When planting a tree, dig the hole bigger and deeper than the bag. A square hole will stop the roots from going round and round. Once that is done, fill the hole with water and leave it to dissipate into the surrounding soil. This will encourage the roots to venture out and this will ensure a strong tree. Mix your compost with some of the dug out soil,superphosphate and fertilizer and place it into the bottom of the hole. The tree goes in next and the rest of the compost mix goes in around the tree. Water well. It is far better to leave the water running in slowly so as to saturate the soil. Water once a week depending on the rainfall. one can plant a 2 L plastic cooldrink bottle in the hole after making holes in the bottom of the bottle. When watering, fill the 2 L bottle and the water will then go down to the roots where it is needed. One can do a similar thing with a 10cm diameter pipe.