Soil plays a large role in farming in Monster Harvest as it influences your Planimals. You can upgrade your soil through the use of heartslime. You can do so at the sign on your farm beside your home.
The only way to get heartslime is through fallen Planimals. Whatever level your Planimal is the amount of heartslime you will receive. This means that even when they die, your Planimals can still support you.
Related: What Do Slimes Do in Monster Harvest
The level of your soil directly influences any new Planimals you grow. When you harvest a Planimal, it normally starts out at level one. Upgrading your soil will cause harvested Planimals to start the level of your soil. This means that if you level ten Planimal were to die and you used its heartslime to upgrade your soil to level ten, all your Planimals harvested would be level ten.
This is a great mechanic that will keep your new Planimals around the same level as your current team. This cuts out the need to grind at lower levels to keep your Planimals' levels on par with one another. So while losing one of your team members can be sad, just remember that even six feet under they still have your back.