The experiment was conducted for 155 days from June to
November and was replicated twice (in 2009 and 2010). Four plastic
bottomless culture containers (30 cm
×
75 cm
×
30 cm deep each)
were filled with sand to a depth of 30 cm and set in a water tank
(130 cm
×
80 cm
×
20 cm deep) (Fig. 1). Ten sweet potato cuttings
were planted at 15 cm intervals in each container. A nutrient solution
was then supplied to the soil surface every day for 2 weeks.
After 2 weeks, when the roots were developed, one side of the water
tank was lifted to a height of 15 cm, so that the distance between
the water table and the soil surface was 15 cm, 20 cm, 25 cm, and
30 cm for each of the four containers. The nutrient solution was
a half-strength of standard nutrient solution (Otsuka formula A,
Otsuka Chemical Co., Japan).