If you have a friend or neighbor with a large, healthy lychee tree, you can request to dig some soil to inoculate your soil with any beneficial microbes it may have. Also, Lychees love a thick layer of mulch beneath their canopy and out past the drip line. The thick layer of mulch and the deterioration of plant matter and whole microbial ecosystem it supports significantly increases the rate of growth of Lychee trees. The mulch also holds in more moisture and can increase the ambient humidity which may benefit leaf growth.
If the edges of your Lychee leaves look burnt, your water quality is probably poor. Rain, pond or RO water is very good for Lychees. Most hose end water filters are useless. Better water quality and a wind break for younger trees is extremely beneficial.
Once a lychee tree gets some size, city water is fine and a wind break is not necessary.
Lychees love Iron and I drench my trees with chelated Iron throughout the growing season.
Simon