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Required Soil Depth For Forest Garden?  RSS feed

 
Shane Quackenbush
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chicken duck forest garden
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Hi all, i have about 60 acres that i'd like to slowly turn into a forest garden, but the soil depth to bedrock is as little as 5 ft at points, are there trees that i won't be able to grow here because of taproots? I'm think Black Walnut, White Oak etc? Or will they be able to adjust? I'm just looking for trees that i need to avoid with that limitation. The soil itself is quite good, and has very good drainage. (We have the hay taken off right now)
 
Todd Parr
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Location: Wisconsin, zone 4
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Most trees roots are in the top foot or so.  They are many areas around here that the rocks are sticking out above the surface and have no appreciable topsoil and the trees look to be doing just fine.  There are many trees growing directly on huge rocks with the roots wrapping entirely around just to get to soil at all.  I think you'll be just fine.
 
Marco Banks
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books chicken food preservation forest garden hugelkultur trees urban woodworking
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5 feet of soil is way more than you need for almost any species of tree.  2 feet is usually enough.

Oaks have the capacity to go through limestone bedrock by exuding a chemical compound through their roots that softens the stone and allows their roots to penetrate.  There are fissures and cracks throughout most bedrock.  Roots will find their way through those cracks, further splintering them and finding moisture as needed.  When it rains, all that water is percolating through somewhere.  The tree roots will simply follow the natural flow of your hydrology.  And fungi has been shown to decompose rock in order to mine the minerals therein.  As the soil beneath your trees becomes increasingly fungal dominated and colonized by symbiotic root-associated fungi, the rock below will not be a negative thing, but a positive thing for your trees.

Fruit trees rarely root much deeper than 2 or 3 feet.  Just dig around lightly along the drip line of an apple, peach, plum, apricot . . . and you will see the thick feeder roots that are only 6 to 12 inches below grade.  Their tap root doesn't go that much deeper -- maybe 3 feet or so.  This is because most of the soil life and nutrients that the trees depend upon are in those upper 2 feet.  That's where the worms, bacteria, fungi, and other biota live. 

So plant away and celebrate the 5 feet of amazing soil you have.  It will only get better as your food forest matures.

Best of luck.
 
Shane Quackenbush
Posts: 3
chicken duck forest garden
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Great info! Thanks a gazillion!

Marco Banks wrote:5 feet of soil is way more than you need for almost any species of tree.  2 feet is usually enough.

Oaks have the capacity to go through limestone bedrock by exuding a chemical compound through their roots that softens the stone and allows their roots to penetrate.  There are fissures and cracks throughout most bedrock.  Roots will find their way through those cracks, further splintering them and finding moisture as needed.  When it rains, all that water is percolating through somewhere.  The tree roots will simply follow the natural flow of your hydrology.  And fungi has been shown to decompose rock in order to mine the minerals therein.  As the soil beneath your trees becomes increasingly fungal dominated and colonized by symbiotic root-associated fungi, the rock below will not be a negative thing, but a positive thing for your trees.

Fruit trees rarely root much deeper than 2 or 3 feet.  Just dig around lightly along the drip line of an apple, peach, plum, apricot . . . and you will see the thick feeder roots that are only 6 to 12 inches below grade.  Their tap root doesn't go that much deeper -- maybe 3 feet or so.  This is because most of the soil life and nutrients that the trees depend upon are in those upper 2 feet.  That's where the worms, bacteria, fungi, and other biota live. 

So plant away and celebrate the 5 feet of amazing soil you have.  It will only get better as your food forest matures.

Best of luck.
 
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