Before I jot down the recipes that serve as good starting points for the 5:1:1 and Gritty Mix, I'll explain a simple concept that will help those of you who build their own media and help others understand some things about what happens when we try to amend media we feel is too water retentive.
In order to take advantage of media that offer fast drainage and superior air porosity, it's essential that the the media are based on a very large % of coarse material. The reason for this centers on something called 'the threshold proportion'. The easiest way to explain how it works is by providing an example that can be 'seen' in the mind's eye. Start with a quart jar, half full of marbles. We can all 'see' the large pores between the marbles. Now, start adding peat/ coir/ sand/ other fine material (PCSO) to the jar. What happens? All the large pores between the marbles are beginning to become filled with PCSO. If we keep adding PCSO to the jar until there is exactly enough fine material to fill all the spaces between the marbles, we have arrived at 'the threshold proportion'. The threshold proportion is the most difficult proportion of media ingredients to grow in because the marbles reduce the amount of space available for root colonization and the only part of the medium available for roots to grow is in the PCSO; so, where the roots must grow, the level of aeration is the same as what it would be without the marbles, meaning the coarse material (marbles) has not contributed at all to aeration; and as we'll see in a moment, it has actually reduced o/a aeration on a per volume basis.
Let's do another experiment using the same ingredients. We have a jar filled with PCSO, and we know there is some air between the particles. We'll need to pretend that PCSO is fluid enough so when we drop a marble into the filled jar, a volume of PCSO equal to what the marble displaces overflows the rim of the jar. Start dropping in marbles. As each marble is dropped in, PCSO and the air between the particles is displaced, so with every marble dropped into the jar, o/a air porosity decreases. The same thing happens when we try to "amend" an overly water-retentive medium based on all or nearly all fine components. When you start adding coarse ingredients to water retentive media in hopes of increasing aeration, aeration actually decreases until we get beyond the threshold proportion, until there is no longer enough fine material to fill all the pores between the coarse ingredients; only then, does air porosity begin to notably increase.
The 5:1:1 mix works well for 2 main reasons. 1) If made correctly it holds only a small amount of excess (perched) water, which means the soil column is well aerated from the top of the soil column to nearly the bottom. 2) The use of pine bark and perlite work differently than adding perlite to fine ingredients. Perlite mixed with peat provides very little additional air porosity because the peat particles completely surround the perlite particles as the PCSO particles above surround the marbles. When perlite is mixed with the flat platelets of pine bark, one piece of perlite wedged between 2 flat pieces of pine bark provides a great deal or air porosity - imagine a BB wedged between 2 dimes to get a 'mind's eye visual'.
The Gritty Mix is screened to a size that ensures almost all water is held on the surface of soil particles, in the internal pores of porous ingredients (bark/ Turface MVP), and at the interface where soil particles contact each other. When combined, soil particles screened to a size that will not pass a 1/10" mesh cannot hold perched water, and perched water is the primary enemy of air porosity and root health.
5:1:1 Mix
5 parts pine bark screened so it passes through a 3/8 or 1/2" screen (fines included)
1 part sphagnum peat (chunks broken up - ok to include small sticks)
1 part coarse perlite
garden (dolomitic) lime (1-1/4 cups/ cu ft of soil or 1 tbsp per gallon of soil
Gritty Mix
1 part Turface MVP or Allsport (screen and use what doesn't pass aluminum insect screen)
1 part crushed granite (grower grit), or #2 Cherrystone/ quartzite, or Manna-Pro Poultry Grit
1 part pine bark screened 1/8-3/8" or fir bark screened 1/8-1/4"
FWIW - I have never grown a plant that didn't to exceedingly well when fertilized with Foliage-Pro 9-3-6. It has all nutrients essential to normal growth, including Ca, Mg, and all the micros; it derives >2/3 of it's nitrogen from nitrate sources and uses no urea (helps keep plants more compact and full); and, it comes in a ratio that closely matches that at which the average plant actually uses the nutrients (offers the grower greater control and allows the grower to fertilize at the lowest rate possible w/deficiencies or toxicities.
Al
