After putting potatoes into the soil Mark poured some yellowish liquid into the soil.
What was that liquid and why did he do it?
In the film's source novel Mark is still struggling to work out how he's going to come up with the needed water for the potatoes.
I cut each potato into four pieces, making sure each piece had at least two eyes. The eyes are where they sprout from. I let them sit for a few hours to harden a bit, then planted them, well spaced apart, in the corner. Godspeed, little taters. My life depends on you.
Normally, it takes at least 90 days to yield full-sized potatoes. But I can’t wait that long. I’ll need to cut up all the potatoes from this crop to seed the rest of the field.
By setting the Hab temperature to a balmy 25.5°C, I can make the plants grow faster. Also, the internal lights will provide plenty of “sunlight,” and I’ll make sure they get lots of water (once I figure out where to get water). There will be no foul weather, or any parasites to hassle them, or any weeds to compete with for soil or nutrients. With all this going for them, they should yield healthy, sproutable tubers within forty days.
But he later makes the point that plants will process urine directly with no ill effects.
Do I generate 10 liters of water per day? No, I’m not the urinating champion of all time. It’s the crops. The humidity inside the Hab is a lot higher than it was designed for, so the water reclaimer is constantly filtering it out of the air.
I’m not worried about it. If need be, I can piss directly onto the plants. The plants will take their share of water and the rest will condense on the walls. I could make something to collect the condensation, I’m sure. Thing is, the water can’t go anywhere. It’s a closed system.
That being the case, the grape juice that you saw him drinking in the previous scene is probably what he's using (after he's peed it out) to moisten and activate the newly planted potatoes. Note how dry the soil is. Without additional moisture the potato will simply dry out and die.