If your plants that are in soil are only 2 weeks old then they dont need any fertz just yet .
Need abit more info , like soil type .
Most soils carry enough nutes so as you wont need to feed them for 4-5 weeks and the seed also has enough feed in it to see it through a few weeks so if i were you then i would just give water .
If you start nutes now there is the possability that you will burn your plants .
Remember cannabis plants are weeds that grow without any nutrients , And people that grow weed add nutes to get the best out of the plants to enhance potency and yield , so the nutes come in to play round about week 6-7 veg time for me . As for your measurements of feed , i would say its per gallon as on the bottle it says 3tsp , and if you were to be doing your measurements in ltrs it would say 3ml .
Were are you from ??? In England we use ml to ltrs and across the pond they use gallons