Between my drivewaw and the fence with my neighbor I have a strip of dirt about 3.5 feet wide where I plant some ornamental plants. I have 2 australian tree ferns planted about 14 feet apart.I bought them 2.5 years ago at Home depot.

One was 12 inches tall. The other was 18 inches tall. Both of them grew a lot.The one that was 12 inches tall is a monster.It has huge size leaves and keeps growing fast.The one that was 12 inches tall keeps growing but at a slower rate.They both have received the same treatment as far as watering and fertilization.The one that was 12 inces tall is a little yellowish and shows some burns at the tips of the leaves.I have added ammoniam sulfate and iron granules.The one that was 18 inches tall is very green keeps putting out branches or leaves at a faster rate than the other. Any ideas as to what might be causing these differences between both.

Note:The soil is very permeable in both cases and the PH is slightly different.