The phosphates are washed off by rainwater and diluted into the watershed (eventually the oceans). In the ocean, it is very slowly converted by bacteria and eve

The phosphates are washed off by rainwater and diluted into the watershed (eventually the oceans). In the ocean, it is very slowly converted by bacteria and eventually brought back by geologic processes uplifting sediment. For a while, we could mine the deep ocean floor...which might be comparable to mining on Mars.