From Bulbapedia, the community-driven Pokémon encyclopedia.

Loamy soil is very fertile soil made of loam found in Hoenn and Sinnoh, where berries are able to grow. After berries are picked from a plant, the soil becomes loamy again. If berries go unpicked, they will fall and replant themselves in the loamy soil. However, they will only replant themselves a number of times before disappearing altogether, and the Berry originally planted will be completely lost.

Since Generation IV, mulch can be used on loamy soil to control the way Berry plants can grow.

Locations

Hoenn

• Route 102 -- Room for 2 plants.
• Route 103 -- 3 plants, Cut required.
• Route 104 -- 3 plants south of Petalburg Woods, 7 on the north.
• Route 110 -- 3 plants south of Mauville City.
• Route 116 -- 4 plants, Cut required.
• Route 117 -- 3 plants.
• Route 111 -- 4 plants, north of Desert Ruins.
• Route 112 -- 4 plants, north of Fiery Path.
• Route 114 -- 3 plants.
• Route 115 -- 2 plants require Strength, 3 require Surf
• Route 118 -- 3 plants.
• Route 119 -- 3 plants near Fortree City, 2 require Waterfall, 2 farther south.
• Route 120 -- 3 plants near Fortree City, 7 at the south end.
• Route 121 -- 4 plants near Lilycove City, 4 at the farther west.
• Route 123 -- 7 plants require Surf. 12 plants at the Berry Master's house.
• Mirage Island -- Room for 1 plant.

Sinnoh

• Route 205 -- 2 (outside Floaroma), 2 (under bridge), 4 (south of Eterna Forest), 4 (outside Eterna City)
• Route 206 -- 2 (west, under middle of Cycling Road), 2 (west, south of Cycling Road)
• Route 207 -- 4 (west side south of Cycling Road)
• Route 208 -- 4 (outside Hearthome, next to Berry Master's house)
• Route 209 -- 2 (outside Hearthome, east of grassy area), 2 (before Solaceon, south of lost tower)
• Route 210 -- 4 (outside Solaceon, immediate right), 4 (outside Celestic Town, immediate north stairs)
• Route 211 -- 4 (outside Celestic Town)
• Route 212 -- 4 (outside Pastoria City), 2 (west of Pokémon Mansion), 2 (middle of route, west side, past pond, before rain, Surf required)
• Route 213 -- 4 (outside Pastoria City, north)
• Route 214 -- 4 (south of lake on east side)
• Route 215 -- 2 (middle, Cut required), 2 (outside Veilstone City)
• Route 218 -- 4 (behind fishermen)
• Route 222 -- 4 (north, follow yellow ground)
• Route 226 -- 4 (top of hill, Rock Climb required)
• Route 229 -- 4 (east, outside of Resort Area)
• Fuego Ironworks -- 4 (south of Fuego Ironworks building)
• Floaroma Town -- 2 (front of flower shop)
• Solaceon Town-- 4 (south of Pokémon Center)
• Pastoria City -- 4 (across from Pastoria Gym)
• Fight Area -- 4 (beside Pokémon Centre)
• Resort Area -- 4 (south section)