Production Practices and Quality Assessment of Food Crops. Vol. 1

4 R. M. Madakadze and J. Kwaramba
to develop from vegetative stage to flower initiation. Examples of such vegetables
include most Brassica sp, Allium sp and Daucus carota.
2.2. Rainfall and water availability
Adequate soil moisture is essential to the production of vegetables. Many vegetables
are about 80% water in terms of thesis composition. Vegetables require a lot
of water to grow and develop normally. Water dissolves plant nutrients in the soil,
plays an important role in biological activities, keeps the plant cool and transports
food nutrients in the plant. Less than 1% of the water that passes through the
plants is used in the photosynthetic process, however, in plants under water stress,
photosynthesis and growth are reduced. Transpiration has a cooling effect on leaves.
A rapidly transpiring leaf can lower the leaf temperature as much as 8 °C.
The total amount of rainfall received annually and its distribution is very
important. For most vegetable crops little or no moisture stress during the entire
growth period generally gives high yields and good quality. Some vegetables require
water at particular critical stages in their growth and development (examples given
in Table 1 below) while others consistently require water at all stages (e.g. tomatoes).
In most places rainfall distribution is not even and so irrigation during dry periods
is required for successful crop growth. The quality of the water must be acceptable
as irrigation water has been implicated for several diseases, toxicity and some
insect pests outbreaks. Saline or brackish water leads to death and retards growth
of many vegetables. Toxicity of several salts can lead to leaf scorch and also
retards growth. Usually water can be from wells, dams, streams and rivers. The
irrigation system used by the farmer is usually determined by the crop requirements,
resources available for irrigation equipment, the terrain and the source and quality
of water available. Both excessive water and drought can affect the quality of vegetables.
Too much water leads to water-logging of which many vegetables are
susceptible. Lack of efficient water in the vegetables will affect taste, appearance
and increase incidences of certain pests (insects and diseases).
Relative humidity is the amount of water present in air as a percentage of what
could be held at saturation at the same temperature and pressure. High humidity
increases the incidence of many diseases and increases insect population. It can
Table 1. Moisture sensitive stages for some selected vegetables.
Crop Moisture sensitive stage
Cabbage Head formation to harvest
Cauliflower All stages of growth
Radish Root enlargement
Turnip Root enlargement to harvest
Lettuce Heading stage to harvest
Onion Bulb formation
Peas Flowering through pod formation to harvest
Irish potato Tuber initiation to enlargement
Sweet corn Silking and ear development