Effect of drainage layers on water retention of potting media in containers

Avery Rowe

doi:10.1371/journal.pone.0318716

. 2025 Feb 25;20(2):e0318716. doi: 10.1371/journal.pone.0318716

Effect of drainage layers on water retention of potting media in containers

Avery Rowe ^1,^*

Editor: Somayeh Soltani-Gerdefaramarzi^2,²

PMCID: PMC11856292 PMID: 39999133

Abstract

Excess water retention in the potting medium can be a significant problem for plants grown in containers due to the volume of saturated medium which forms above the drainage hole. Adding a layer of coarse material like gravel or sand at the bottom is a common practise among gardeners with the aim of improving drainage, but some researchers have argued that such layers will raise the saturated area and in fact increase water retention. Two different depths and four different materials of drainage layer were tested with three different potting media to determine the water retention in the container after saturating and draining freely. For loamless organic media, almost all types of drainage layer reduced overall water retention in the container compared to controls. For loam-based media, most drainage layers had no effect on the overall water retention. Two simple models were also used to estimate the water retention in the media alone, excluding the drainage layer itself. All drainage layers reduced water retention of loamless organic media, according to both models. There was disagreement between the two models applied to loam-based media, and further study is required to determine the most accurate. Both models showed that some drainage layers with smaller particle sizes reduced water retention in loam-based media, but disagreed on the effect of drainage layers with larger particle sizes. Overall, any drainage layer was likely to reduce water retention of any medium, and almost never increased it. Thicker drainage layers were more effective than thinner layers, with the most effective substrate depending on the potting media used. A 60 mm layer of coarse sand was the most universally-effective drainage layer with all potting media tested.

Introduction

It is widely recognised that soil in a container behaves differently from soil in the field [1–3]. One of the most significant differences is the presence of a saturated volume of soil at the bottom of the container after irrigation and draining, known as a perched water table [4–6]. The depth of this saturated volume is constant for any specific soil, and unaffected by container size and shape. As a result, soil in a shallower container has a greater percentage water-holding capacity (WHC) than the same soil in a deeper container [1, 7–9]. This effect has led to the concept of container capacity, a measure of the total water retained in a specific container filled with specific soil after saturation and drainage [4, 9, 10].

Adding a layer of rocks or gravel at the bottom of plant pots is a common practice among home gardeners, as well as some researchers [11], and is generally intended to reduce this perched water table and improve drainage. However in recent years the recommendation has been disputed, with arguments based on principles of water movement through soil suggesting that a drainage layer will in fact raise the perched water table and increase water retention overall [12, 13]. Despite frequent discussion of this practice, there has been little formal research into the effect drainage layers have on the water-holding capacity of potting media in containers.

A number of areas of research may be applicable to this subject in part. One of the oldest relevant studies is the tests of water movement through soil interfaces conducted by Hsieh and Gardner [14]. This research demonstrated that when the wetting front encounters an interface between soils of different porosities, it will not advance until the soil behind is nearly saturated. Applying this to the interface between potting medium and drainage substrate in a container suggests that water will not move down into the drainage layer until the medium is almost saturated, and therefore the perched water table in the medium will be raised and the overall water retention increased.

More recent studies have demonstrated that a layer of coarse material acts as a barrier to water movement in field soils [15], and that more moisture is retained in root zone soil when a layer of gravel is added below it [16]. Khire et al. [17] also found that more water is stored above a capillary barrier layer when it consists of coarser material than when it consists of finer material. Smesrud and Selker [18] conducted theoretical analysis which predicted that the effectiveness of a capillary barrier is maximised when the contrast of particle sizes between the soil above and the substrate below is infinite - in other words, when the soil is suspended over a void of air. This would suggest that the perched water table created by soil over any other substrate would be less than that created by soil over air.

Another area of recent relevant research which can be applied to this subject is the construction of green roofs, which are roofs made from a layer of growing medium topped with living plants [19]. This shallow layer of medium is surrounded by an impermeable base and sides, with a small number of drains for excess water to escape through - physical conditions which are similar to a typical plant pot. In the construction of green roofs a drainage layer of coarse substrate below the growing medium is always considered essential [19, 20]. She and Pang [21] found that water begins to drain out from the soil of the green roof after it has reached field capacity (container capacity), but before it is fully saturated. This result contrasted with predictions that water only escapes at full saturation, showing the need for further research to understand the movement of water through soil in containers with layered contents.

Ruter [22] found that when a fabric-bottomed container was placed on a column of sand to drain, the perched water table in the medium was completely eliminated. This suggests that water moved more easily from the medium to sand than from the medium into air, and therefore that adding a drainage layer of sand to the bottom of a container may aid drainage. However, the column of sand in this study was many times deeper than the container itself, so this may not predict the behaviour of a thin drainage layer. In addition, the fabric bottom of the container may have an effect on the water movement from the medium to the sand which would not occur if the two materials were in direct contact.

