what cuases leaves to turn yellow in small plants?

on all 3 of my plants, the first set of true leaves, the bottom most true leaves are all somewhat saggy. why is this?

but on just one of my plants, both bottom, or first true leaves have turned yellow. one is worse, its turning yellow and now the tip has dried and will probably crumble off.
why is this?

also, the soil i got, had everything in it, organic compost, pearlite etc. but it is a soil thats for flowering. i planted it anyways, my brother brought me the wrong kind of soil. i figured, well it will grow the plants anyways. i thought maybe because its a flowering soil that it might have a nitrogen defeciancy? but its only happening in one plant. also the plant is in a small cup right now, could a small cup make the difference? but the plant is sitll small i dont think it would matter. but idk.

thanks for the help

A pic would be nice but I have experienced the same problem and it was either due to N deficiency or the PH was too low. PH your water and maybe use a very low strength organic fert on them.

ah ok i see, yeah im not that terribly worried about it right now, its just a few leaves on one plant, but you know, i just wanna know for the future in case the problem gets worse. thanks for the info.
so then what would happen if the ph is too high? and also how can i raise soil ph? thanks again

Most of the time the ph is to high. High ph will cause nute lock out where your plants can't extract the nutes out of the soil.
Yellow leafs coukd also be too much water.

I just had a 1.5 week ld seedlin that turned pretty yellow so I added a full compliment of nutes to my hydro grow and she cleared up in 3 days. Keep on top of Temps, PH and feeding schedules and 90% of the work is done. The bottom most leaves (sprout leaves) WILL die of and shrivel so don't sweat them, it's the true leaf tips u want to pay attention to for early signs of def. or disease. Hope this helps, good luck w your grow!