1- If you blanch aplany, you keep it in the dark, or reduce the amount of light it gets to keep it whiter. 'Fill in' basically means fill the trench. 'Earth up' is to pile soil up to block the light.
2- 'to hand' means near you, in a convenient place
3- It's liquid plant food to give extra nutrients, usually added to the water
4- It means that is is ready early in the seaon, before the majority of types of celery.