Many of the above studies were conducted on mineral field soils. In contrast, container growing often utilises loamless potting media based on organic materials like peat, coir, and bark, with mineral amendments like perlite and vermiculite [2, 4, 5, 9, 13, 23, 24]. These media typically have very different physical properties from loams, including higher total porosity and lower water-holding capacity [4, 25]. As a result, the water movement through loamless media in containers may not be accurately predicted by applying research conducted on field loams.

Further, researchers have demonstrated that the water movement and other behaviour of soil in containers is highly dependent on the geometry of the container [3], and can differ greatly from the behaviour of even the same soil in the field [2]. Thus, much of the research into the behaviour of water at soil interfaces cannot be applied directly to the situation of container growing because it was conducted in the field or in containers of very different sizes and shapes to those generally used for live plants.

A typical filled plant pot consists of an impermeable container with a small number of holes in the base, full of a single uniform potting medium. When the saturated medium is allowed to drain, the only interface the water passes through is the drainage hole where medium meets air. In contrast, a pot with a drainage layer is a more complex system. First the water passes through the interface between the medium and the drainage substrate, and then it passes through the interface between the drainage substrate and the air under the drainage hole. These multiple interactions make it more difficult to predict the final outcome using only theoretical analysis of individual soil interfaces.

In recent years, some growers have begun to stratify different growing media within containers, to deliberately adjust the vertical moisture profile [26]. Criscione et al. found that substrate stratification significantly changes the movement of water through containers [27, 28], as well as the growth of plants in these containers [29]. These studies were all conducted using two layers of substrate each half the height of the container, and therefore the conclusions may not be applicable to the use of shallower drainage layers in the base of a container mostly filled with one medium.

This study aimed to determine the effect on water-holding capacity of drainage layers in containers, and compare the effect using different potting media, different drainage substrates, and different layer depths. The intention was to obtain results which could be applied by commercial nurseries and home gardeners in practice, so the conditions were chosen to closely replicate real growing containers. The study did not attempt to determine the effect of drainage layers on growing plants directly. Experiments and theoretical models were used to determine the water-holding capacity of the entire container, and of the medium alone, so that both results could be considered by readers when predicting the effects on plant growth.

Materials and methods

Three different types of potting media were used, as follows: JI was a loam-based John Innes “No. 1” compost (Bathgate); CV was a mix of 70% coir (Coir Products Coco Peat) and 30% vermiculite (Westland Gro-Sure) by volume; CPB was a mix of 60% coir, 20% perlite (Vitax 2–6 mm), and 20% pine bark (Melcourt Propagating Bark 2–7 mm) by volume. The two loamless mixes were chosen to represent some of the most common loamless media used by growers according to Bunt [4]. Such media typically contain at least 50% peat or fine bark, with the remainder composed of mineral amendments (such as perlite, vermiculite, or sand), compost, or coarse bark. Coir was chosen as an acceptable alternative to peat in these mixes [30, 31], due to the UK government’s intention to reduce or eliminate horticultural use of peat.

Four different drainage substrates were used: gravel (Melcourt Horticultural Gravel 4–10 mm), leca (LECA Lightweight Aggregate 4–10 mm), grit (LECA Lightweight Aggregate 2–4 mm), and sand (Dehner Aquarienkies 1–2 mm). This range of substrates was chosen to examine the influence of particle size on drainage effects, as well as to compare the effect of porous substrates with non-porous ones.

Test pots were made from clear plastic containers with a single 10 mm hole cut in the centre of the base. The pots had a total capacity of 1,290 ml, and were an inverted truncated cone shape with height 175 mm, base diameter 86 mm, top diameter 118 mm, and dry weight of 53 g. The base had a slight domed area raised by 4 mm relative to the edges, common to nursery containers to allow water to escape when placed on a flat surface. For trials with sand, a piece of fibreglass mesh (16 x 18 mesh count, approximately 1 mm spacing) was placed over the drainage hole to prevent the sand escaping. The size and friction of particles of the other drainage substrates was sufficient to prevent them escaping from the drainage hole. The drainage hole was temporarily sealed with tape before filling.

For each potting medium, trials were conducted with a layer of each drainage substrate to depths of 30 mm and 60 mm (giving a total of 24 combinations). Control trials were also conducted without drainage substrate with the pot filled with media to the top, to 30 mm from the top, and to 60 mm from the top. In addition, trials were conducted of all the 30 mm and 60 mm drainage layers with no media. Ten replications were conducted for each condition, with the exception of two conditions (CV-leca-30 mm and sand-60 mm) which each had one invalid trial for a total of nine results.

