PROBLEM - CORDYLINE AUSTRALIS PEKO

Firstly, from the title, this sounds like I know something about these plants. I can assure you I don't. I merely typed in what it said on the label!

I recently had a new patio laid and purchased four of these plants mainly for their architectural interest. I replanted them in individual pots one foot square by 16inches deep. For effect I covered the soil with a breathable plastic membrane and arranged large white ornamental pebbles on the membrane. The plants grew well and looked splendid. Recently, the leaves, closest to ground level, (new growth) of all four of the plants have started to turn yellow, then brown and eventually die. The rest of the plants, about a metre high, have perfectly healthy green leaves. If you're not familiar with this plant it looks similar to a palm with thin, spiky, tough leaves. The plant is a native of Australia/New Zealand but grown here in the UK. Now, when the leaves turn yellow I cut them off as they look unsightly. 

Is it normal for leaves closest to ground level to turn yellow and die while the rest of the plant looks perfectly healthy? Instructions on label; Feed with a suitable granular fertiliser in Spring. Planted in multi purpose compost with Growmore and chicken manure.