Q: I plan on building a large planter and growing roses. I have access to horse manure and also composted fish waste from a hatchery. Would it be a good idea to fill the planter with this stuff or mix it with something else?
A good growing soil is porous, so water can penetrate and exit the dirt easily. The soil should contain a moderate amount of nutrients. If there is too much nitrogen or potassium present, tender roots will “burn” and shut down. My bet is that a soil made exclusively of horse manure or fish compost would have too much fertilizer present. I’d mix the rich materials you have with something more coarse. Consider filling the container with a 3:2:1 mixture of porous woods dirt, river sand and manure (or fish compost).