For each of the three potting media, a baseline moisture level was defined according to the mass of a fixed volume of medium. Before each condition, the medium was air-dried until it reached this moisture level, confirmed by weighing a fixed volume of medium to ensure it had the correct baseline mass. All the containers were then filled with equal masses of medium (as recommended in the procedure Bilderback [32] gives for measuring soil porosity in the nursery) to within 5 g precision, using a scale with 1 g precision. The medium was added by hand, and no evidence of layering or spaces was observed through the clear containers. Each filled container was irrigated from above using a watering can with a rose until the water level reached the top of the pot (or to the specified height for the tests involving pots filled to below the top). Finally the drainage hole was unsealed and the container allowed to drain suspended on a grate and uncovered, as a growing pot would be. The containers were monitored until water was no longer visibly draining, which took between one minute and three hours, depending on the condition. No evidence of blockages or inconsistencies was observed during drainage. The pot was then weighed a final time (to 1g precision) with its wetted contents. The mass of the empty container was subtracted from the measurements to determine the initial dry mass (m_i) and drained mass (m_d) of the contents alone. See Fig 1 for diagrams of the experimental procedure.

The saturating and draining process caused all the potting media to compact. The compacted depth was measured from one representative sample for each medium with each depth of drainage substrate, and the final compacted volume calculated (v). None of the drainage substrate layers compacted measurably, so their final volume was taken as equal to the initial volume.

The water-holding capacity (WHC) was then calculated for each trial (see Eq 1). ANOVA post-hoc Tukey tests were used to determine the significance of differences between the means of different conditions (see S1 Appendix for ANOVA tables). Different potting media were not compared directly to each other, because their compacted volumes and initial moisture levels were not standardised for this study.

\begin{matrix} W H C = (m_{d} - m_{i}) ∕ v \end{matrix}

(1)

Two simple models were created to estimate the WHC of the medium itself in containers with drainage layers. In both models, the initial mass of the medium in each trial ( $m_{i (m e d)}$ ) was calculated using the initial mass of the full container ( $m_{i (f u l l)}$ ) and the initial mass of the corresponding drainage layer ( $m_{i (d r n)}$ ) (see Eq 2).

\begin{matrix} m_{i (m e d)} = m_{i (f u l l)} - m_{i (d r n)} \end{matrix}

(2)

Model A was intended to estimate the highest possible WHC for the medium itself. The model was based on the assumption that the drainage layer held the same amount of water after draining when in a container filled with medium as when in a container alone. Thus, the drained mass of the medium in each trial according to model A ( $m_{d (A)}$ ) was calculated by subtracting the drained mass of the drainage layer ( $m_{d (d r n)}$ ) from that of the full container ( $m_{d (f u l l)}$ )(see Eq 3).

\begin{matrix} m_{d (A)} = m_{d (f u l l)} - m_{d (d r n)} \end{matrix}

(3)

Model B was intended to estimate the lowest possible WHC for the medium itself. The model was based on the assumption that the drainage layer remained fully saturated with water after draining when in a container filled with medium. Thus, the drained mass of the medium in each trial according to model B ( $m_{d (B)}$ ) was calculated by subtracting the saturated mass of the drainage layer ( $m_{s (d r n)}$ ) from the drained mass of the full container (see Eq 4).

\begin{matrix} m_{d (B)} = m_{d (f u l l)} - m_{s (d r n)} \end{matrix}

(4)

The combination of these two models provided a range of possible values for WHC of the medium alone in each condition, to use as a basis for comparison against the control condition using the same depth of medium. This enabled the water retained in the drainage layer itself to be eliminated as a factor, and the effect of the drainage layer on the medium above it to be considered in isolation.

Results

Coir-perlite-bark medium

The CPB mix showed the least compaction, with the effective volume of mix in a full pot decreasing from 1,290 ml to 1,133 ml. The control container filled to 30 mm from the top compacted from 1,030 ml to 838 ml, and the container filled to 60 mm from the top compacted from 780 ml to 672 ml. The WHC of the controls increased as the depth of medium decreased.

The results of the CPB drainage layer trials are presented in Fig 2 by combined drainage layer type. Post-hoc Tukey comparisons are presented in Table 1 (by combined drainage layer type), Table 2 (by drainage substrate only), and Table 3 (by drainage layer depth only). Compared to the control, every container with a drainage layer had a lower overall WHC (p < 0.001). There were significant differences between many of the drainage layer conditions, with grit-30 mm condition having the highest WHC and leca-60 mm the lowest. Analysing only according to the drainage substrate, all had lower overall WHC than the control (p < 0.001). Leca had significantly lower WHC than gravel (p < 0.05) and grit ( $p < 0.001$ ). Combining all substrates, both 30 mm and 60 mm drainage layer conditions had lower overall WHC than the control ( $p < 0.001$ ) with no significant difference between them.

Table 1. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer types.

