Sweet potato (Ipomoea batatas (L.) Lam, cv. Kokei No.14) cuttings
were planted at 40 cm intervals on soil ridges (30 cm in height,
80 cm in width, and 5 m in length) and were cultured for 98 days
from May to September, 2000 in the experimental field of Osaka
Prefecture University in Japan. The soil was clay loam type and compound
fertilizer was applied as basal dressing with 6 g m−2 each for
N, P and K as chemical compounds of CaNO3, P2O5 and K2O, respectively.
The experimental site was covered with polyethylene film
stretched at 2.5 m above the ground to avoid rain fall. The soil moisture
was controlled by periodical irrigation. The irrigation water
was supplied every day, every three days, or every six days with
an enough amount up to the water holding capacity in this field
(corresponding to 25% volumetric water content) in each irrigation
time. The volumetric water contents in soil ridges were measured
every 12 days by weighing method. Soil samples were collected at
the depth of 10–15 cm just before the irrigation. To measure the
dry biomass of plants after harvest, plants were dried at 80 ◦C in an
oven for more than 4 days