water three times a week.
Sweet potatoes and cucurbits, such as pumpkins, are more drought-tolerant than other crops and require irrigation once a week from month two after planting.
For sweet potatoes, irrigation is crucial after establishment and is needed twice a day for the first week and once a day for the second week. From the third week water should be applied three times a week until storage roots have developed (months one and two).
Pumpkins require frequent irrigation until six to eight weeks after emergence.
Water-efficient vegetables – Cowpeas, amaranth, pigeon peas and bambara are drought-tolerant and can be grown under rain-fed conditions, if the rain is well distributed.
WATER HARVESTING
The principle of infield rain water harvesting is to transfer run-off water from an area that is not cropped to supplement rainfall received on the area where crops are grown. This is more relevant for areas where drought is experienced and rainfall is irregular.
Catchment strips within the boundary of the cultivated area can be altered to direct run-off to cultivated plots.
This information was originaly supplied by Sharon Mulandana; Erika van den Heever; Sunette Laurie, from the South African Agricultural Research Centre (ARC).