Drainage layer type	Comparison	Mean Difference	SE	t	p
Gravel-60 mm	Grit-60 mm	0 . 012	0 . 009	1 . 352	0 . 912
	Leca-60 mm	0 . 035	0 . 009	3 . 773	$0.00 9^{* *}$
	Sand-60 mm	0 . 029	0 . 009	3 . 126	0 . 059
	Gravel-30 mm	0 . 017	0 . 009	1 . 883	0 . 627
	Grit-30 mm	- 0 . 012	0 . 009	- 1 . 360	0 . 909
	Leca-30 mm	0 . 027	0 . 009	2 . 911	0 . 101
	Sand-30 mm	0 . 008	0 . 009	0 . 904	0 . 992
	Control	- 0 . 076	0 . 009	- 8 . 258	<.001***
Grit-60 mm	Leca-60 mm	0 . 022	0 . 009	2 . 422	0 . 287
	Sand-60 mm	0 . 016	0 . 009	1 . 774	0 . 699
	Gravel-30 mm	0 . 005	0 . 009	0 . 531	1 . 000
	Grit-30 mm	- 0 . 025	0 . 009	- 2 . 712	0 . 160
	Leca-30 mm	0 . 014	0 . 009	1 . 559	0 . 823
	Sand-30 mm	- 0 . 004	0 . 009	- 0 . 448	1 . 000
	Control	- 0 . 088	0 . 009	- 9 . 610	<.001***
Leca-60 mm	Sand-60 mm	- 0 . 006	0 . 009	- 0 . 648	0 . 999
	Gravel-30 mm	- 0 . 017	0 . 009	- 1 . 891	0 . 622
	Grit-30 mm	- 0 . 047	0 . 009	- 5 . 134	<.001***
	Leca-30 mm	- 0 . 008	0 . 009	- 0 . 863	0 . 994
	Sand-30 mm	- 0 . 026	0 . 009	- 2 . 870	0 . 112
	Control	- 0 . 110	0 . 009	- 12 . 032	<.001***
Sand-60 mm	Gravel-30 mm	- 0 . 011	0 . 009	- 1 . 243	0 . 944
	Grit-30 mm	- 0 . 041	0 . 009	- 4 . 486	<.001***
	Leca-30 mm	- 0 . 002	0 . 009	- 0 . 215	1 . 000
	Sand-30 mm	- 0 . 020	0 . 009	- 2 . 222	0 . 402
	Control	- 0 . 104	0 . 009	- 11 . 384	<.001***
Gravel-30 mm	Grit-30 mm	- 0 . 030	0 . 009	- 3 . 243	$0.04 3^{*}$
	Leca-30 mm	0 . 009	0 . 009	1 . 028	0 . 982
	Sand-30 mm	- 0 . 009	0 . 009	- 0 . 979	0 . 987
	Control	- 0 . 093	0 . 009	- 10 . 141	<.001***
Grit-30 mm	Leca-30 mm	0 . 039	0 . 009	4 . 271	0 . 002
	Sand-30 mm	0 . 021	0 . 009	2 . 264	0 . 376
	Control	- 0 . 063	0 . 009	- 6 . 898	<.001***
Leca-30 mm	Sand-30 mm	- 0 . 018	0 . 009	- 2 . 007	0 . 543
Leca-30 mm	Control	- 0 . 102	0 . 009	- 11 . 169	<.001***
Sand-30 mm	Control	- 0 . 084	0 . 009	- 9 . 162	<.001***

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 9)

Table 2. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer substrates.

Drainage layer substrate	Comparison	Mean Difference	SE	t	p
Gravel	Grit	- 0 . 009	0 . 007	- 1 . 248	0 . 723
	Leca	0 . 022	0 . 007	3 . 169	$0.01 8^{*}$
	Sand	0 . 010	0 . 007	1 . 417	0 . 619
	Control	- 0 . 084	0 . 008	- 9 . 914	<.001***
Grit	Leca	0 . 031	0 . 007	4 . 417	<.001***
	Sand	0 . 018	0 . 007	2 . 665	0 . 068
	Control	- 0 . 075	0 . 008	- 8 . 895	<.001***
Leca	Sand	- 0 . 012	0 . 007	- 1 . 752	0 . 408
Leca	Control	- 0 . 106	0 . 008	- 12 . 502	<.001***
Sand	Control	- 0 . 094	0 . 008	- 11 . 071	<.001***

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 5)

Table 3. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer depths.

Drainage layer depth	Comparison	Mean Difference	SE	t	p
60 mm	30 mm	- 0 . 009	0 . 005	- 1 . 676	0 . 220
60 mm	Control	- 0 . 094	0 . 008	- 11 . 181	<.001***
30 mm	Control	- 0 . 085	0 . 008	- 10 . 121	<.001***

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 3)

The modelled WHC of the medium itself according to drainage layer substrate is presented in Fig 3 (30 mm drainage layers) and Fig 4 (60 mm drainage layers), compared with the control containers with the same initial medium depth. With both drainage layer depths, both the modelled WHC values were lower than the control values for every condition.

