The setting features biomancers, mages who are like necromancers in that their magic revolves mainly around the use and obtaining or creation of minions or servants or the modification of said servants or themselves, but instead of dealing or interacting with dead things their magic deals with living things.
There are many ways for biomancers to obtain their servants, many of which unspeakable and despicable, but for those who are less morally dark the most common way of obtaining or creating the living things that will do their bidding is through 'growth pods'. Growth pods are modified plants that fulfill similar functions to eggs or wombs in that within them grows whatever organism the biomancer has implanted the 'seed cell' of which into. There is a large amount of biological knowledge required, from anatomical to cellular, to create seed cells that will grow into successful creatures that won't die a mere day or so after exiting their pod, let alone have the correct mental imprints to follow the biomancer's will(if they don't intend to constantly control the creature like a puppeteer), but once they understand how living things work to a well-enough degree they have a frankly ridiculous amount of freedom when it comes to what the pod(or whatever else they use) will grow for them. There are many other things biomancers can do as well but they aren't the focus of this question.
My problem? I don't know if soil alone would be able to provide everything a growth pod needs to grow whatever creature the biomancer wants. Yes everything we eat technically comes from the soil via vegetables and other in-betweens, and yes they may be able to provide the pod with whatever it needs with some ahem fertilizer ahem, but I want there to be a morally 'good' way for biomancers to go about their craft that isn't the equivalent of restricting themselves to plants and being a faux druid.
Would soil alone be able to provide everything necessary to feed their development and grow animal-based living creatures within these growth pods?