RickCorey's blog: Screening Pine Bark Mulch to Lighten Potting Mix and Seed Starting Mix

To loosen and improve the aeration and drainage of commercial potting or seed-starting mix, mix the commercial stuff with lots of Perlite, chicken grit, screened crushed rock or screened medium pine bark mulch or fine bark nuggets. 

To improve drainage, grains should be around 1/8 inch.  Say around 2-4 mm if they are rounded, or longer & thinner if they are chips or shreds or fibers.  Unusual shapes really open a mix up!

That's why "sharp" crushed rock drains better than river sand with similar grain sizes.  But if you have to choose between clean nuggets and soggy, smelly, dirty shreds, use small clean nuggets.

 Bark shreds and chips are better than rounded nuggets for increasing aeration and drainage. They really open a mix up!  But if you have to choose between clean dry nuggets and soggy, smelly, dirty shreds, use the small clean nuggets.

1.  Pine bark chips' largest dimension should be long: 1/8 inch to 3/8".  Say 3 mm to 10 mm.
2. The middle dimension could be anything from 1/16  inch to 1/5 inch.  Say 1.5 mm to 5 mm.
3. The small dimension ideally would be thin  - 1/32" or less up to 1/10 inch. Say less than 1 mm up to 2 mm.
4. In order to have sufficient water retention, you must have some fine bark particles or peat. I add 5% to 20% commercial peat-based seed-starting mix or fine potting mix.

As a rule of thumb:

1. Discard what will not pass through ½" hardware cloth - those chunks are too big for small pots. 
2. Use most of what is held back by ¼" screen. The bigger bits are fine for large pots, but too big for seed-starting in cells.
3.  Discard some of what passes too easily through ¼" hardware cloth: it's too fine to improve drainage. 
4.  Discard most of what WILL pass too easily through ⅛" mesh.  That's dusty. If you want fibers that fine, add a little peat or commercial potting mix instead.

 Screen quickly by tilting the screen 30 or 40 degrees and pouring the mulch over the screen so it runs slowly down the slope with a little encouragement. That's what passing through "easily" means.

Screen slowly by laying the screen flat over a wheelbarrow. Add just 1-2 spadesful. Now rub that around with gloved hands or the back of a steel rake until no more falls through the screen.  Dump the coarse stuff to one side and repeat.

Screen less aggressively by propping the screen at a slight angle and rubbing it around only briefly before dumping the coarse stuff.  You can shake the screen briefly or tap it up and down  a few times. 

Rather than waste hours by starting with ⅛" screen, first pass the mulch or nuggets quickly through ½" and ¼" mesh.  Most of what passes ½" but is held back by ¼" is good to use in containers.  Then use the ⅛" mesh on the small amount of bark that passed through ¼" mesh once or twice. 

Reduce the amount of bark dust by starting with medium mulch, not fine mulch.  Most "medium nuggets" will be too large to use in small containers, but "small nuggets" may have more dust and fines.

 If you have a lot of dust in your final "cut", and a lot of patience, you can lay window screening on your mesh and remove some of the dust that way.   If you find a source of 16 mesh screening - or anything coarser than 24 mesh - please let me know!

Don't use pine bark that was stored wet in a plastic bag if it smells bad - anaerobic fermentation products are acidic and bad for root hairs.  If you must use "smelly bark", flush it with water and let it air out first.  Or compost it for a few weeks if you don’t need clean sterile mix.

Don't waste any too-coarse or too-fine bark.  Coarse bark makes great mulch for top-dressing.  Fine bark can be mixed into outdoor soil, such as a raised bed. You can always clean your lawn mower and use it to chip coarse chunks down to medium chunks.  I think pounding on them with a brick would work, too.

If you plan to mix a huge amount of fine bark into a bed, consider supplementing it with some nitrogen source or composting it first. It will consume a little nitrogen as it breaks down over several years, but this nitrogen deficit is nowhere near as severe as that caused by sawdust.  Bark contains a little nitrogen, and it breaks down slower than wood.

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