I have discovered that, unlike in later games in the series, here one still needs to connect raw material-producing buildings with workshops, even though the manual only states that they need to be in their service areas (p. 21).
However, this causes a problem with plantations: each field not covered by the plantation's plants decreases its efficiency. (I haven't seen any plantations with 100% workload, even if I only take the minimum 2 squares away for a road from the plantation's service area.)

Is there no way to have 100% efficiency for plantations? Or am I missing something? Maybe workload doesn't mean efficiency?

What version in the series are you referring to, 1503?

@Both of you: it is about 1602, but when I posted this, that was the only Anno game on GOG, that's why I didn't specify. I have seen this with various buildings. Some of them don't even require fertilities (for example, sheep farms also don't want to go all the way to 100%). When placing plantations which do require fertilities, I usually make sure to put them on islands where the fertility is 100%, and even when it's only 50%, I don't see the building operating at 50% workload.
Also, I don't have more raw material producing buildings than the workshops require, so it's not like they're at full capacity.

Anyway, thank you for your answer, it's good to know that those few tiles don't detract from the efficiency.

Happy Anno-ing! :)

Thanks :)
After spending I-don't-even-know-how-many-hours with 1701, 1404 and 2070, it's good to be able to play these older gems as well. (Actually, I'm finding 1503 to be surprisingly deep!)

You're welcome! Yes, Anno 1503 is definitely deep in terms of gameplay. I found it notably harder to maintain a positive balance than in the other Anno games because in 1503 you only get money from selling goods/trading. The absence of taxes means one can't just comfortably kick back and let the cash rain down over time like in 1602. Always had plenty of money in 1602. Having to keep an eagle eye on the balance in 1503 is a bit stressful at times. And the demands of those impertinent residents are endless, a bottomless pit!
One of the things that kept happening to me in the beginning was running out of tools before reaching a mountain with iron ore and getting tool production running. Even after figuring out the minimum setup to get to that point, settlers really like to plunder the warehouse for tools even more than booze, if one forgets to turn off tools supply to the settlers at the right point, the mission can end in a dead end. As for the Venetian traders...they are bums, hardly ever selling what you need.

I never completed the campaign and played open scenarios for the most part, maybe I'll try the campaign again some time. Still have the game installed but at the moment I prefer more relaxing games with less micromanaging.