I have 3 crewmembers assigned to the greenhouse, but the last plant I added shows as decaying.

If I remove one crewmember the N2O plant also shows as decaying.

Is it possible to plant 5 plants with 4 atmosphere or is there a limit?

under the greenhouse there's a slot for a module to add another crewmember,these can be found on missions

If memory serves, the plants will show yellow if you have 1 or no attendants.

You need at least 2 attendants for them to show all green, preferably 3. At least for O2 plants.

From the Wiki:
You need to assign at least the same number of engineers as the highest tier number, a plant is, you want to grow in the module. For example: Tier II plants need at least 2 engineers.

So plants have different tiers: genesisalphaone.fandom.com/wiki/Plant

If you want to grow a tier 3 plant like your N2O (Fungoon) you need 3 engineers.

Whats odd though is that your T2 plant is decaying despite having 3 crew on duty.
Maybe it's the mix of different gases that screws things up.