I'm not an experienced pixel artist I'm afraid, but I can attempt to give some advice.
It is important to accentuate certain shapes in the tiles and have them work together to give a more "organic" feel. The tiles for the dirt/earth are not that bad, but the grass doesn't really give off the feel of grass. As for the trees, you need to try and create a more symmetrical balance of sorts. Right now, they kind of lob to the side and stand out more, making the duplicate trees a little more redundant/repetitive.
As for the earth fading into the grass, dithering doesn't really work well here. Try to simplify these tiles and just have tiny patches of grass move into the proper grass tiles.
I suggest you look at plenty of examples and try to emulate their styles for study, after which you can then work on your own style.
I'm afraid that's the best advice I can give since I'm also still learning. Good luck to us both!