I have been adding bentonite, zeolite and spongolite along with compost to our sand when planting since I found homeone about 12-18months ago. My question is regarding some old planting we have. We have a garden bed across the back of our house that is approximate 12m long by 1-1.2m wide. We planted this nearly 2 years ago with hibiscus cottonwood rubra, a frangipani cutting and a lot of dianella streetscape's (hybrid I believe). Originally the only amending to the soil we did was a lot of sheep poo mixed down to about 300mm and let it break down somewhat, Then a layer of black mulch (I know, I know, how despicable).After finding homeone I mulched with fresh tree lopper mulch over the black mulch which was very thin anyway. The growth was fantastic for the first year but since then the hibiscus has slowed a bit (not too much), the frangipani is fine but the dianella's are really struggling. A couple have died completely so I did some digging around yesterday and below the tree lopper mulch the black mulch seems to have broken down and there is a layer of approx 5cm of seemingly ok soil but below that is just sand, all the manure has broken down and been used by then plants/washed away.
I really want to fix the soil without pulling these plants out so what are my options and what sort of quantities should I be adding to the soil as I can't exactly run a rotary hoe through the bed, Or am I best off pulling out all the dianella's, amending the soil properly and starting again.
The pictures below are when first planted and three months on. The hibiscus are now a little over 2m high and the healthier dianellas are around 350-400mm high.
This was it when first planted
http://i20.photobucket.com/albums/b205/rhysy_boi/Firstplanted_zps9e74d750.jpg
And three months later
http://i20.photobucket.com/albums/b205/rhysy_boi/threemonthson_zps402b62bf.jpg