You've probably noticed that the small refinery cannot synthesise Fertiliser. This is because Fertiliser requires 3 different materials combined. To do that you need a large Materials Refinery which allows for 4 inputs. The 3 you want for 1 unit of Fertiliser are

• 2x Ammonia
  
• 2x Potassium
  
• 1x Human Waste.

Ammonia is made by combining Nitrogen and Hydrogen.
Potassium is obtained by splitting Mars Soil on the Large Refinery using option II on the Reactions list,

I'll assume you know how to get Human Waste. ;)

Thankyou!
Where do I get the human waste from?
If it needs 3 materials, why do I need 4 inputs?

Okay, you beat me to it with my edits. LOL

You get human waste by making a toilet, fix it to the ground, fitting it with a barrel and sitting your chap down on it. There are 2 slots but I'm not sure which one is used at the moment. Only one works so you'll soon find out.

Just noticed your second part of the question.

Most Synthesised materials need 3 elements combined. Some need 2 so what I do is double up on their ingredients on the input side. It saves on some running around juggling barrels.

Excellent, ok...I've ♥♥♥♥♥♥ enough workshop bits to get this game playable...now just to solve the food issue & I'm good to go.

Once you get the feel for the game here's something that makes Human Waste more useful,
https://gtm.you1.cn/sharedfiles/filedetails/?id=848356991&searchtext=human+waste
There are lots of other mods that can enhance your experience but be careful what you go for. It's all subjective anyway, but some are really old and not maintained and could just cause problems. Some are straight out cheats and may spoil it for you. It's your choice in the end so don't feel you have to do anything anyone says.