But seriously, I think implementing fertilizer is a great idea. It could even increase soil quality

in forested area since soil quaility isn't implemented on farm land.
Now to unnessicarly complicate the idea....
If Loftar and Jorb are leaning towards realisim then the fertilizer could be anything that is high in the main nutrients that plants require(Nitrogen, Phosphorus, Potassium, Magnesium, Sulfur, Calcium, ect.) Thus excrement from animals and (if your like the Syrians who grow HUGE cabbages in the stuff...) humans could be used as well as left over residues from any plant could be used as a fertilizer if properly composted.
Also one could destroy ones enemies and then drain their blood into a bucket and crush their bones in the quern and scatter it across your fields to feed your crops. This would be great for the viking marauder who doesn't like to be wasteful...
The possiblities are really endless with this game and that is why it is so amazing.