IMO the invariance penalty update did wonders for the learning system. Before it the optimal strategy was just grinding the same inspirational (spinning top, most likely), which was quite boring and bot friendly. Now that we need a variety, there's a proper reward in engaging in all sorts of different content.
The reason that I mention this is because the farming system suffer from the same problem the learning system had. Influence is the main thing, I'm sure everyone will agree, and upkeep is the trash stat. So everyone just spams a ton of hay, then a bit of humus, and finishes off with chips because they're really easy to get en masse. Fields with full meters just need a ton of lime, ofc, but that's easy enough. So, the main grind in fertilizing is collecting hay. I'd say it's as much as 3/4 of the time investment in farming. This is fairly monotonous and bot friendly work. This also takes away from importance of the lesser influence fertilizers.
The learning and the farming problems are similar, so I think they should have similar solutions. IMO fertilizer types should have diminishing returns on top of the already existing ones for the fertilizer quantity. So you might open with two hay, but by then the hay penalty would get high enough for turkey droppings to give more influence than another piece of hay. This means you'd need to engage in a variety of tasks to fill the "good" slots in the fertilizer queue. And in the "bad" slots, ie where you currently use wood choppings, the combination of the quantity and the variety penalty would make chopping spam too inefficient. Naturally we'd need some new upkeep fertilizers to keep these slots useful. IMO it would be best if the new fertilizers gave different combos of effects rather than just one like hay or choppings.
When introducing these new fertilizers needed for this change, some of them could be made region dependant, to encourage trade. In another thread I suggested tying purity pushing for crops in with the fertilizing system. If this was implemented then this trade incentive would be even more powerful.