Coir-vermiculite medium

The CV mix showed intermediate compaction, with the effective volume of mix in a full pot decreasing from 1,290 ml to 1062 ml. The control container filled to 30 mm from the top compacted from 1,030 ml to 816 ml, and the container filled to 60 mm from the top compacted from 780 ml to 615 ml. The WHC of the controls increased as the depth of medium decreased.

The results of the CV trials are presented in Fig 5 by combined drainage layer type. Post-hoc Tukey comparisons are presented in Table 4 (by combined drainage layer type), Table 5 (by drainage substrate only), and Table 6 (by drainage layer depth only). Compared to the control, the gravel-30 mm, grit-30 mm, and leca-30 mm drainage layer conditions did not have significantly different overall WHC. The grit-60 mm ( $p < 0.01$ ), gravel-60 mm, leca-60 mm, sand-60 mm, and sand-30 mm conditions (p < 0.001) all had lower WHC than the control. There were significant differences between many of the drainage layer conditions, with the 30 mm grit condition having the highest WHC and 60 mm gravel the lowest. Analysing only according to the drainage substrate, conditions with grit did not have significantly different overall WHC, while leca ( $p < 0.05$ ), gravel ( $p < 0.01$ ), and sand ( $p < 0.001$ ) all had lower WHC than the control. Sand had the lowest WHC, and was significantly different ( $p < 0.001$ ) from grit which had the highest WHC. Combining all substrates, 60 mm layers had lower overall WHC ( $p < 0.001$ ). 30 mm layers were not significantly different from the control, but had significantly higher WHC than 60 mm layers ( $p < 0.001$ ).

Table 4. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer types.

Drainage layer type	Comparison	Mean Difference	SE	t	p
Control	Gravel-60 mm	0 . 104	0 . 014	7 . 607	<.001***
	Gravel-30 mm	0 . 014	0 . 014	1 . 052	0 . 979
	Grit-60 mm	0 . 058	0 . 014	4 . 291	$0.00 2^{* *}$
	Grit-30 mm	- 0 . 011	0 . 014	- 0 . 802	0 . 997
	Leca-60 mm	0 . 073	0 . 014	5 . 371	<.001***
	Leca-30 mm	0 . 024	0 . 014	1 . 718	0 . 733
	Sand-60 mm	0 . 082	0 . 014	6 . 043	<.001***
	Sand-30 mm	0 . 076	0 . 014	5 . 613	<.001***
Gravel-60 mm	Gravel-30 mm	- 0 . 089	0 . 014	- 6 . 555	<.001***
	Grit-60 mm	- 0 . 045	0 . 014	- 3 . 316	$0.03 5^{*}$
	Grit-30 mm	- 0 . 115	0 . 014	- 8 . 409	<.001***
	Leca-60 mm	- 0 . 030	0 . 014	- 2 . 236	0 . 393
	Leca-30 mm	- 0 . 080	0 . 014	- 5 . 686	<.001***
	Sand-60 mm	- 0 . 021	0 . 014	- 1 . 565	0 . 821
	Sand-30 mm	- 0 . 027	0 . 014	- 1 . 994	0 . 552
Gravel-30 mm	Grit-60 mm	0 . 044	0 . 014	3 . 239	$0.04 4^{*}$
	Grit-30 mm	- 0 . 025	0 . 014	- 1 . 854	0 . 647
	Leca-60 mm	0 . 059	0 . 014	4 . 319	$0.00 1^{* *}$
	Leca-30 mm	0 . 010	0 . 014	0 . 695	0 . 999
	Sand-60 mm	0 . 068	0 . 014	4 . 991	<.001***
	Sand-30 mm	0 . 062	0 . 014	4 . 561	<.001***
Grit-60 mm	Grit-30 mm	- 0 . 069	0 . 014	- 5 . 093	<.001***
	Leca-60 mm	0 . 015	0 . 014	1 . 080	0 . 976
	Leca-30 mm	- 0 . 034	0 . 014	- 2 . 458	0 . 268
	Sand-60 mm	0 . 024	0 . 014	1 . 752	0 . 713
	Sand-30 mm	0 . 018	0 . 014	1 . 322	0 . 922
Grit-30 mm	Leca-60 mm	0 . 084	0 . 014	6 . 173	<.001***
	Leca-30 mm	0 . 035	0 . 014	2 . 499	0 . 248
	Sand-60 mm	0 . 093	0 . 014	6 . 845	<.001***
	Sand-30 mm	0 . 087	0 . 014	6 . 415	<.001***
Leca-60 mm	Leca-30 mm	- 0 . 049	0 . 014	- 3 . 509	$0.02 0^{*}$
	Sand-60 mm	0 . 009	0 . 014	0 . 672	0 . 999
	Sand-30 mm	0 . 003	0 . 014	0 . 242	1 . 000
Leca-30 mm	Sand-60 mm	0 . 058	0 . 014	4 . 163	$0.00 2^{* *}$
Leca-30 mm	Sand-30 mm	0 . 052	0 . 014	3 . 745	$0.01 0^{* *}$
Sand-60 mm	Sand-30 mm	- 0 . 006	0 . 014	- 0 . 430	1 . 000

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 9)

Table 5. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer substrates.

