When you water, make sure you water enough so the water sinks below the rootball. You can judge that by building a soil berm around the tree at least as wide as the width of the tree. Dig a hole in the same area and fill it with water. Let it sink in then dig down to see how far the water has penetrated into the soil. Now you can calculate how many times you will need to fill the basin around your tree so the water reaches below the roots.
For example: Your berm in 3 inches tall. You dig a hole 3 inches deep, fill it with water then dig down to find the water has penetrated 6 inches. If the bottom of the rootball is 24 inches down, you will have to fill the basin at least 4 times to water your tree thoroughly. There's nothing that can kill a JM faster than lack of water. The other option is to set a small sprinkler to run and let it until water starts to puddle.
To measure how much moisture is in the soil, find a long skinny stick (I use bamboo plant stakes or wood dowels) and push it into the soil as far as you can. Leave it a minute and pull it out. You will be able to see/feel moisture on the stick and so judge if the soil is staying too wet or too dry. You don't want things to progress this far, but an overwatered tree dies from the bottom up but an underwatered tree dies from the top down. JMs need to stay damp but they also need good drainage.
Don't fertilize again until next spring. JMs are slow growers - an excess of fertilizer forces them to grow too fast and will kill them.
