Irrigation
The best way to maintain the desired moisture content in the hills is to monitor the soil moisture with tensiometers. A shallow tube at 15 to 20 cm into the hill will indicate the timing of irrigation, and a deeper tube just below the base of the hill at 40 to 50 cm will determine the length of irrigation.
Water requirements will vary with soil type and increase as storage roots develop within the hills. It is desirable to maintain moisture content in the hills at or near field capacity. Both tensiometers should remain within the 10 to 20 kPa range on sandy soils. This is especially important from 10 weeks onward as roots have been initiated and are starting to fill out. This is also the period of increased water use by the plants. Fluctuating soil moisture levels during this stage will reduce yield and cause cracking of roots.
Fertilizer application
The recommended fertilizer rate for sweet potato production is based on crop removal figures. Research has shown that this recommendation will produce high yields when used in conjunction with yearly soil nutrient testing and petiole sap nutrient monitoring.
Estimated crop removal in kg per ha is:
-100 kg Nitrogen (N)
- 90 kg Phosphorus (P)
-200 kg Potassium (K)
- 200 kg Calcium (Ca)
All the phosphorus may be applied in the basal along with 50 kg of N and 50 kg of K.
The remaining 50 kg N and 150 kg K should be divided into two side-dressings at 4 to 6 weeks and 10 to 12 weeks from planting. Some calcium will be supplied by the lime or dolomite used to adjust the soil pH, and any additional calcium may be applied in the basal as gypsum. Petiole sap nutrient monitoring is advisable so that the desired nutrient levels for different growth phases can be checked. Any trace element deficiency would be detected by regular petiole testing, but generally, two foliar applications around the time of side-dressing should maintain adequate levels. Sprays should include zinc, copper, manganese, iron and boron.
