Mulch
From Bulbapedia, the community-driven Pokémon encyclopedia.
Mulch is a type of item introduced in Generation IV. Mulches are basically fertilizers used before planting Berries in loamy soil.
According to the lady in the Berry Master's house in Sinnoh, mulches are made "by mixing high-grade soil with Pokémon...um...you know...".
Mulches promote healthy Berry growth, and there are four types of mulch items:
The Growth Mulch lowers the amount of time for Berry tree maturation to 75% of the normal time, while the Damp Mulch raises it to 125% of the time. For instance, if a Bluk Berry is planted in loamy soil with Damp Mulch, it will bear fruit in 10 hours instead of 8, while if a Sitrus Berry is planted in loamy soil with Growth Mulch, it will also bear fruit after 24 hours, though instead of 32. Using these two mulches, it is therefore possible to cause some Berries with different growth times to grow in the same amount of time.

