These are sediments that come in contract with the water column.

As the level of reduction goes down to more negative values, say 100 mv's, the FePO4 bond is now broken and relases the Fe and PO4 into the aerobic water column, some will be oxidized and fall out of solution back to the sediment again.

Some will make it to plants, and algae.
Search for CEC of soils for more.
clays have a lot of CEC's, they can bind metals etc and then release them later under the presence of plant roots or reductive conditions.



Regards,
Tom Barr