Lesson 1. 7: Cabbage Water Requirement
This module provides guidance on the proper timing and techniques for harvesting sweet potatoes. It also covers post-harvest handling, including sorting, grading, and storage to ensure high-quality roots.
Roots Maturity
Root maturity can vary between varieties, and root development is slower during cooler weather.
Growers need to monitor the development of roots with regular checks of root size after 18 weeks.
Marketable grades of roots are between 0,25 and 1 kg. If harvested at the correct time, around 60 to 70% of total roots should be within this grade.
If grown during the dry season, most varieties should be ready for digging up at about 20 to 22 weeks from planting. If left too long in the ground, the roots can become oversized and unmarketable.
It is important to note that roots kept for too long in ground upon maturity, they may become oversized and unmarketable. Quality of the roots also deteriorates.
Below are indicators that should be looked into to determine the roots maturity
- Right size of the tubers.
- Variety maturity date.
- Leaves start yellowing and dying back.
- Cracks appear in the soil above the tubers.
Harvesting methods
Harvesting sweet potatoes can be very labour intensive, and requires suitable equipment for commercial production. Before harvesting, most of the top growth needs to be removed or it will become entwined in the digging machine. Vine removal is best done with a swinging pulverizer where the flails are shaped to the contour of the bed. This will chop the vine into pieces and leave the hills bare.
A standard slasher or pulverizer can be used, but will not remove material between the rows. Chopping into the top of the hill should be avoided at all costs as this may damage the roots. Following this, any remaining vines can be cut on both sides of the hill with large, sharp coulters mounted on a toolbar. This vine removal should be done a week before digging to toughen the skin of the roots.
Roots are lifted from the soil using a single row potato digger. To avoid digger damage, this should be done while the hills are still moist so that the soil travels up the digger bars with the roots. The digger elevator should be moving only slightly faster than ground speed.
The dug roots are then manually collected into bulk bins and transported to the shed. The harvested crop must be kept away from lengthy exposure to the sun, and skin damage will be less if the roots are kept wet during handling.
Research trials have shown that 20 to 40 t/ha of marketable roots is achievable, depending on variety and management.
NB: Sweet potatoes, which bruise easily, are harvested by hand with mechanical aids. Vines are mechanically removed. Large tractor-drawn platforms that have a digger chain running in the center are frequently used to lift the sweet potatoes from the ground. The sweet potatoes are then carefully removed from the chain by hand and placed either in wooden boxes holding 18 to 23 kg or in a bin that measures by 1.2 by 1.2 m and holds 454 kg.
Root curing
Sweet potatoes to be stored for later marketing or for seed stock must be cured immediately after harvesting to minimize storage losses. Curing involves controlling temperatures and relative humidity and providing ventilation for seven to ten days.
Curing is a wound-healing process which occurs most rapidly at 26 to 32 °C, a relative humidity of 85 to 90% and good ventilation to remove carbon dioxide from the curing area. Wounds and bruises heal and a protective cork layer develops over the entire root surface. In addition, suberin, a waxy material, is deposited. The cork layer and suberin act as a barrier to decay-causing organisms and to moisture loss during storage.
This process involves the forced hot air treatment of roots at 30 °C with 90% relative humidity for between 4 to 6 days. This must be done immediately after harvest, and will result in the formation of a wound skin, which heals any mechanical damage suffered during harvesting. Post-harvest rot infections are minimized and excessive moisture loss prevented. Curing can also improve eating quality by increasing sweetness.
Root curing is not a standard commercial practice, but is worth considering if roots need to be stored for a prolonged period. Subsequently, harvested roots are placed in buildings to cure (30-35 °C, 90% RH) and then stored (10-15 °C; 85-90% RH) until needed for the market.
Curing promotes wound healing and provides a barrier to prevent bacteria and fungi from entering damage that results during harvesting and handling. Properly cured roots will store for 12 months or longer with 15 to 25% losses under the best conditions.
Temperature must not drop below 12 °C in order to prevent physiological cold damage to which sweet potatoes are particularly susceptible. Relative humidity should remain between 80 and 90% to prevent dehydration as the storage roots continue to respire. As they are needed for marketing, roots are removed from storage rooms, processed through a mechanical washer/grader and packed into boxes of about 15 kg. Wash water may contain chlorine or other approved fungicides to reduce infection of wounds generated by the grading procedure.
In rural areas farmers can store the roots on underground pits covered with moist soil.
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