Drainage layer substrate	Comparison	Mean Difference	SE	t	p
Control	Gravel	0 . 059	0 . 016	3 . 607	$0.00 5^{* *}$
	Grit	0 . 024	0 . 016	1 . 453	0 . 595
	Leca	0 . 050	0 . 017	3 . 025	$0.02 7^{*}$
	Sand	0 . 079	0 . 016	4 . 855	<.001***
Gravel	Grit	- 0 . 035	0 . 013	- 2 . 637	0 . 073
	Leca	- 0 . 009	0 . 014	- 0 . 671	0 . 962
	Sand	0 . 020	0 . 013	1 . 529	0 . 547
Grit	Leca	0 . 026	0 . 014	1 . 932	0 . 309
Grit	Sand	0 . 056	0 . 013	4 . 166	<.001***
Leca	Sand	0 . 029	0 . 014	2 . 180	0 . 197

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 5)

Table 6. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer depths.

Drainage layer depth	Comparison	Mean Difference	SE	t	p
Control	60 mm	0 . 079	0 . 014	5 . 879	<.001***
Control	30 mm	0 . 026	0 . 014	1 . 923	0 . 138
60 mm	30 mm	- 0 . 053	0 . 009	- 6 . 209	<.001***

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*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 3)

The modelled WHC of the medium itself according to drainage layer substrate is presented in Fig 6 (30 mm drainage layers) and Fig 7 (60 mm drainage layers), compared with the control containers with the same initial medium depth. With both drainage layer depths, all the modelled WHC values were lower than the control values, with the 60 mm drainage layers showing the greatest difference.

John Innes medium

The JI mix showed the most compaction, with the effective volume of mix in a full pot decreasing from 1,290 ml to 925 ml. The control container filled to 30 mm from the top compacted from 1,030 ml to 800 ml, and the container filled to 60 mm from the top compacted from 780 ml to 590 ml. The WHC of the controls was lowest for the full pot and highest for the pot filled to 60 mm from the top.

The results of the JI trials are presented in Fig 8 by combined drainage layer type. Post-hoc Tukey comparisons are presented in Table 7 (by combined drainage layer type), Table 8 (by drainage substrate only), and Table 9 (by drainage layer depth only). Compared to the control, the leca-30 mm drainage layer condition had higher overall WHC (p<0.05), the sand-60 mm drainage layer condition had lower overall WHC (p<0.01), and all other conditions were not significantly different. Analysing only according to drainage substrate, sand had lower overall WHC (p<0.05), and other substrates were not significantly different. Sand had the lowest overall WHC, and was significantly different from all other substrates, while leca had the highest WHC. Combining all substrates, neither 30 mm or 60 mm drainage layers had significantly different overall WHC from the control.

Table 7. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer types.

Drainage layer type	Comparison	Mean Difference	SE	t	p
Control	Gravel-60 mm	0 . 021	0 . 013	1 . 619	0 . 792
	Gravel-30 mm	- 0 . 004	0 . 013	- 0 . 306	1 . 000
	Grit-60 mm	- 0 . 027	0 . 013	- 2 . 104	0 . 478
	Grit-30 mm	- 0 . 010	0 . 013	- 0 . 766	0 . 997
	Leca-60 mm	- 0 . 001	0 . 013	- 0 . 098	1 . 000
	Leca-30 mm	- 0 . 044	0 . 013	- 3 . 359	0.031*
	Sand-60 mm	0 . 055	0 . 013	4 . 229	0.002**
	Sand-30 mm	0 . 021	0 . 013	1 . 645	0 . 777
Gravel-60 mm	Gravel-30 mm	- 0 . 025	0 . 013	- 1 . 925	0 . 599
	Grit-60 mm	- 0 . 049	0 . 013	- 3 . 723	0.010*
	Grit-30 mm	- 0 . 031	0 . 013	- 2 . 385	0 . 307
	Leca-60 mm	- 0 . 022	0 . 013	- 1 . 717	0 . 735
	Leca-30 mm	- 0 . 065	0 . 013	- 4 . 978	<.001***
	Sand-60 mm	0 . 034	0 . 013	2 . 610	0 . 199
	Sand-30 mm	3.436×10⁻⁴	0 . 013	0 . 026	1 . 000
Gravel-30 mm	Grit-60 mm	- 0 . 023	0 . 013	- 1 . 798	0 . 683
	Grit-30 mm	- 0 . 006	0 . 013	- 0 . 460	1 . 000
	Leca-60 mm	0 . 003	0 . 013	0 . 208	1 . 000
	Leca-30 mm	- 0 . 040	0 . 013	- 3 . 053	0 . 071
	Sand-60 mm	0 . 059	0 . 013	4 . 535	<.001***
	Sand-30 mm	0 . 025	0 . 013	1 . 951	0 . 581
Grit-60 mm	Grit-30 mm	0 . 017	0 . 013	1 . 338	0 . 917
	Leca-60 mm	0 . 026	0 . 013	2 . 007	0 . 543
	Leca-30 mm	- 0 . 016	0 . 013	- 1 . 255	0 . 941
	Sand-60 mm	0 . 083	0 . 013	6 . 333	<.001***
	Sand-30 mm	0 . 049	0 . 013	3 . 749	0.010**
Grit-30 mm	Leca-60 mm	0 . 009	0 . 013	0 . 668	0 . 999
	Leca-30 mm	- 0 . 034	0 . 013	- 2 . 593	0 . 206
	Sand-60 mm	0 . 065	0 . 013	4 . 995	<.001***
	Sand-30 mm	0 . 031	0 . 013	2 . 411	0 . 292
Leca-60 mm	Leca-30 mm	- 0 . 043	0 . 013	- 3 . 261	0.041*
	Sand-60 mm	0 . 056	0 . 013	4 . 327	0.001**
	Sand-30 mm	0 . 023	0 . 013	1 . 743	0 . 718
Leca-30 mm	Sand-60 mm	0 . 099	0 . 013	7 . 588	<.001***
Leca-30 mm	Sand-30 mm	0 . 065	0 . 013	5 . 004	<.001***
Sand-60 mm	Sand-30 mm	- 0 . 034	0 . 013	- 2 . 584	0 . 210

