One way to envision this boundary effect: If you have identically sized spheres their closest packing is about 74%, leaving about 26% voids. If you then place much smaller spheres in contact (think gravel and sand), the sand will infiltrate the gravel voids, taking up 74% of this 26% =19%. So, now the contact zone is 93% solid and 7% void. Tim -----Original Message----- From: pbs [mailto:pbs-bounces@lists.ibiblio.org] On Behalf Of Alberto Sent: Friday, January 23, 2015 8:51 AM To: Pacific Bulb Society Subject: Re: [pbs] gethyllis cold tolerance (Rick Buell) Putting ingredients of a potting mix in layers slow down drainage instead of improving it. This has been researched for long. Water will percolate by gravity through a layer but when reaching the next it will collect making the mix stay wet for longer than planned through capillarity. This s the reason why the best potting mixes are a combination of not water retaining materials, so excess water is rapidly forced out.