Thursday, April 4, 2013

Potting Soil

My Dad and little sister transplant Coleus
 This time of year is dirt making time.  We need potting soil for starting seedlings, planning decorative combination pots and hanging baskets and I always need some on hand for dividing perennials and rounding up volunteer seedlings.  I remember at my family’s greenhouse, in late winter, coming home to the smell of Dad “cooking dirt”.  He had a steam bed system set up in an old manure spreader.  He would mix up large batches in the spreader, cover it with plastic and sterilize it.  From there he shoveled endless yards into flats and stacked them for the transplanting crew who moved from house to house transplanting seedlings from the seed trays into the flats, comfy women in calico aprons with dirty fingernails and names like Beverly and Betty Lou.  They toted their radio from place to place, perched on old stools and marked their progress in a ledger whose pages were yellowed and crinkled from the moisture.  The warm earthy smell filled the greenhouses and our attached home and brought visions of spring to the bitter lingering winter.

Some people, I’m sure, have decent luck just shoveling regular old dirt from their garden into a pot but around here, with our clay based soil, if you do that, by the end of the summer, what you end up with is a pot shaped clay brick.  You need to amend the soil to lighten it to retain moisture and add nutrients to meet the demands of the plants with their root base limited.  You also need to solarize or sterilize the soil to reduce soil born disease which is the same reason why you should wash and bleach your pots every year.

I’m reducing my large decorative combination pots by about half this year because after last year’s extraordinarily long hot summer I’m really tired of watering pots.  Plus, the raccoons can’t seem to resist playing in them. There are several “self-watering” systems available that really do cut down on watering and I’ll show you my results further down this blog.  But these require “soilless” potting mixes which is basically peat moss or similar substance (that will wick water from a reservoir) amended with nutrient additives.  There are several recipes available on the web.

Of course you can purchase the ready mixed potting soil or soil less mix.  My favorite was Miracle Grow’s Moisture Control mix and I used to make several stops at the Home Depot each season loading my trunk with 3 or 4 of the large bags until I had carted home close to a dozen.  However, for the sustainable heirloom gardening crowd, Monsanto Corporation is the devil incarnate, and Scotts, the owners of Miracle Grow brand, are in cahoots with Monsanto so buying a dozen large bags of Miracle Grow potting soil holds as much gardener guilt as going to the bad part of town and buying the same quantity of crack.  It’s an ethical no no.  Another sustainable ethics no no, is basing your potting medium on Peat Moss, which is not a renewable resource and probably comes from far far away.  You can substitute your own compost for Peat Moss.  I mixed mine about half peat and half compost.

Once you have established your wicking base, you need to add nutrients to the soil.  The recipe I use contains Peat Moss, Perlite (used to improve drainage), Vermiculite (holds moisture), Gypsum (adjusts the pH), Blood Meal (nitrogen), Bone Meal (phosphorous) and Greensand (potassium). 

I love shopping for these ingredients and having them on my shelves in case I need them for a spot in the raised beds later on in the season.  For instance, bone meal, scattered at the roots of tomato plants and watered in with Epson Salts seems to halt blossom end rot in its tracks.  This satisfies the Mad Scientist in me.  It will also earn you the respect of your local garden center workers.  I choose organic brands and Non-Miracle brands whenever possible.  It always takes me a while to collect everything.  For instance, the only place I’ve found locally, which carries greensand, is Tractor Supply.  So I buy a year ahead when I spot the hard to find items.

I mix my soil in the loader of the tractor.  I use cup measures and cut up gallon milk jugs to measure and scoop my ingredients.  I keep the garden hose handy to dampen the ingredients to keep the dust down, use gloves, and avoid breathing the dust.


Then I fill all my planting containers, first making sure I have drilled holes in the bottoms of those which did not come predrilled.  If you think that there is no possible way that a 20 gallon pot could ever fill up with rain and flood, you are sorely mistaken.

 Now, about those self-watering systems.   I bought the ones from   They consist of a reservoir with a fill spout and the reservoir has a grid lid which allows the water to overfill and wick from the bottom of the pot midway up your potting mix coaxing the roots downward and eventually straight into the reservoir itself.

The spout has a cap, but I ended up removing the cap by the end of the season.  It’s hard enough to find the spout in a bushy pot of sweet potato vine.  You don’t need to fiddle with the cap too.  Make sure you leave the top of the tube far enough above the soil surface to make it easier to find. This looks awkward in the spring when the plants are small, but in a month, the pot will have filled in well enough to disguise the spout.  Disguise it very well.   The spout comes with a tube attached to a foam float which is supposed to tell you when you have filled the reservoir.  Just get rid of that.  You will know the reservoir is full when the water comes back out the top of the spout.  Obviously…

 My pots ended up being a bit too deep so to keep the spout high enough, I had to raise the reservoir up by placing used pots under it.  You could also use packing peanuts (not the eco-friendly kind which dissolve in water) or other plastic scrap that won’t compress too easily. Don’t use gravel.  Pots are heavy enough without it. 

You also want to avoid filling the reservoir with soil.  I used scraps of burlap to cover the grill.  The soil did bypass the side flanges and fill around the edges of the reservoir and pots below.
Last year was a very dry summer.  Through July I watered almost every morning.  The pots with the reservoir were much less maintenance. 

  I would fill the reservoir once or twice a week depending on how the plants looked.  And I did not just fill the reservoir up and walk away.  I found if I waited 30 seconds or so, the dry soil would immediately wick the reservoir almost dry.  I kept filling until the soil stopped wicking and the reservoir would take no more.  On very dry days, especially early in the season when the roots weren’t very deep, I also topped off the pot with some water to restore anything that may have wilted in the afternoon sun.  But overall I was very satisfied with the difference the reservoirs made.

I was also curious as to what was going on in there.  When I emptied the pots in the autumn, I found the roots had grown right into the reservoir and formed a thirsty mass.  They also grew down around the pots underneath, carrying soil with them and utilizing the entire depth of the pot.At the end of the season, you may be left with a root bound mass which appears to have devoured all available soil.  Some pots will have virtually untouched soil in the bottom.  I may reuse this in my pots the next season, or amend my beds with it. 

 The roots masses I generally throw into the compost pile.  The soil which appears used up, I dump into my raised beds.  The high percentage of peat and compost serve to lighten the dirt that I filled the beds with to begin with.  There will always be a “dead” spot somewhere in your beds you think could use a little help.  I dump the used soil there and cover with a thick layer of compost so it will be ready to go in the spring.