Bob, 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 following: 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. Tim 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 [mailto:firstname.lastname@example.org] 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?