引用自 Daulken：

Here's an example, let's say I start with a coal deposit, and I plop down a mine there, cause a bunch of pollution and then mine it out entirely. Then I destroy the mine and I want to put a plantation there. Let's say it's completely ruined and I get 0% efficiency. But, if I put a manure spreader, will it raise it back to whatever was baked into the map, or will 0% be my forever max for that plantation?

The only beneficial thing that manure spreaders do within the game is counteract 'soil degradation,' and the only things that cause soil degradation within the game are plantations and ranches operating on the default work mode. As such, the only benefit to building manure spreaders in farmland which was formerly a mine or other industrial site is that doing so will allow you to use the default work modes on ranches and plantations in the area if that is something that you want to do.

Soil conditions will return to the 'natural' level over time if whatever caused the deviation from the natural level is removed.

引用自 Daulken：

What is the actual cap of efficiency for a plantation?

Depends on plantation upgrades (which is a function of crop type) and work mode. Off the top of my head:
- Natural fertility and current soil condition gives at best a 0-point penalty.
- The Multiculture work mode at best gives a 40-point bonus.
- Nearby Sugar Plantations or Sugar Hydroponic Plantations can give up to a 20-point bonus.
- A nearby Beekeeper can give a 10-point bonus.
- A nearby Cattle Ranch or Cattle Factory Ranch can give another 10-point bonus.
- Agricultural Subsidies give up to a 35-point bonus.
- Budget settings give up to a 25-point bonus.
- One of the ministers you can appoint at the Ministry gives a 7-point bonus to buildings on max budget.
- Some crop types have upgrades that improve the plantation's efficiency. Coffee can get a 10-point bonus, Corn and Rubber can get 20-point bonuses, Cocoa can get a 30-point bonus, and Tobacco can get a 50-point bonus from upgrades while Pineapple can get a 3-point bonus for every "nearby" pineapple plantation.

Assuming I haven't forgotten any efficiency bonuses, this puts the cap at 247% efficiency for Banana and Cotton, 257% for Coffee, 267% for Corn and Rubber, 277% for Cocoa, and 297% for Tobacco, though these numbers are somewhat unlikely to be achieved in practice.

Biggdrasil (Return to Nature wonder) and Water Wells / Water Towers (Going Viral service buildings) can improve soil conditions above the natural level, but it appears that these cannot improve the local conditions beyond 'ideal' and so these can only remove a penalty.