3 week old from seed, northern sole

hi 3 week old from seed, northern sole..
everything is fine, just round the edges on the larger leaves they are browning a little..

wat could this be,

i presume my soil has to many nutes in and has burned it a little..

but ive grew 2 other strains in same soil few months ago and they took off fine..

i wunder wat it could be its a hybrid strain light green northern sole indica..
cheers