Keeping Potting Media Airy

[Ray]

When we water our plants, most just pours through. Some is immediately absorbed by the plant and medium, and some (“bridging water”) is held by surface tension in the gaps between media particles. It is that fraction, that if too extensive (too fine, or old, decomposed media) that leads to root suffocation, death and rot.

Many people put pot shards or foam packing peanuts in the bottom of a flower pot, thinking it will keep the potting medium more airy. While there is no doubt that having the entire pot diameter available for draining and air exposure is a good thing, reducing the height of the column of potting medium actually increases the amount of bridging water held in the medium. You can prove it with an ordinary kitchen sponge.

Hold a sponge under water and repeatedly squeeze it until saturated. Keeping it horizontal and without squeezing it, lift it out of the water and let it completely drain. Once it has stopped dripping, turn it on its edge. More water will drain out.

I have had a long-running friendly disagreement with another grower here over this, so recently ran an experiment in which I compared a 6” tall, 3” diameter container 1) completely filled with potting medium (I used Grodan rock wool mini cubes for uniformity), and 2) half filled with foam packing peanuts and half medium. If the amount of water held was independent of media column height, the “half” pot would have held half the amount of water that the “full” pot did. In fact, the “half” pot held about 30% more water than that.

I have published the details of the experiment here, if you’d like to read more.

[MCD]

I think your point is valid; you can also see this when you tilt a pot to a 45 degree angle after watering, which causes quite a bit of additional water to drain out. At that angle the height of the media is somewhat higher.

But in your experiment, I would expect that the peanut layer would hold some small amount of water, from clinging to the surface and nooks and crannies? In other words I would expect the weight of water to be slightly higher than half, even without accounting for the effect that you're trying to show. Maybe an experiment with just the peanuts would show how much that is, probably very small.

[ebrookmi]

The styrofoam does not hold any water, packing peanut/styrofoam is a hydrophobic polymer - a reason why it's thought to help when adding it to the bottom of a pot. It seems Ray has been able to use the styrofoam as a control instead due to the nature of the material, and any results would be purely based on the planting media alone. The shorter height allows for more surface area for water adhesion which would prevent the media from drying as fast as a deeper pot full of media.

[Ray]

Actually, I decided to check that - the full container of foam peanuts held 10 grams of water droplets, so I went back and adjusted the wet weight of the Grodan cubes by 5 grams.

So…instead of the “half pot” holding just under 30% more water, it’s a shade over 25%.

[MCD]

Thanks for checking; figured it would be small but not 0.

Personally I like to put a (thin) layer of something inorganic like leca at the bottom of the pots, under a bark mix, but not for aeration. I find that depending on the pot shape and drainage hole locations, water can pool at the bottom a little bit, and bark sitting there can start to degrade earlier than the rest.

[tmoney]

oh man, a science discussion! i so want to chime in,,,

so, ray, i think this is of course an interesting topic. however, i have to take issue with a couple things in your report. namely, you used inches in your intro but reported your data in metrics!!! dude, you can't be doin that!

but also i think it would be worth it to design a test for individual particles of each media type. perhaps i will take on such an experiment in the fall (in my copious amounts of spare time)...

also, i would criticise my students for not thoroughly acknowloging the multitude of other variables that could be influencing your final conclusion. not sure you have addressed these fully to draw the conclusion you have. perhaps. i have no skin in this game (and don't know the history you are referring to) but i am not sure i could jump to the same conclusion based on the evidence....

mind you, i don't care either way since we don't grow in foam or grodan, i'm only speaking from a science perspective! so....carry on gentlemen!

[Dimples]

Originally Posted by MCD

I find that depending on the pot shape and drainage hole locations, water can pool at the bottom a little bit, and bark sitting there can start to degrade earlier than the rest.

I combat the pooling by placing the pots on an absorbent towel after watering. The towel wicks out the water pooling at the bottom of the pot and it also removes some of the water held in void spaces higher in the pot due to the cohesive properties of water. It's not a method suitable for very large collections but it works very well.

[sunfire]

So, I watched the SFOS lecture from Bill Thoms and that is how he pots his bulbos. I guess then it would make sense why it is a successful method? Retaining more water in a shallower layer of media would be optimal for these short rooted, moisture loving plants?

[estación seca]

Yes. Bulbos in growth are water hogs.

[Ray]

Originally Posted by tmoney

but also i think it would be worth it to design a test for individual particles of each media type. perhaps i will take on such an experiment in the fall (in my copious amounts of spare time)...

I don’t have the materials to do the same evaluation for everything…

also, i would criticise my students for not thoroughly acknowloging the multitude of other variables that could be influencing your final conclusion. not sure you have addressed these fully to draw the conclusion you have,

What other variables?

Interestingly, the materials of the container (a plastic document storage tube with cap) do not allow it to get wet and collect droplets. The cubes did hold water between the sidewall and themselves, but as that would be common to both samples, I figured that would be part of the result and present in a real-life situation, either way.

I was concerned about packing density, which is why I weighed the cubes, measured the cylinder, then compacted the cubes to fill the volume.

A “compaction gradient” was possible as a compressed the cubes in the volumes, but tried to minimize that by using a standard weight every inch to compress it. Besides, it seems that the average packing density would be a constant, whether I had done that or not, so the bridging water retention would have averaged out, as well.

Honestly, if you see other potential variables I haven’t, please let me know. At the very least, it gives me a reason to rationalize!

Oh - the change in units came about because the container was a free sample, with inches dimensions, but metric calculations are so much clearer…