The kidneys are paired retroperitoneal structures that are normally located between the transverse processes of T12-L3 vertebrae, with the left kidney typically somewhat more superior in position than the right. The upper poles are normally oriented more medially and posteriorly than the lower poles.
Growing Comfrey in Pots. Comfrey will grow in pots (at least 3 gallon), containers such as plastic tubs, or 5-gallon plastic buckets but they do better when not grown in a pot. They have deep roots so prefer growing in the garden where they get more nutrients. The containers need drainage holes in the bottom. Add fertilizer to the potting soil.
This is because comfrey is perhaps the most efficient of all British plants at sucking up nutrients. Its deep roots - 10ft or more - take in all available goodness and store it in the large, hairy leaves. When cut, these break down very fast, so transferring the nutrients back into the soil. I like the specificity of this - the comfrey is tapping into the unique brew of this garden's nutrients and then releasing it back to plants also growing in this precise soil. I use comfrey in three ways. The first is simply to add it to the compost heap, where its nutrients will both enrich the whole heap and encourage decomposition. Comfrey has a high carbon to nitrogen ratio - ideal for most plants. The second way is to spread the leaves as a mulch beneath, say, your tomatoes.