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

30 R. M. Madakadze and J. Kwaramba
is greatly increased when sun scalded roots are stored (Clark, L. Moyer, 1988). Sweet
potatoes left in containers to dry for brief periods in the field can be covered with
vines to prevent sunscald.
4.4.4. Soil structure effects
Sweet potatoes require well prepared soil worked to a depth of 15 cm, supported
by a heavy clay loam subsoil with good drainage. In loose or too deeply ploughed
soil the roots have a tendency to become long and slender (Nonnecke, 1989). A firm
subsoil beginning at 15 cm provides the right conditions for tuber development.
Sweet potatoes produce better quality tuberous roots when following soybeans or
other legumes in the rotation. Heavy soils or those with organic matter exceeding
2% are not recommended since the shape may be poor and scurf and black rot
fungi persist in such soils. Scurf is a disease characterized by roughened skin.
Long rotations should be used to decrease the incidence of scurf and infection
from Fusarium wilt (Kemble et al., 1996). Heavy clay soils result in rough,
irregular roots (Peet, 2002).
4.4.5. Soil moisture effects and related disorders
Sweet potato is considered drought tolerant, and an even moisture supply throughout
the growing season will enhance the yield and market appearance. Maximum yields
have been reported at 20% available moisture on fine soils to 50% in coarse sands.
Water logging and fluctuating soil moisture in sweet potato production significantly
affect the quality of the roots. Poor aeration caused by poor drainage decrease
yields of sensitive cultivars and can cause either souring (tissue breakdown in the
storage roots) with severely impeded drainage or water blisters (enlargement of
lenticels on the periderm) if the drainage problems is less severe (Peet, 2002).
Wet soil conditions at harvest lead to an increase in tuber rots and adversely affect
yield, storage life, nutritional and baking quality (Ton and Hernandez, 1978;
Akparanta, Skaggsa and Saunders, 1980).
4.4.5.1. Souring
Souring is caused by roots being in water – saturated soils for prolonged periods
prior to harvest. Souring can result in complete crop loss. Sweet potato roots
sustain a high rate of metabolic activity. When soils are saturated with water, the
exchange of oxygen and carbon dioxide is inhibited and the roots become
asphyxiated. Ethanol and carbon dioxide accumulate in such roots. The result is a
high percentage of roots that decay during curing, and surviving roots undergo a
greater amount of shrinkage. Excessive soil moisture may also reduce quality factors
such as carotenoid pigments, dry matter content and baking quality. Tolerance to
flood damage may be due to either less ethanol production (Ahn et al., 1980) or
the ability of the storage roots to metabolilize ethanol during and immediately
following flooding (Corey et al., 1982).