There are technologies that you need to get hold of to make a planet grow i believe, one of the earliest of which is called "soil enhancement", it allows you to build more improvements on the planet and i'm thinking should up the population in the process.
The improvements you're talking about are farms, which increase planet population capacity by increasing the amount of food in mt/wk increments. 1 mt/wk = one billion (1.00 b) more population capacity.
Soil Enhancement/Habitat Improvement/Terraforming open up Yellow/Orange/Red un-buildable tiles respectively, and make them Green buildable tiles, which
can be used to build those farms, just like you said.
But...
"Naked" Mars, with no improvements (no farms, no nothing)
except "Initial Colony," which all 'fresh' colonies start off with... can support 8 billion (8.00 b) people. This is because the "Initial Colony" building produces 8 mt/wk of food (among other things).
All colonies... unless you do any of the following:
- have the Super Breeder ability
- build 'growth' buildings
- manually send more colonists from another planet
- manually stuff more people into your Colony Ship/Transport in the first place
start off by growing VERY slowly. Then... as the population rises... their growth rate rises. Provided, of course, that you have high enough approval (Mars was at 100%), and the capacity to feed them (Mars produces 8mt/wk food). Natural enough.
Yet not all of those options are always great, though I suppose I 'could' spare some of Earth's early pop to fill out Mars. But anyway, as a technical matter, I shouldn't have to do any of those things. They'll just speed things up, which is fine if the problem is
slow growth. My problem is
stopped growth. According to what the game says... and in fact
does on other planets, Mars shouldn't just
stop growing at 2.63 b, then randomly decide to reach capacity later, even though it's had the food and the approval to grow to 8.0 b the whole game long.
My problem is that Mars completely
stops growing at 2.63 b (8.00 b). After it grew just fine all the way to that number. It just
stops. For a looooong time, dozens to hundreds of turns. Every game.
Even though it has the food to feed an 8.00 b capacity, and is at 100% approval. Approval being the factor that affects a planet's growth rate... in other words 'how fast' it can reach its current pop capacity.
Earth's approval rating is much lower, but that doesn't affect Mars's growth rate. Earth's approval rating only affects Earth's growth rate. Therefore, it isn't because of farms or morale.
In any case, it eventually solves itself and starts growing WAY further into the game, even though I've done nothing to change it's situation, including build any farms. I don't know if this is due to discovering a tech, but that would seem odd since other Planets seem to grow to 8.00 b fine, or 16.00 b if they were someone's Capital (The "Civilization Capital" building produces 16mt/wk of food). Just Mars likes to sit at 2.63 b for a long time until it decides to grow.
In fact, I've only ever once or twice built a farm ever in any game of Gal Civ II at all, but it was on Earth, not Mars... and only because Earth had a magic double food tile. I used the extra population to keep invasion Transports cranking out almost all game long, because if I didn't keep offloading the extra population, I would have had to demolish some of Earth's factories and build approval buildings to make all the extra people happy.
although to be honest i think i was probably only accidently helpful as i'm still very much learning the game myself hehe.
