Sounds like a N deficiency to me if it's just the bottom leaves and if they aren't curling or anything.

You can add N to soil by using some slow release fertilizers that are high in Nitrogen. You can tell the nitrogen content of a fertilizer by looking at the three big numbers on the bag (also known as the NPK; nitrogen, phosphorus, potassium). If you are going to general fertilizer, use slow release fertilizer so it won't burn your plants. Also, though human urine is full of soluble nitrogen, I wouldn't pee on my plants because I wouldn't want them to absorb my piss. In the end, it is your decision.

If you want a super easy way to grow soilless, search "hempy buckets" on this site.

Peace,

SMG