that is WAY too small for 20 days old...and that pot is WAY too big to start a seedling in, and that soil has very little drainage...
Unless you paid big bucks for that seed I'd chuck it and start over...this time use a small pot to start the seedling in, no nutrients in the soil and plenty of drainage(lots of perlite), you want soil that doesn't just sit there for days wet. Good way to test is to take a party cup(plastic solo cup), cut holes in it in the bottom, fill it with soil, wet it down and set it in a windowsill that gets sun. Notice the weight of the cup before and after putting the water to it, let it sit for 24hrs, lift the cup, does it appear about the same weight as before watered or even a little lighter? Then its probably good to go for soil. If its still as heavy as the previous day after 24hrs add more perlite and try again...
Now comes the nutrients to your soil...you don't want something thats going to fry the shit out of your little seedling...it doesn't need any nutrients for the first 2-3 weeks of its life...it has those first two leaves that come up that give it the nutrients it needs(yours are browning at the moment so its been sucked dry)...now if the root system didn't get a chance to develop before those die off yer pretty fucked and your plant will be stunted(ergo why i said just start over)...if you have no idea what is in your soil for NPK pickup a soil test kit that includes telling you how much nutrients are in the soil...a little more expensive but it tells you what you are working with. If you just want to grow, pickup some general potting mix at any hardware store and it should be fine as long as you add extra perlite to it.
So, start your seeds in smaller containers, with no nutrients in the soil if you can(they make seedling soil for this), then transplant to a larger container in 2-3 weeks after the root system has really started to grow...there is also a guide in the newbie section that is a sticky near the top that says "How to Grow Marijuana" read down through that