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planting trees & shrubs in poor soil

Big Al
5 years ago
last modified: 5 years ago

Not a newbie to gardening, but a newbie to starting in poor soil as well as to soil building theory is here.

We are planning to plant a tree garden. The original top soil was 1 ft humus-rich black forest soils with boulders. All native tree roots were within that 1 ft top soil, even of 50-ft high trees. That topsoil was removed during construction. The subsoil is super-compacted mix of sand, silt, and clay. The fact that native tree roots were all within that 1 ft of topsoil removed makes me convinced that the remaining soil as it is now is totally UNINHABITABLE for tree/plant roots due to compaction and/or lack of organic matter.

So I am planning to buy a couple of truck loads of compost to improve that soil fertility. I reckon compost is a better investment in terms of organic matter vs questionable topsoil mix. Now the questions.

1) How deep and wide shall I dig the holes for apple trees, dwarf apple and cherry/peer trees, and shrubs? (the size of every hole will still be subject to encounters with boulders that are unmanageable for a mini backhoe i am planning to use). The rest of the area I would till probably 6"-8" deep for veggies/flowers/grass/etc.

2) How thin (% wise) can I mix the compost with the native subsoil within those holes that it would still make a difference?

3) What else could I mix in to improve the soil quality? E.g. would adding 5% by volume of wood chips (low enough so it wouldn't settle much when decomposing) soaked in an organic fertilizer increase aeration while preventing nitrogen grab from those wood chips?

4) Would it be correct to say that whatever I do with mixing of compost as long as it is enough for tree roots to go there and aerate and build humus, that deep soil could be improved later virtually as much as I want on by applying organic fertilizers to the top? Or is that a one time deal and you get what you paid for? Do I understand the soil building process correctly that any roots would build organic matter within soil via nutrient exchange with microbial population, or does it only happen via root or plant matter decay?

5) Would it be correct to say that whatever I don't dig out and mix with compost will never in a lifetime become fertile due to compaction and lack of humus? I.e. the roots will never be able or want to penetrate beyond the excavated and mixed hole and would never aerate and add carbon and sugars for microbes to leave on?

6) considering all the above, what should be the good balance between quantity (want bigger root mass thus need bigger holes) and quality (bigger holes means spreading compost much thinner thus decreasing the quality)

Feel free to refer me to any posts that have previously discussed this (but i could not find any).

Thank you in advance

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