In the context of the entire recitation and those around it, this refers to the dead king being one with Ra and having his powers. Whether that particular line is translated as making lapis lazuli grow, or cultivating lapis lazuli, it means that he is creating lapis lazuli, just as he creates the acacia in the next sentence, and controls Upper and Lower Egypt, and the Ennead. Recitation 320 further shows that this is about the power of Ra. One of the lines can be translated in more normal language as Ra stating, "I am greater than all other gods, mortals tremble at my might". So, generally, he creates minerals and vegetable matter,controls all of Egypt, the other gods, and mortals are in awe of him. It may be that lapis lazuli is mentioned because the blue colour may be referencing the blue lotus, on which Ra was born up out of Nun, and the Acacia has a function in other parts of the PT regarding the resurrection of the king.