Redstone circuits in spawn chunks in Minecraft SMP and no players online

I tried to search this question, but it seems to be rather specific.

The problem I have is the following:

We have a small SMP server in Minecraft and I set up an iron golem farm
according to

(only redstone in the hopper sorter system) and an automatic wheat farm according to

(minecart running beneath the farm and a redstone circuit for the unloading as well as a hopper sorting system). Both farms work very well as long as a player is online.

Of course, they stop working after the somewhat 60s after the last player has logged out. So I went and read a bit about that topic and came up with the idea to send an item through a nether portal every 45s via a piston/hopper clock plus a dropper. This kinda seems to work, since my iron golem farm is drowning itself in iron. However, the wheat farm seems to stop working when no one is online, as well as any minecart/redstone builds in the spawn chunks. Is there any mechanic at work here that I overlooked?

Answer

To start things with, let me clarify something from the comments on the question. Placed(fixed), in the world, blocks, items and even flowing water or lava, are not considered entities.

Per the minecraft wiki

Entities encompass all dynamic, moving objects throughout the
minecraft world.

Here is a list of entities in minecraft:
Types of Entities

The reason your wheat farm is not working, is because crops require at least one player to be in their chunk update radius.

Basically, your farm system works, but because the crops never grow, the villagers can’t replant anything.

Your iron golem farm is working because, it does not require any player to be in its chunk update radius.

Events processed in spawn chunks

Some extra information on crop growth requirements:
Crop growth requirements

Something to note about spawn chunks:

Spawn chunks will not be unloaded from memory, regardless of how far away a player moves, so long as there is at least one player in the
overworld.
All overworld chunks (including the spawn chunks) are
unloaded 60 seconds after the last player in the overworld enters the
Nether or the End.

In your case, I believe that the chunks do not get unloaded, due to your sending an item through the nether portal(items on the ground are considered entities).

Attribution
Source : Link , Question Author : Prof. Farnsworth , Answer Author : Swtos

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