Open in a new tab

*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 9)

Table 8. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer substrates.

Drainage layer substrate	Comparison	Mean Difference	SE	t	p
Control	Gravel	0 . 009	0 . 012	0 . 686	0 . 959
	Grit	- 0 . 019	0 . 012	- 1 . 499	0 . 566
	Leca	- 0 . 023	0 . 012	- 1 . 806	0 . 377
	Sand	0 . 038	0 . 012	3 . 069	0.023*
Gravel	Grit	- 0 . 027	0 . 010	- 2 . 676	0 . 066
	Leca	- 0 . 031	0 . 010	- 3 . 052	0.025*
	Sand	0 . 030	0 . 010	2 . 919	0.035*
Grit	Leca	- 0 . 004	0 . 010	- 0 . 375	0 . 996
Grit	Sand	0 . 057	0 . 010	5 . 595	<.001***
Leca	Sand	0 . 061	0 . 010	5 . 970	< .001***

Open in a new tab

*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 5)

Table 9. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer depths.

Drainage layer depth	Comparison	Mean Difference	SE	t	p
Control	60 mm	0 . 012	0 . 014	0 . 879	0 . 655
Control	30 mm	- 0 . 009	0 . 014	- 0 . 671	0 . 781
60 mm	30 mm	- 0 . 021	0 . 009	- 2 . 452	0.042^*

Open in a new tab

*p < . 05, **p < . 01, ***p < . 001 (P-value adjusted for comparing a family of 3)

The modelled WHC of the medium itself in the drainage layer conditions is presented in Fig 9 (30 mm drainage layers) and Fig 10 (60 mm drainage layers), compared with the control containers with the same initial medium depth. For 30 mm drainage layers, grit and sand had lower WHC than the control according to both models, while gravel and leca had modelled WHC values both above and below the control. For 60 mm drainage layers, sand had lower WHC than the control according to both models, while all other substrates had modelled WHC values both above and below the control.

Discussion

The results show that adding a layer of coarse substrate below potting media will usually reduce water retention of the container overall, sometimes have no effect, and is very unlikely to increase water retention. This supports the recommendation of adding drainage layers to increase drainage, and contradicts previous theoretical predictions that such layers worsen drainage [12, 13].

The addition of drainage layers caused a greater reduction in water retention when the potting media was coarser and more porous. This may be due to the similarity in pore size improving the capillary movement of water through the interface between different materials [18]. This possible explanation is also supported by the fact that the loam-based John Innes media had the greatest reduction in water retention when using a substrate of 1–2 mm sand, which has the smallest particle size and therefore a pore size closest to the JI medium itself.

Thicker layers of drainage substrate tended to increase drainage more than thinner layers. This could be due to the perched water table of the drainage substrate itself making up a smaller proportion of the volume of the deeper layer. There was no apparent difference in drainage effects between porous and non-porous drainage substrates of the same particle size (leca and gravel). This is likely because leca is slow to absorb water, and holds onto water at high tensions [33]. Thus, the leca in the drainage layer would not absorb significant quantities of water in the short time the container is saturated before drainage, and would therefore behave similarly to non-porous gravel of the same particle size.

