In real life, the Haber–Bosch process for creating nitrogen fertilizer from air + coal + electricity is one of the biggest revolutions in humankind, allowing for the drastic population growth that we've seen in the 20th century.

Aside from its historical significance, I think this would have some benefits to the game, namely requiring farmers to buy more recurring products (coal, electricity) from other players, and increasing the CO2 emissions associated with modern farming.

Pesticides are also another major part of modern farming. This one is a little trickier because Eco doesn't have a chemistry profession / chemical industry, but it could go under the fertilizers profession for now.

Fertilizer and pesticides should be essential to large-scale farming, allowing players to maximize yields from limited farming areas (since land ownership can be a challenge). IIRC the fertilizer system already does this somewhat, but the benefits of fertilizer should probably have an even bigger multiplier vs pre-industrial farming (which should probably have less than 100% yield).

As noted in https://feedback.play.eco/improvement-suggestions/p/too-much-sewage we should get rid of waste filter compost as it's unrealistic and unbalanced. Compost could still be made with rotten food as an eco-friendly alternative to industrial fertilizer.