Best soil for a raised bed?

Tim Eck
Mon, 21 Jul 2014 06:32:57 PDT
Air porosity isn't the only reason roots fail to transition into denser
soils.  With the help of other volunteers, I have planted over ten thousand
American chestnuts and (American x Asian) x American BC1s and noticed the
1. They will grow in well drained sand or clay but not in swales.  
2. Nearly 100% success rate direct planting seed into clay and nearly 100%
failure rate transplanting seedlings with soil into clay.
While I have heard a lot of myths regarding this phenomenon, I think we need
to resort to 'plant psychology' to fully explain it.  
The single most urgent purpose of a root is to secure a reliable water
supply.  For this reason, they will follow a moisture gradient toward higher
moisture and they will be growing primarily during high (meaning adequate)
moisture periods. When they encounter an obstacle (e.g. a rock), the growing
tip will divert around the rock with one exception - they generally will not
turn an obtuse angle, but will bud another lead if the growing tip grows
into a corner or nook.
The above motivations explain the behavior of roots in plastic pots versus
clay (or fabric) pots.  Plastic pots are prone to 'spin', where the roots
will circumnavigate the lower periphery of the pot several times, seeing the
impervious plastic as a rock.  (Earthenware) clay pots and fabric pots would
also generate this behavior except that their periphery wicks and evaporates
moisture, removing the moisture gradient motivation.  Non-porous stoneware
pots will also exhibit spin as will a transition from a porous potting
medium to a denser clay hole where the dense clay will resemble a rock to
the growing tip.  
Chestnut trees planted with potting soil intact would circle the hole with
their roots several times until a dry spell killed them.  Now we know to
plant everything bare root.

P.S.  It's worth noting that the 'porosity' is greatest -up to 50% - when
the particle size is all the same.  Although absolute size of the aggregate
doesn't enter into the calculation, it will determine the ratio of 'air
porosity' to 'water porosity' with larger aggregate favoring air porosity.
Depending on your pH requirements you may want to look at 'sand bed'
aggregate from a local quarry since it is washed of dust and fines to
promote optimum porosity.

-----Original Message-----
From: pbs [] On Behalf Of penstemon
Sent: Sunday, July 20, 2014 1:20 PM
To: Pacific Bulb Society
Subject: Re: [pbs] Best soil for a raised bed?

At this point, I feel like I almost have to try a sand bed, just to see
what'll happen. Will the roots grow throughout the sand, dive down into the
clay, or descend to the top of the clay and then spread horizontally?

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