Keeping potting media airy

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Ray

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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 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 disagreement with another grower 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 the details.
 

orchid527

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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 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 disagreement with another grower 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 the details.
Have you considered the possibility that the Grodan cubes, being compressible, get more water squeezed out by a taller, heavier column. Your experiment may not translate well to bark or other less compressible media. Mike
 

Ray

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Mike, I weighed the Grodan and when adding it to the container, every inch I put a standard compression on it (a full Krylon can - a highly technical piece of apparatus). I did that dry for both samples, so the packing density was very close to the same (as close as I could get it), and upon wetting and letting them drain, I saw no sinking that would have suggested additional compacting.
 
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