Three months ago I planted an Afgan Pine (Pinus elderica), Juniper (Junipcrus chinensis), Navel Orange (Citrus sinensis), and a Thuja Green Giant Arborvitae, all between 3 and 4 feet tall. Using a moisture meter, wooden dowel or my finger, I recently noticed that starting about 12" away from the trunk, the closer you get to the trunk, the drier the soil becomes. Is that because the root ball dries out quicker then the surrounding soil and if so should I water when the soil about 6" away from the trunk is 'low moist'? I use a 1/2" soaker hose wrapped twice around the trunk at about 6" from the trunk. The soaker hose dripping for two hours dispenses between 18 and 24 gallons depending on which tree I am watering. Generally I water every 3 to 4 days now because in southeast Arizona, USA, the temperature reaches 100 t0 105 F during the afternoon. All trees are in well draining soil and have 3" of compost mulch.
I'm an amateur at trees, but definitely an aspiring expert. So your question interested me. One immediate thought comes to mind, could the lack of moisture nera the tree be caused by the tree canopy? The root ball would seem to be more likely but possibly not.
With the temperatures you describe and the value and commitment associated with those three trees, I would measure the moisture in the soil as near the root ball as possible in the early stages of growth. Next year, the roots will have spread, this year they are unlikely to have spread much.
Thanks for finally clearing that up. It has to be the differance of the media in the rootball compared to the surrounding soil since the trees are too small to have a canopy. I will use the reading I get near the rootball now, because with the soaker hose most of the water goes down instead of around the trunk.