Pokémon Diamond and Pearl/Berries

Battle Uses
Berries are the only items Pokémon know how to use by themselves during battle. Therefore, they are mainly used for hold items, though they can be given to Pokémon on the field.

Berries main uses are for healing status problems, such as burn, freeze, or paralysis. They can also be used to restore small amounts of HP.

Planting Uses
Berries can also be used to grow berry trees. You can do this by planting them in the soil where a previous berry tree was (you can also fertilize the soil, a new feature in Diamond and Pearl). You should water the seed to begin with, and a few times after that as well. You know when the soil is dry when it turns light brown; this means it needs more water to continue growing.

After about 24 hours, assuming you've been watering it, you will have a nice and tall berry tree. Now, the number of berries you receive depends on how often you water it. If you don't water it at all, or very seldom, it will only grow two berries. If you water it frequently, you can get three or more berries.

Also, a minor note; if you leave a grown berry unpicked for too long, it will eventually shrivel up and deteriorate on the ground. When this happens, you have a chance that it will continue to grow another tree.

Fertilizer/Mulch
Fertilizer can be bought from the Berrymaster's daughter at their house to the left of Hearthome City. You spread the fertilizer on the soil before planting the berry. There are four different types, each sold for 200.

Growth Mulch speeds up the growth of Berries but makes the soil dry up faster.

Damp Mulch slows the growth of Berries but the soil takes longer to dry out.

Stable Mulch extends the amount of time fully grown Berries will stay on their plants before falling off.

Gooey Mulch increases the chances of a new plant growing where an old plant withered.