hole for tree

[kbolis]

I live in Pokhara, Nepal and I work with an NGO and we have a few trees of which some are fruit trees! the soil is sandy and rocky! some native trees are Sisou and bombax ceiba commonly known as cotton tree.
I want to dig a big hole and fill it with nice rich soil then plant some fruit trees there! I was thinking at least 10cu. feet. I thought it would give the tree a good starting point knowing that the roots, of course, will reach out far more than 10 cu. feet! What is a good soil/ fertilizer mix??? And what size of a hole is your recommendation for such a feat? I have some trees that are a little too big for removing so I thought i could dig around it and replace some of the soil for a better quality soil!!?

 

[BC WetCoast]

The usual planting hole size is twice the size of the root ball, however, the larger the better. If your natural soil is sandy, you may want to amend it with some silt or clay to increase the water holding capability. Then you should also add some organic matter, such as well rotted compost. I would add some slow release fertilizer. We generally use 30-10-7. You should also add some bone meal to help with root growth. I usually just spread a few handfuls with the backfill soil being amended.

For the bigger trees you could do what is called vertical mulching or radial trenching. In radial trenching you dig small trenches radially away from the base of the tree. Try to not cut any roots, then back fill the trench with amended soil. With vertical mulching, you dig 15cm holes about 30 cm deep on a 1m grid and then backfill them with amended soil. Do that just past the edge of the canopy.

Once you've amended the the soil, you should mulch the soil surface.

You can find useful tips at the ISA website, www.treesaregood.com