After you return the Water Seed to the Water Palace, you can go to the Upper Land to find the next Mana Seed.
Help the Moogles
Popoi hit his head when you landed and has forgotten how to reach his home.
Talk to a Moogle to learn that their village has been overrun by Pebblers.
Watts is in the north part of this area. He can upgrade any weapons that you have orbs for, as usual.
Use the south exit to go to an area with pink trees. Neko is in this screen. Get two Quill Caps from him and equip them on Primm and Popoi. Now go east to where the trees are green, and then use the northwest exit.
This is the Moogle village. The hedgehog-like creatures are the Pebblers that have overrun the village. Kill the Pebblers, then leave the area. Instead of actually leaving, you will stay in the area and the Moogles will return. Watts and Neko will relocate to the Moogle village.
Forge Your Weapons
There are chests in the Moogle village that contain weapon orbs: the axe's orb and the glove's orb. Get those orbs and have Watts forge your weapons.
Reach the Sprite's Village
Talk to a Moogle. Popoi will regain his memories again. He says to walk the seasons from spring to winter, then go to spring again. The area with the pink trees is the "spring" area, the area with the green trees is "summer". There is an area northeast of the summer area that has brown trees for autumn, and west of autumn is winter.
Exit the Moogle Village and go west to the springtime area. Then go east, northeast, west, and south. If you did it correctly, you will hear a sound to the right. Go to the northeast area of the springtime screen. The path there will be open. Exit northeast.
The village has been destroyed. Go north and you find a huge monster bird, Spring Beak. It's weak to Earth spells, so use Earth Slide against it. It moves around a lot, so long-range weapons are the best choice, but it has high speed, so most attacks will miss. Popoi's magic should be enough to do the trick.
You will receive the Boomerang's Orb after defeating Spring Beak.
Seal the Wind Seed
Go north from the area where you fought Spring Beak. Here you find the Wind Palace. Use your sword on the Wind Seed to receive its power.
Talk to the old man behind the altar. It's Popoi's grandfather. He will talk about the fate of the sprites, then will give you the power of Sylphid. One of the abilities you gain with Sylphid is Analyzer, a spell that tells you enemy weaknesses and reveals the type of magic to use on crystal orbs.
Popoi's grandfather says that there is an orb in the Upper Land forest that opens the way to Matango.