Place an armour stand on top of a bone block and he will become a bone meal sprayer. Crops around will be upgraded randomly. This does not work for nether wart or sugar canes, as they can't be grown with bone meal. To remove the bone meal sprayer, thrown an iron hoe on him (you will get back the hoe). You can't destroy manually the armour stand for lag reasons. The bone block beneath will be replaced with grass (which you can hoe, by the way).
Sorry for eventual spelling or grammar mistakes, I am not a native speaker.
The command is 14.000 characters long and can cause lag, so caution laggy computers.