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A given volume of large soil particles has less overall surface area when compared to the same volume of small particles and therefore less overall adhesive attraction to water. So, in media with large particles, GFP (remember the weight of the water column pushing down on water lower in the pot is the Gravitational Flow Potential) more readily overcomes capillary attraction. They simply drain better and hold more air. We all know this, but the reason, often unclear, is that the height of the PWT is lower in coarse media than media comprised of primarily fine particles. The key to good drainage is size and uniformity of particles. Mixing large particles with small is often very ineffective because the smaller particles fit between the large, increasing surface area which increases the capillary attraction and thus the water holding potential. An illustrative/ rhetorical question: How much perlite should we add to pudding to make it drain well?
I already stated I hold as true, the grower's medium choice when establishing a planting for the long term is the most important decision he/she will make. There is no question roots are the heart of the plant, and plant vitality is inextricably linked in a hard lock-up with root vitality. In order to get the best from our plants, we absolutely must maintain roots in a healthy state.
If we start with a water-retentive medium, we cannot effectively amend it to improve aeration or drainage characteristics by adding larger particulates. Sand, perlite, Turface, calcined DE ...... none of them will work effectively. To visualize why sand and perlite can't change drainage/aeration, think of how well a pot full of BBs or would drain; then think of how poorly a pot full of pudding would drain. Even mixing the pudding with perlite and BBs at a 1:1:1 ratio in a third pot yields a mix that retains the drainage/aeration characteristics and PWT height of the pudding. It's only after the BBs/ perlite becomes by far the largest fraction of the mix (80-95%) that aeration begins to improve. At that point, we are growing in BBs/ perlite amended with a little pudding .
We cannot add coarse material to fine material and improve drainage or the ht of the PWT. Use the same example as above and replace the example of pudding with play sand, peat moss, or a peat-based potting medium - same results. The benefit in adding perlite to heavy (water-retentive) soils doesn't come from the fact they drain better. The fine peat or pudding particles simply 'fill in' around the perlite, so drainage and the height of the PWT remains roughly the same. All perlite does in heavy soils is occupy space that would otherwise be full of water. Perlite simply reduces the amount of water a medium is capable of holding because perlite is not internally porous. IOW - all it does is take up space. That can be a considerable benefit, but it makes more sense to approach the problem from an angle that also allows us to increase the aeration AND durability of the medium. Adding ping pong balls which are full of air adds how much aeration to a bucket of soil? It would actually reduce aeration. That is where Pine bark comes in, and I will get to that soon.
If we wish to profit from a medium offering superior drainage and aeration, we need to start with an ingredient or ingredients as the basis for your medium that already HAVE those properties by ensuring that the medium is primarily comprised of particles much larger than those in peat/ compost/ coir/ sand/ topsoil ….., which is why the recipes I suggest as starting points all direct readers to START with the foremost fraction of the medium being coarse particles. From there, if you choose, you can add an appropriate volume of finer particles to increase/ adjust water retention. We do not have that option with a medium already extremely water-retentive right out of the bag.
I fully understand that many might be happy with the results they get from using a commercially prepared soil, and I'm not trying to twist any arms to convince anyone to change anything. My intent is to make sure that those who are having trouble with issues related to their choice of soil, understand why the issues occur, there are options, and what they are.
We have seen that adding a coarse drainage layer at the container bottom does not improve drainage. It does though, reduce the volume of required to fill a container, making the container lighter. When we employ a drainage layer in an attempt to improve drainage, what we are actually doing is moving the level of the PWT higher in the pot. This simply reduces the volume of soil available for roots colonization. Containers with uniform particle size from top of container to bottom will yield better/ more uniform drainage and have a lower PWT than containers using the same medium with added drainage layers. The coarser the drainage layer, the more detrimental to drainage it is because water is more (for lack of a better scientific word) 'reluctant' to move downward because the capillary pull of the medium above the drainage layer is stronger than the GFP. As mentioned, the reason for this is, there is far more surface area on particles for water to be attracted to in the medium located above the drainage layer than there is in the drainage layer, so the water 'perches'. I know this goes against what most have thought to be true, but the principle is scientifically sound, and experiments have shown it as so. Many nurserymen employ what are termed the 'pot-in-pot' or the 'pot-in-trench' method of growing to capitalize on simple science to be rid of the limitations associated with PW.
