There are only two ways for soluble salts (fertilizers, bio-carbonates, sodium etc) to get out of a container. Either the plant absorbs them, or they are flushed out with a lot of water. If the water used to flush the container is not either rain water, or distilled water, it too could be adding additional soluble salts with each watering. If the small tree's root system does not fill the complete volume of the container, these soluble salts quickly begin to accumulate causing many problems for the tree. If the medium is frequently flushed to remove the harmful ingredients, the unused portion of the medium is constantly water longed and compacted.