Does Gravel in Pots Enhance Drainage

Logic would have you ever consider that including gravel or pot shards within the backside of a pot or container will improve drainage and this has been a standard follow for a very long time. Extra lately some gardening consultants and scientists have declared this to be a delusion, claiming that gravel doesn’t improve drainage when positioned within the backside of a pot. That’s the present pondering.

New science has now been revealed that debunks the claimed delusion and concludes that drainage materials within the backside of a pot or container does improve drainage.

Authentic Considering About Drainage

Earlier than going any additional it is very important outline what drainage means from a gardener’s perspective. It seems that defining this isn’t so easy. I requested our Fb Group, Backyard Fundamentals this query and received a number of solutions. A part of the confusion lies in the truth that the time period is used in a different way when discussing actual soil within the floor in comparison with soil in pots and containers.

When speaking about backyard soil outdoors we take into consideration drainage when it comes to how briskly water runs away. The “soil perc take a look at” consists of digging a gap, filling it with water, and timing how rapidly the water runs away. Good draining soil is one the place water runs away rapidly.

In pots, nonetheless, time just isn’t the problem. In pots, we’re extra excited by “how moist” the soil is after extra water runs out of the pot. A poorly draining soil is one the place the remaining soil could be very moist after watering. A well-draining soil is drier.

Since pots are the main target of this dialogue, I’ll outline drainage as the quantity of water held within the soil after watering. Due to this fact good drainage ends in soil that holds much less water.

Traditionally, gardeners put massive inert materials within the backside of pots to extend drainage. This normally consisted of clay pot chards or rocks as proven within the above image.

This appears very intuitive. These massive supplies don’t maintain water very properly and extra water ought to merely run by them, leading to drier soil above the drainage materials. It was believed that drainage materials elevated drainage.

Notice: this publish is all about gravel under the soil and never about gravel combined into the soil.

The Fantasy: Gravel Does Not Enhance Drainage

Various years in the past the concept gravel elevated drainage was declared a delusion, and this declare has turn out to be extra broadly identified.

To grasp the parable it is very important perceive the thought of a Perched Water Desk which I’ve mentioned totally in What’s a Perched Water Desk.

As water is added to a pot of soil, it creates a saturated space close to the underside of the soil, referred to as the perched water desk. The parable claims that including drainage materials within the backside of the pot doesn’t change the perched water desk. As a substitute, it merely strikes this saturated layer increased within the pot.

Including a drainage layer creates three layers: a drainage layer, a saturated soil layer (i.e. perched water desk), and an unsaturated soil layer.

Because the drainage materials doesn’t eradicate the perched water desk there is no such thing as a worth in utilizing it. Many individuals have tried to demonstrated this with sponges, however these have by no means been a passable rationalization to me.

Motion of Water

A few of the scientific supporting proof for debunking the gravel delusion was the motion of water between two totally different layers.

What occurs whenever you layer one sort of soil above one other? You would possibly count on that water simply strikes from one layer to a different however that’s not what occurs. Water has issue transferring from one soil to a different when the particle sizes of the soils are totally different. This was proven a few years in the past in a easy experiment. A container had a decrease degree of sand (massive particles), and an higher degree of silt loam (smaller particles). Water was dripped onto the highest. You’ll be able to see within the image that water (the black areas) moved down by gravity till it reached the sand layer, picture (a). At that time, water began transferring sideways as an alternative of transferring down (b). As extra water was added, the silt loam turned saturated and capillary forces may now not overcome gravity and the water began transferring into the sand layer (c).

The motion of water on this experiment is not any totally different than water transferring in a pot. Water in a pot stuffed with soil doesn’t simply go away the pot as a result of the soil and air have totally different particle sizes. This experiment additionally illustrates why it is best to by no means layer several types of soil within the backyard. All the time combine them collectively.

Nonetheless Not Satisfied?

For those who nonetheless doubt the existence of a perched water desk, strive the easy experiment on this video. My technique to reveal that is higher than the sponge demoes you might need seen.

New Science Debunks the Claimed Fantasy: Gravel Does Enhance Drainage

The parable-busting mentioned within the earlier part was based mostly on a idea that integrated the above diagram displaying water transferring between silt and sand, in addition to different identified information about soil and the way water strikes in it. It appeared like an inexpensive rationalization.

Nevertheless, The idea had by no means been examined in a lab. That has now modified.

A current examine by Avery Rowe measured water ranges in three totally different potting media, each with and with out a wide range of drainage supplies. This examine clearly reveals that drainage materials does improve drainage. It debunks the claimed delusion!

The creator has produced a abstract of the scientific examine on his web site, https://tradescantia.uk.

Experimental Setup

1. Fill the container with the drainage layer and a measured quantity of potting combine, then weigh the entire container.
2. Briefly seal the drainage gap, fill the container with water to the brim, and weigh it once more.
3. Take away the seal and let the surplus water run out.
4. Weigh the drained container one final time.

The identical process was repeated with solely drainage materials to get the burden of water collected within the drainage layer. This worth was then used to calculate the burden of the water within the soil.

Three soil sorts have been examined.

• CPB is a mixture of 60% coir, 20% perlite (Vitax 2–6 mm), and 20% pine bark (Melcourt Propagating Bark 2–7 mm) by quantity.
• CV is a mixture of 70% coir (Coir Merchandise Coco Peat) and 30% vermiculite (Westland Gro-Certain) by quantity.
• JI is a loam-based John Innes “No. 1” compost (Bathgate) (i.e. potting media).

A spread of drainage supplies have been used to guage totally different particle sizes and porosity. This materials was additionally layered at two depths: 30 and 60 mm.

• Gravel (Melcourt Horticultural Gravel 4–10 mm)
• Leca (LECA Light-weight Mixture 4–10 mm)
• Grit (LECA Light-weight Mixture 2–4 mm)
• Sand (Dehner Aquarienkies 1–2 mm)

Outcomes

The unique analysis paper studies outcomes when it comes to water-holding capacities. I’ve taken the uncooked knowledge from the paper and calculated the quantity of water held within the soil layer.

This desk reveals the amount of water, in milliliters, within the soil layer in addition to the % change between solely soil (i.e. no drainage layer) and soil with a drainage layer.

CBP soil is extra porous than CV soil, which is extra porous than JL soil. The drainage layer had the best impact on essentially the most porous soil.

The creator has commented privately that based mostly on a visible examination of the containers the above values for soil above drainage layers are most likely on the excessive facet. Discuss with Mannequin B within the examine for an in depth dialogue of this.

It must be famous that the method of including water was not the standard technique utilized by gardeners. To enhance the consistency of the method the container was fully crammed after which drained. Gardeners usually tend to pour water into the pot and permit extra to empty as they’re pouring. This may water inconsistently and add air bubbles, each of which have been eradicated within the present examine. It isn’t clear how this distinction in watering would have an effect on the information.

Conclusion

The ends in the above desk clearly present that the soil holds much less water when a drainage layer is used. That is true of all three soil sorts and the entire drainage supplies examined.

Drainage layers have been more practical with coarser potting mixes, and thicker layers have been more practical than thinner ones. Total a 60mm layer of coarse sand was the best.

These outcomes debunk the above-described delusion. Drainage materials within the backside of a cup will improve drainage.

This testing didn’t measure the existence of a perched water desk nonetheless the idea of the perched water desk remains to be legitimate, however drainage materials might alter its depth.

The examine additionally didn’t take a look at the impact of a drainage layer on plant development. Crops might or might not develop higher with a drainage layer. Keep in mind that plant development can also be affected by the porosity of the potting soil and selecting the right one could also be extra vital than the drainage layer.