If you discover you need to increase drainage, you can simply insert an absorbent wick into a drainage hole & allow it to extend from the saturated medium in the container to a few inches below the bottom of the pot, or allow the wick to contact soil below the container so the earth acts as a giant wick, pulling all or most of the perched water from the medium in the container (see images in the OP). Eliminating the PWT has much the same effect as providing your plants much more medium to grow in, as well as allowing more, much needed air into the root zone.
In simple terms: Plants that expire because of drainage problems either die of thirst because the roots have rotted and can no longer take up water, or they suffer/ die because there is a volume of air in the root zone insufficient to insure normal root function, so water/nutrient uptake and root metabolism become seriously impaired.
To confirm the existence of the PWT and how effective a wick is at removing it, try this experiment: Fill a soft drink cup nearly full of a water-retentive medium. Add enough water to fill to the top, being sure all the medium is saturated. Punch or melt a drain hole in the bottom of the cup and allow the water to drain. When drainage has stopped, insert a wick, or even a toothpick into the drain hole. Take note of how much additional water drains from the cup. Even touching the medium with a toothpick through the drain hole will cause substantial additional water to drain. The water that drains is water that occupied the PWT. A greatly simplified explanation of what occurs is: The wick or toothpick 'fools' the water into 'thinking' the pot is deeper than it is, so water begins to move downward seeking the 'new' bottom of the pot, pulling the rest of the water in the PWT along with it (because water sticks to itself). If there is interest, there are other simple and interesting experiments you can perform to confirm the existence of a PWT in containers, and I can expand later in the thread.
I always remain cognizant of these physical principles whenever I build a medium. I have not used a commercially prepared medium in many years, preferring to build or amend one of my 2 basic mixes to suit individual plantings. I keep many ingredients at the ready for building soils, but the basic building process usually starts with conifer bark and perlite. Sphagnum peat moss plays a secondary role in my container media because it breaks down and compacts too quickly to suit me, and when it does, it impedes drainage and reduces aeration.
Size matters. Partially composted conifer bark (pine is easiest to find and least expensive) works best in the following recipes, followed by uncomposted bark in the smaller than 3/8" range. Bark fines of pine, fir or hemlock, are excellent as the primary component of your media. The lignin (it's what makes woody plants woody) contained in bark keeps it rigid and the rigidity provides air-holding pockets in the root zone far longer than peat or compost mixes that too quickly break down to a soup-like consistency. Conifer bark also contains suberin, a lipid sometimes referred to as nature's preservative. Suberin, more scarce as a presence in sapwood products and hardwood bark, dramatically slows the decomposition of conifer bark-based media. It contains highly varied hydrocarbon chains and the microorganisms that turn peat to soup have great difficulty cleaving these chains - it retains its structure.
Note there is no sand or compost in the media I use. Sand, as most growers think of it, can improve drainage in some cases, but it reduces aeration by filling valuable macro-pores in a medium. Unless sand particle size is fairly uniform and/or larger than about BB size, I leave it out of anything I grow in.
Compost is too fine and unstable for me to consider using in any significant volume as well. The small amount of micro-nutrients it supplies can easily be delivered by one or more of a number of chemical or organic sources that do not detract from drainage/ aeration.
The basic blends I use:
The 5:1:1 mix:
See dry 5:1:1 mix in the center below. The bark products at 3, 6, and 9 o'clock are ideal for the 5:1:1 mix. The bark at top/ below is prescreened fir bark in 1/8 - 1/4", and what I use for the gritty mix.
5 parts pine bark fines, dust - 3/8 (size is important)
1 part sphagnum peat (not reed or sedge peat, please)
1-2 parts perlite (coarse or all-purpose, if you can get it – not super-coarse)
garden lime
controlled release fertilizer (if preferred)
Big batch:
2-3 cu ft pine bark fines
5 gallons peat
5 gallons perlite
2 cups dolomitic (garden) lime
2 cups CRF (if preferred)
Small batch:
3 gallons pine bark
1/2 gallon peat
1/2 gallon perlite
4 tbsp lime (or gypsum in some cases)
1/4 cup CRF (if preferred)
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Al’s 5-1-1 and Gritty Mix
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