Ones urine and feces are all the fertilizer a person needs. We only need to buy food for one year of life to sustain us until death from old age, and even then, that corpse has enough nutrients to continue the cycle to make another.

Edit: the downvotes show me that people here do not understand nutrient cycling, closed loop systems, regenerative agriculture, or how composting and pathogens work.

Urine is easy and safe to use, there have been plenty of articles and studies posted on reddit about it so we can skip this part of the conversation

With poop, I can only assume the downvotes are from people who immediately imagine growing potatoes in poo, which is not how its dealt with.

aerobic septic systems render feces safe to use, this can be bolsterd with UV-C lights for extra insurance.

anaerobic systems are cheaper and less effective, but are used almost everywhere outside of the cities already

If you buy conventionally farmed food, you can bet you are already eating food fertilized with municipal sewage

Half the worlds population does not have running water to flush feces away, meaning **they need simple on site solutions**. For this, dry-toilets or compost toilets are used.

Composting is a simple way people can process feces from dry toilets. Poop is anaerobic, and composting is an aerobic process (unless you do it wrong, but there is a clear difference). Aerobic microbes will consume and destroy anaerobic pathogens

Do not plant root crops in poo, its a pretty simple rule. But It can be safety used to feed an apple tree, with layers of mulch on top.

If they colonize mars do you really think we are just going to jettison our sewage nutrients away so we can ship more in from earth?

Each and every person has and makes enough fertilizer for themselves technically speaking, just because we flush away out feces away with potable water is the normal, does not mean it is smart, efficient, or sustainable.