Vegetables need regular watering from planting until harvest. The practice of water conservation is useful especially if there is a dry spell after sowing, emergence and transplanting.
The secret of 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 makes it possible for plants to reach moisture and nutrients more easily than plants with shallow root systems that have developed because of shallow watering.
The techniques described below will help to cut down on water use per unit area and reduce evaporation from the soil surface.
Drip or trickle irrigation
This method wets the soil slowly, allowing for 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 application rate is too high and water is wasted.
Irrigate at night or in the early morning when it’s cooler and humidity is higher
This will reduce losses through evaporation.
Plant veggies at optimal spacing to get quality produce and high yields
Optimal spacing ensures the best coverage, maximises yields and minimises production costs.
Use drought-tolerant plants or cultivars
Separate water-conserving plants from plants with a high water demand. Then plant according to your regional rainfall or to the seasonal prediction. (Zambian farmer Wisdom Mababe chooses his maize cultivars using this information.)
Group plants that have similar water needs together
This helps to prevent under- or over-irrigation.
MULCHES
A mulch is a thick layer (5cm to 15cm deep) of organic material spread over bare soil. Mulches help to:
• regulate soil moisture and temperature
• prevent erosion
• suppress weeds
• prevent crop damage
Don’t mulch wet, low-lying soil and remember that mulch (especially dry grass) may attract termites, is not long-lasting and can be a fire hazard.
Mulch can be used after sowing, and removed just after crop emergence to stop the plantlets from becoming leggy. It can also be placed around the stems of growing plants. Plantlets can be placed in growing holes in plastic sheeting, used as an inorganic mulch.
Mulch sources
• Grass clippings – mowings from most lawns contain more than 1% nitrogen and 2% potash. Lawn clippings are also useful as a green manure to be worked into the soil a few weeks before planting, or for making compost.
• Straw – is a clean source that rarely contains weed seeds. It can be bought off farmers who cultivate crops and is quick and easy to lay down. After harvest, dig it in and by the next planting season it will be part of the soil.
• Wood shavings and sawdust – are generally safe and effective soil improvers that won’t acidify soils. They help to aerate the soil and increase water holding capacity. Sawdust should be well decomposed before it is used.
• Newspaper – 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.
• Plastic mulches – these are inorganic and very effective, but mainly compatible with drip irrigation. Plastic mulches are also useful for raising the soil temperature in the early growing season.
CROP REQUIREMENTS
Water needs vary with crop type and age. Below are some general guidelines that may be adapted to suit conditions in your vegetable field or 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 especially in hot weather. Water twice a day in the first week and once a day in the second week.
Leafy crops: Leafy crops and brassicas have high water requirements and needs about 25 l/m² 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.
Roots and tuber crops: These crops need 10 litres to 15 litres per m2 every week during the first month after planting. From then on until the crop begins to mature, 30 litres a week per m2 is enough. If there is no rain water regularly and make sure that the 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 and solanaceous crops, like tomatoes and peppers need about 25 litres per m² per week. During week 1 after planting or sowing, water twice a day; during week 2 water once a day; from week 3 onwards, water three times a week. Sweet potatoes and cucurbits such as pumpkins are more drought tolerant than other crops and need irrigation once a week from the second month after planting.
For sweet potatoes, irrigation is critical after establishment, twice a day for the first week and once a day for the second week. From the third week water three times a week until the storage roots develop at months 1 and 2.
Pumpkins need frequent irrigation until 6 to 8 weeks after emergence.
Water-efficient vegetables: Cowpea, amaranth, pigeon peas and Bambara are drought tolerant and can be grown under dryland conditions where rain is well distributed.
