Nutrient burn's are normaly caused by spilling water with nutes in on the leaves,i feed every water apart from when the 4 weeks of vegging is up then i flush with just water.This is just before putting the plants on 12/12 off.Then i leave the plant's to dry under the light's then add the flowering nutes,and the 12/12 off phase starts then flush again at week 4 and again at week 7 but do not add any more feed after this time.But continue to feed the plants there is no reason to water feed water feed,you will lose valuble growing time by doing this and more important valuble weight.