Whether it's OK to leave a Sonotube in the ground or not depends on what you're using it for. If you're using it as part of a structural foundation, then leaving the Sonotube in place is generally considered safe and acceptable. However, if you are planning to use the Sonotube as more of an ornamental feature such as a planter box or bird feeder stand, then it's best to remove it after installation and fill the hole with soil instead. Leaving the tube exposed can cause problems over time due to weathering and erosion, so removing and replacing with soil is advised in those cases. Furthermore, if you are using your Sonotube for outdoor projects that require concrete pours like patios or pool decks, then make sure that all visible edges of your tube are well-protected against water damage by either painting them with waterproof sealant or wrapping them tightly with tar tape. This will help ensure maximum durability for years to come!