Does anyone know why my bottom leaves of my plants are turning yellow & crispy?

[PestoMonster]

I have 2 troughs of nasturnans which are blooming marvellously, but their bottom leaves are going yellow & some of them seem to be turning crispy & dying. Also the bottom leaves of my runner beans are going yellow (but not crispy). Is this normal or am I doing something wrong?

Loads of mine are like that, I just pull themoff

Ok, a bit like dead-heading then? Anyone else any the wiser?