rafter size for grass roof
Typical loads on a residential floor are either 30 or 40 pounds per square foot (psf) live load, plus about 5-10 psf dead load, certainly not pounds per square inch (psi). So either the OPS made a typo, or the design is for load which is 144 times too high. As for soil, a commonly used unit weight would be between 100 and 130 pounds per cubic foot, so each foot of soil will add between 100 psf and 130 psf to the load. The OPS has not indicated how they derived the psi figure, but if the roof is to be 2 feet thick, it would carry between 200 psf and 260 psf dead load.
This is a very large load compared to normal residential floor loading. It is also much larger than conventional snow loading, so it is not a surprise that standard tables do not cover this type of load. It is similar to the load that might be imposed by a concrete roof deck on a commercial structure, so that would certainly be one way to design the framing, i.e. look at commercial design rather than residential design table.
I do not recommend that design of a heavily loaded roofing structure be performed by an internet chat room. My recommendation would be that the OPS either hire a professional, or if that is not in the budget, perhaps discuss the project with a lumberyard that will include the engineering design fee in their cost to supply appropriate sized framing lumber.