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

e beneficial to the water economy as transpiration is decreased. Pollen viability
increases in certain crops as humidity increases.
2.3. Soil
Generally vegetables grow in a wide range of soil types. It is however, necessary
that these soils be fertile, well drained, and have a high water holding capacity.
The soil must be fertile enough to provide the essential nutrients (N, P, K, Ca, S,
Mg, Fe, Cl, Mn, Cl, Mn, B, Zn, Cu and Mo) required by the plants in the right
amounts or fertilizers must be added to supplement the soil content. The critical
mineral elements affecting quality of vegetables are covered in more detail in Section
3.3.
Heavy clay soils are very difficult to work, especially in the wet season. Organic
matter must therefore be added to such soils. Sandy soils are also not good as
they contain very little nutrients and have poor water holding capacity. Organic
matter must therefore be added as well to improve humus content and water holding
capacity. The ideal soil for most vegetables is the medium clay loams supplied
with organic matter or other nutrients.
The acidity and/or alkalinity of the soils is very important in production of all
vegetables. A pH range of between 5.5 and 7.5 is generally accepted by most vegetables
although tomatoes and peppers can tolerate slightly more acid soils. At
this pH range most important nutrients are available to plants as long as they have
been applied to the soil. Table 2 below shows optimum pHs for a few selected
vegetables. Calcium, P and Mg maybe unavailable at an acid pH below 6, and
Mn, B, Zn, Fe and Al may become so available to the plant that they become
toxic. Sandy soils are host to nematodes that hinder growth of many vegetables
and are very difficult and expensive to control. Soil depth in the production of
vegetables must be at least 40 cm as most vegetable roots can reach that depth.
2.4. Topography
Effect of Preharvest Factors 5
Topography is very important in relation to temperature and temperature fluctuation.
The diurnal or daily night temperature variations which occur at elevations over
Table 2. Optimum pH values for selected vegetable crops.
pH Crops
6–8 Asparagus
6–7.5 Beet, cabbage, muskmelon, peas, spinach, summer squash
6–7.0 Celery, chives, endive, lettuce, onion, radish, cauliflower
5.5–7.5 Sweet corn, pumpkin, tomato
5.5–7.0 Snap beans, lima beans, carrots, cucumbers, peppers
5.5–6.5 Eggplant, watermelon
4.5–6.5 Potato
Adapted from Splittstoesser (1990).