Bab 152: Master

The spells Emery had learned had taken from a couple of minutes to two hours at most. At first, Emery believed he had fully mastered [Darkness Infusion], however, according to Magus Xion, learning how to cast a spell was different from mastering the flow of spirit energy. This was what Emery was trying to learn by repeatedly casting the infusion spell into the long blade of grass. 

Normally, when an inexperienced acolyte casts a spell, they either put all they can or only a fraction of their spirit energy into the spell. This resulted in a highly inefficient method. The reason why the grass was shattering or not lighting up at all with the darkness element was due to the unstable flow of energy.