Soft Mattress
Soft mattresses have gotten a bad name over the years from back sufferers. We were always told that firmer mattresses are better for us to sleep on. This is not the case, firmer mattresses are mainly suggested for people that sleep on their backs.
The firm mattress will assist in keeping the spine and neck strait for back sleepers. A firm mattress with a little padding may be the healthiest for back sleepers to fill in curves and prevent lower back pain.
Side sleepers actually need a softer mattress to give a little more pressure relief to the shoulders and hips. When someone sleeps on their side pressure is relieved on the lungs and other vital organs which makes breathing much easier. That is part of the reason that in late pregnancy doctors suggest sleeping on the left side.
For stomach sleepers it has been told that this is a very unhealthy position to sleep in. A lot of times people that sleep on their stomach have back problems and wake up stiff and achy more often than with other sleep positions.
Also when you sleep on your stomach you put pressure on your diaphragm which inhibits the lungs from fully expanding. A soft mattress for a stomach sleeper may sag in the middle which may result in the body naturally arching in a unhealthy posture.
Soft mattresses are not for everyone health wise or comfort wise. The more important issue is that you need to find a mattress that is physically comfortable for you and the normal position you sleep in.
Remember that there is a big difference between a soft mattress that was designed that way and an old saggy mattress that is worn out. Even a brand new mattress can sag if you are to heavy for the support system underneath.
Ideally you should always be in proper alignment regardless of the firmness or softness of your mattress. A general rule is that the heavier you are the firmer support system you need and the lighter you are the softer support system/
So in summary a soft mattress is neither good or bad for your back by itself it depends on your body type, either heavy or light, and sleep position. Just keep in mind that you do need some pressure relief to be comfortable while you sleep.