The volumetric water contents in soil were 23.4 ± 0.3%
(mean ± SD), 15.3 ± 2.9%, and 10.2 ± 2.5%, respectively, during the
culture period in plots irrigated every day, every three days and
every six days. The water was mostly saturated in the soil irrigated
every day, because the water holding capacity in this field
was corresponding to 25% volumetric water content. Each of the
dry weights of tuberous roots and shoot (leaves + stems) in the
plot irrigated every three days were greatest among all treatments
(Fig. 3). From the supplementary soil ridge experiment with
Brussels sprouts, CO2 concentrations (Cs) increased linearly with
increasing volumetric water contents (Ws) in the soil ridge following
the equation, Cs = 0.102Ws − 1.018 (r2 = 0.929, n = 42). Therefore
CO2 concentrations in soil ridges with sweet potato irrigated every
day (the volumetric water content: 23.4%), every three days (15.3%),
and every six days (10.2%) were estimated to be 1.4%, 0.5%, and
0.02%, respectively.