I have had a bit of a disaster with my cauliflowers

I grew 9 in a large growing container - they all developed heads and began to grow - we like our veg young so I cut one when it was about 10cm across - the next day I noticed that one had started to turn brown? i immediately removed this as it looked diseased - a couple of days later I cut another and saw that the other 6 had turned brown - had to ditch the lot - anyone know what caused this?

 

Sounds like Boron deficiency

 

I presume Boron is a trace element - Not being very experienced as a gardener I'm still on a steep learning curve. I am growing in pure compost & vermiculite, the other vege seems to be doing Ok, Broad beans, peas, parsnips even celery ??