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RE: Does my 5:1:1 mix look right? (Follow-Up #6)

posted by: tapla on 01.10.2013 at 05:22 pm in House Plants Forum

Ideally, the inorganic soil particles would be sized from about 1/10" to no larger than 3/16", and the bark fraction slightly larger, say from 1/8" to 1/4" to allow for some reduction in size over the life of the soil due to breakdown/composting.

PWTs disappear as particle size becomes larger than just over 1/10", so a soil with particles around that size maximizes the amount of water a soil can hold without it holding water between particles. This ensures a healthy root environment, free from the deleterious effects of perched water, from the bottom of the container to the top - even when the soil is at container capacity (holding all the water it can).

This is an interesting (physics?) question: adding bark to the gritty mix. I might wonder if it would actually make your water retention worse? It might not be exactly true, but bark holds about as much water as the average between Turface and granite combined. The aim of screening materials for the gritty mix to certain sizes is to ensure that as much water as possible will be held on the surface of soil particles and inside particles that are internally porous. What bark does to water retention depends on the size of the bark in relation to the other materials in the soil, how wet the soil is at watering time, and what mix of other ingredients are in the soil.

If you made a gritty mix of equal parts of screened Turface and granite, adding bark wouldn't change water retention much; but if the soil favored one or the other (Turface or granite), the bark could change water retention. If your soil was 4 parts Turface and 2 parts granite plus 3 parts of bark, the bark would decrease water retention; but it would increase water retention if the mix contained 4 parts granite and 2 Turface.

Also, even though it's very difficult to over-water anything in the gritty mix, letting your soil dry down some between waterings allows the bark fraction to work as a water reservoir and a sponge. When you water an almost dry gritty mix, the Turface sucks up water on contact - VERY quickly; but water pouring through the soil isn't absorbed as quickly by the bark. This is a benefit because any perched water quickly diffuses as gas (water vapor) and can be absorbed by the bark that didn't get fully saturated when you watered. If you water while the soil is still wet, the bark can become fully saturated and unable to absorb (sponge up) any excess water that might tend to want to perch near the bottom of the container.

I'm still trying to wrap my head around the concept of perched water and water retention as a result of particle size (rather than how absorbent a material is!). There is a difference between how absorbent a soil particle is and how absorbent a conglomeration of particles is. A tiny particle of sand holds water only on it's surface. Let's call that amount of water 'x'. You might expect that a conglomeration of 100 particles to hold a volume = to 100x, but actually it's much more than that because of the added water held between the particles. Once particles reach a certain size (about .1") water can't be held in the air spaces between the particles, only in the immediate area of contact between particles and on their surface. It's the size of the spaces between particles that determine whether or not they can hold water against the force of gravity.

All tiny particles (like fine sand/peat/compost) = small spaces between particles and lots of water in those spaces - more accurately - a tall PWT. All large particles (like BB size) = no water in the air spaces between particles and NO PWT. When large particles are mixed with fine particles, the fine particles surround the large particles, so the HT of the PWT and soil aeration is largely unaffected. This is why it's impossible to effectively 'amend' soils based on fine particulates by adding larger material, like pine bark or perlite. Unless the larger particles make up a very large fraction of the soil, the fine particles simply 'filter in' around the large particles and negate the effort - except as it relates to o/a water retention. You can see that if you add a fist full of marbles to a pint of peat moss, that the marbles will take up space and reduce o/a water retention without having any impact on drainage or aeration.

We know now that particle size has the most significant impact on o/a water retention, but only up until the point that the PWT disappears. A pint of Turface particles the same size as particles of fine play sand will hold more water than the sand because the Turface has internal porosity, but because most of the water is held BETWEEN particles, the increase in water retention isn't as great as it would be if the particles were larger, say .1". Then, there would be no water in the air spaces between particles and much water inside the very porous Turface particles, making the increase in water retention significant.
Got it? ;-)


This post was edited by tapla on Thu, Jan 10, 13 at 21:17


clipped on: 04.06.2013 at 11:43 pm    last updated on: 04.06.2013 at 11:44 pm