Shohin bonsai may dry out faster than other bonsai because they simply have a very limited amount of soil. This small amount of soil will dry out because the tree uses water to cool down by leaf transpiration at hot days, and it will dry out because the surface may be large compared to pot depth at some trees too.
The soil structure of course also influence on the ability to keep a moist soil, were a well draining soil needs to be watered much more often to keep a fair amount of humidity throughout the day. The health of the tree is therefore relying on the factors of the weather, the soil structure, the depth of the pot, and the volume of the leaf mass transpiring water taken up by the roots.
Keeping trees healthy demands some considerations depending on the area where you grow your bonsai. Right now the summer is in front of us here in Denmark in Northern Europe. Already we have had a very warm period of sunny weather, demanding especially taken care of the smaller trees in the shohin collection. The water needing trees like Chinese Elm and Japanese maples e.g. needs to be kept from drying out totally. Conifers needs to slightly dry out between watering, so different tasks for different types of trees. A White Pine who prefers dryer conditions will be place at a more airy spot without avoiding humid surroundings. By the way, it is worthy to notice that trees needing strong growth in general needs a strong draining soil, and needs more attention to avoid drying out too much on hot days, because it is a delicate balance not to harm the tree one way or the other. Moister soil structures holds back growth but also may harm a tree during rainy periods. One type of soil being right for one type of growth, can be harmful in other conditions than purposed. The solution is simple though. It is mostly a matter of watering correctly depending of the weather and the needs of the trees.
But the theme now is the heat, and therefore I have arranged some simple trays of clay were some trees are placed during these warm sunny days, having a small water basin beneath the bonsai pot . A few of these trees have had new moss surface decoration applied recently because they are been made ready for exhibition next weekend. The humidity trays will help keeping a moister climate just around the pot and tree, helping the moss kept green for this purpose. The water tray will help keeping a higher air humidity just around the tree, and depending on how high the tree is raised from the water surface the humidity will help the tree kept more healthy. Small effects but of value.
In a rainy period the tray is either removed or wood sticks are levelled higher to avoid water contact with the pot.