"Fat soluble" only means the food/nutrients are broken down by using fat as a solvent. For example, broccoli is fat soluble.

This means that if you don't eat broccoli with a little fat mixed in (ie. olive oil, butter, lard), your body won't efficiently break down the vitamins in the broccoli, and it will just pass through your system without the nutrients being utilized by your body (ie. poop it out)

Actually, there is a difference between being detectable in the blood stream and detectable in a fat cell. Blood tests check for metabolites in your blood not your fat. It seems to be the general consensus that Marijuana (THC) remains in the blood stream for 7-11 hours. However, based on the articles I've read, there is conflicting information about what happens within the fat cells. Some sources state that Marijuana and it's byproducts remains in fat cells for days, others months, and one stated decades. A person doesn't have to be against marijuana to wonder about the long standing effects the chemicals used to process it could have. That also has nothing to do with the plant itself. My bigger concern is what happens with chemicals such as pesticides, herbicides, and other byproducts.