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

4.4.5.2. Water blisters
Storage roots may develop small raised bumps at the lenticels called water blisters
following extended periods of flooded soil. The bumps are white at first and turn
brown to black after the roots are harvested. They often develop prior to souring.
Cultivars differ in their tendency to develop water blisters.
4.4.6. Storage root cracking
Storage roots crack or split in the field, at harvest or even during storage in response
to many diverse factors. Growth cracks which develop during root enlargement in
the field are probably the most frequently encounted disorder. These are mostly
longitudinal and are more common on large than on small tubers. A strong association
has been observed between fluctuations in soil moisture and cracking. The
incidence of cracking is high when prolonged periods of dry soil conditions are
followed by continued wet soil conditions (Clark and Moyer, 1988). Low temperatures
(12–16 °C) during the period when storage roots are being set can also lead
to increased cracking. It has also been suggested that Boron deficiency or excessive
N or lime can lead to cracking. Cracking is more common when sweet potatoes
are planted in the same field successively for several years (Clark and Moyer, 1988).
4.4.7. Soil nutrition
Effect of Preharvest Factors 31
Soils used to grow sweet potatoes are often of relatively low native fertility or subject
to rapid losses of some nutrients by leaching. Nutrient deficiencies are therefore
encountered more frequently. Toxicities are also possible in acidic soils, due to an
excessive availability of certain elements such as aluminum and manganese. Lime
is often used in acid soils to improve overall nutrient availability and plant growth.
However, sweet potatoes are subject to serious damage by Streptomycin impomoea
when the soil pH is above 5.2. The damage caused by this pathogen generally
exceeds the potential benefits of liming. Fertilizer recommendations can only be
relied on when based on knowledge of the local soil and sweet potato cultivar
being grown.
4.4.7.1. Nitrogen
Sweet potatoes usually require moderate amounts of N in the presence of other
macronutrients and micronutrients for good vine growth and optimum yield of
roots of marketable size and shape. Storage roots grown in a soil low or deficient
in N may have abnormal skin colour for example white skinned cultivars may
develop tan skins (Clark and Moyer, 1988). An adequate supply of N has been found
to improve the grades, total yield and quality (protein, carotenoids, low fiber) of
sweet potato. Excessive N may increase the number of jumbo roots (Pierce, 1987).
4.4.7.2. Phosphorus
Sweet potato uses P efficiently and can extract sufficient amounts of it from soils
well supplied with the element. Storage roots are often small and irregularly shaped
under P deficiency. Any purple pigmentation present in the storage roots becomes