First tip: Post images directly in threads.

Generates a lot more responses (and faster) than links. May require hosting them in the Dakka gallery, though, as some sites don't take kindly to hot-linking from forums.
As for the base, it's very clean, which is good. Sometimes, extending the basing over the lip can be a characterful effect, but globs of sand and splotches of paint all over the rim usually just scream "lazy," destroying any efforts put into the top.
It is, however, a bit flat and bland. Your layer of sand is so even that it looks unnatural - with the black coloration, it could very easily be mistaken for an attempt (and a reasonably good one) at coarse asphalt, in which case grass poking out of the otherwise pristine surface makes little sense. Small height variations (just smush the stuff around a little, while the glue is still wet - may take two layers, with very fine grit, to have enough depth to introduce variation) will give a far more natural effect. For sand, a consistent texture makes sense, so surface undulations are needed to give it character.
For soil, varying the grit a little works just as well. Small additions of larger grains - coarser sand, fine gravel, cork chips, kitty litter, etc. - to your grit mix will do wonders, in terms of naturalism. It's pretty rare to find perfectly flat, level ground without at least a few small rocks or dirt clods breaking up the surface. Even if it's not impossible, it
is a bit boring, visually.
You can really go nuts with your basing, putting as much effort into it as the model it supports, but that's neither common nor necessarily advisable. From where you are, now, a touch more variation is all that's really needed to get you into a comfortable position. Luckily, it won't take much more effort per base (depending on how you go about it, of course) to get it. That said, it's always worth considering the terrain you want to represent, first. For the sake of fluff, sometimes very flat and simple basing
is the obvious choice. It won't have the same impact (or score as many points with the judges, most likely), but any basing is better than none and it's
your army to treat as you will.
[edit:] Multiple ninjas, basing theme unveiled. I would focus, then, on slight surface variation and color. A simple pass or two of drybrushing will, as that person suggested, up the contrast and let the texture really stand out. For surface variation, think about the terrain a bit more - what
exactly do you want? Ash fallen over normal ground? Black sand desert (amidst dunes? mesas?)? That will tell you whether to add rocks, ripples, etc.