Thank you Cyrille! I just used the trick you suggest me for my trees, but the problem is different; the trees tile has a uniformity in alpha channel that my wheat has not. For example we could take an area of six tiles, distant from other resources:
ABC DEF
The overlaying of B tile (over A tile that outfits its border) is complete if we use tree tile. If we are talking about the wheat tile, it's different, for the wheat texture is made of small elements separed each other by spaces. This means (in few words) that instead of viewing the grass at the base of wheat, you see other wheat. Too much wheat in the phrase ;) isn't it?To resolve the pb I've to find another graphical way.
In a few I'll show some solutions I'm experimenting. Bye!