  We experimentally examined the amount of water retention in a model soil
composed of aggregated glass beads. The model soil was characterized by two
size parameters: size of aggregates $D$ and size of monomer particles
(composing aggregates) $d$. In the experiment, water was sprinkled on the
model-soil system that has an open top surface and drainable sieve bottom. When
the sprinkled water amount exceeded a threshold (retainable limit), draining
flux balanced with the sprinkled flux. The weight variations of retained and
drained water were measured to confirm this balanced (steady) state and
quantify the retained water. We defined the weight of the retained water in
this steady state as $W_0$ and examined the relationship among $W_0$, $d$ and
$D$. As a result, it was revealed that $W_0$ increases as $d$ decreases simply
due to the capillary effects. Regarding $D$ dependence, it turned out that
$W_0$ becomes the maximum around $D\simeq 500$~$\mu$m. The value of $D$
maximizing water retention is determined by the void formation due to the
aggregated structure, capillary effect, and gravity.
