A new take on potting soil

There's a Goldilocks Principle for everything, including potted plants.  Realizing this, I have a new (for me) philosophy for potting plants.  I needed one since I had a dangerously cavalier attitude to the potting soil I used: Recycled soil from other potted plants?  OK.  Running low?  Add some garden soil.  Fertilizer?  Yes or No, but probably not at the right times.

I've never been great at keeping plants in pots. Short term I was ok, but long term I nearly always failed, or at least never met with rousing success. Several potted citrus have languished and never looked healthy after the first year.  Natives didn't last, succumbing too easily to root rot or at best existing without vigorous growth.  Root systems on the surviving plants were often shallow.  Maybe my soil wasn't draining well enough?  The signs were there. Of course my containers had drain holes, but I had noted stunted root development in some older plants that might be due to compressed, saturated, or boggy soil at the bottom of their containers. I also noted some containers that dropped their soil level by 50% in a year.  Was that due to loss of critical interstitial spaces that provide passage for water and air?  Of course I watered based on the top inch of soil and plant appearance, but I had noted some pots where the soil in the top half of the container was dry, but the soil near the bottom was wet.

I undertook a repotting job at Juli's that coincided with some online browsing and I began to possibly get a little smarter.  Potted plants are particularly vulnerable to water issues.  There's a Goldilocks principle for best root and plant development and I'd been completely ignoring it, assuming that it mattered far less than it apparently does.  The philosophy that I've adopted now is that potted plants need porous media to grow in - not any old bag of potting material and definitely not garden soil.  And really I should have known it all along.

Pores in soil provide good drainage and air circulation so that roots develop optimally.  Happy roots = happy plant.  If a soil is too saturated with water or the soil too densely packed, then roots will be oxygen starved and/or water-saturated and they will not flourish.  In your yard, apparently the connection between the root zone of a plant and the rest of the universe is sufficient to keep plants happier.  In a pot, the connection to the rest of the universe is through the drain holes at the bottom and the open space at the top, so the majority of the space in the pot had better be good living.

Good living = good drainage with adequate water retention and airflow.  You get this by creating interconnected voids/pores/interstices within the soil by using many relatively large and irregularly shaped objects within the soil.  I stumbled across some posts on Gardenweb extolling the virtues of Al's Gritty Mix and Al's 5-1-1 mix which have these properties, using a uniform mix of irregular particles of about 1/8" in size.  A useful starting point and a treatise on potting soil is located on GardenWeb in the container gardening forum: Container Soils - Water Movement & Retention XIII.  I recommend it. Container Soil Basics: a compilation is a useful adjunct which answers commonly asked questions.

I ended up following simpler guidelines from Pete Vielleux who owns a native plant nursery, East Bay Wilds, in northern California and contributes to the Gardening With Natives listserv that I follow.

Pete's Soil Mix for California Native Plants in Containers

1 part 5/8-1/2” horticultural pumice [can substitute with same size red lava]
1 part fine or sharp sand [Felton Sand]
2-3 parts potting soil [soilless mixes sold everywhere are fine]. Pete uses American Soil Products’ Ultra-potting mix for drainage and low-nutrients.  The ingredients are "Coconut Coir, Horticultural Sand, 1/8″ Red Lava, Dolomite Lime, Nitroform 38-0-0, Iron Sulphate, Triple Superphosphate, Calcium Nitrate"

In contrast to the Gardenweb recommendations, Pete's mix has a varied size distribution of particles.  For example, Felton Sand has a fairly uniform size distribution with particles less than 1/8" in size.  I'd like to think that I made a smart choice by substituting Builders Sand, which is irregularly shaped with a wide size distribution up to about 1/2" and should therefore contribute more to drainage, but the truth is that it was what was available at the big box store. One might think that the Felton Sand recommended by Pete will tend to fill in soil interstices rather than create them.  I also found the pumice impossible to find in the recommended size, even at my favorite nursery, and used red lava rock instead, again from the big box store.  My mix ratio was 1:1:2 by volume ratio with EarthGro potting mix. 

I'll be (re)potting plants with this mix and will hopefully have a success story to tell.

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