For a very coarse loamless medium such as the CPB mix, the results show that any drainage substrate of any depth can be used to good effect to increase drainage. For a finer loamless medium such as the CV mix, a 60 mm drainage layer is recommended, but any substrate is effective. For a loam-based medium such as John Innes, a 60 mm layer of coarse sand is recommended for the greatest increase in drainage.

The two different models for WHC of the potting medium alone were intended to provide the widest possible range of values, as a starting point for analysis. From visual examination (Fig 11), the drainage layers in containers with loamless potting media appeared to have approximately the same perched water table depth as when alone. Thus, for the CV and CPB media, the predictions of model A are likely to be most accurate for the WHC of the medium alone. For these media both models showed that all drainage layers reduced WHC, so the choice of model for analysis does not significantly change the conclusions.

Fig 11 — In the left image, the perched water table (PWT) in the medium is completely eliminated, and the low perched water table in the grit is visible.

In contrast, the drainage layers in containers with JI medium appeared to remain almost completely saturated after drainage (Fig 12). Thus, for JI medium the predictions of model B are likely to be most accurate for the WHC of the medium alone. The predictions of model B showed that all substrates decreased WHC, whereas model A showed a decrease for some substrates and an increase for others. Thus if model B is considered to be more accurate, it can be concluded that all drainage layers reduced WHC in the medium itself, whereas only some drainage layers reduced WHC in the container as a whole. More detailed study is required to confirm these predictions and validate the models.

The present study had a narrow focus, aiming to analyse the real-world behaviour of water moving through potting media in containers with drainage layers. The individual trials were controlled to ensure validity of the results for the values being measured. However there were no further laboratory investigations into the pore and grain size, quantitative water potential, air-entry value, or other characteristics of the media containers in isolation. These characteristics have a significant impact on the water behaviour of media [7]. Therefore a valuable direction for future research would be to replicate the experiment alongside further laboratory analysis of the media containers, to deepen understanding of why the results of the present study were found. Replicating the experiment using a range of different potting media, drainage substrates, and container shapes and sizes would allow quantitative analysis of how these factors affect the impact of drainage layers on water retention.

The choice to emulate real-world conditions also prevented laboratory controls being applied to the methods of filling, watering, and draining the test containers. Instead, simple practical methods such as weighing media and visual observation of the container contents and drainage process were employed to control the conditions as well as possible. The consistency between results within each experimental condition suggests that these controls were sufficient to produce valid results. However, further replications of the study will be valuable to confirm its validity.

In the present study, containers were fully saturated before being drained, and were assumed to have 100% of their pore spaces filled with water before drainage began. This allowed analysis of the drainage behaviour of the different media combinations in isolation from their wetting behaviour. However in practical use containers do not reach full saturation when they are watered from above, because water begins to drain out before the point of saturation [21], and because some air becomes entrapped in isolated pore spaces. Investigations of soil water hysteresis have shown that soils typically retain more water at a given suction level while drying than while wetting [34, 35]. This is largely governed by the ’ink-bottle effect’, in which large pores with small openings remain filled with water during drying. Additionally, many studies have shown dynamic non-equilibrium effects in which a given medium at a given suction level can have differing water content depending on whether the water is static, moving with a steady flow, or moving with a variable flow [36]. There are many possible causes for these effects and they are still poorly-understood, with predictions differing between research conducted in different fields [37]. Therefore further study is required to investigate the effects of hysteresis and dynamic non-equilibrium on the water movement through containers with drainage layers during irrigation.

The present study did not test the response of living plants when grown in containers with drainage layers. Although the results can be applied effectively to decrease water retention of a given container of media, they cannot be used to predict whether a given plant will benefit from this intervention. Recent studies have shown that stratified containers with a more porous medium in the lower half can improve crop growth, quality, and yield in experimental conditions [29]. Controlled growing trials are required to study the growth of plants in containers with shallow drainage layers, of the type considered in this study and commonly employed by horticulturists and home growers. These trials could investigate a range of factors such as crop yield and quality, irrigation requirements, shoot and root behaviour, and the differing responses of different plant species.

Conclusions

Despite being frequently discussed by horticulturists and home gardeners, there has been little applied research into the effects of drainage layers on the water-holding capacity of container media. Related research in different fields has led to conflicting predictions about whether drainage layers will increase, or conversely decrease, the amount of water stored in the soil above. The object of this study was to experimentally determine how the addition of a layer of coarse substrate at the bottom of a container would affect the water-holding capacity of the potting medium above. Three experiments were conducted using three different common potting media. Each experiment varied the depth and substrate of the drainage layer, and measured the amount of water retained in the container. Two simple models were used to estimate the water retention in the medium alone by subtracting the water retained in the drainage layer itself. The potting media, drainage substrates, containers, and watering methods were chosen to emulate the materials and methods typically available to home gardeners, to ensure the results would be valid for practical application.

Drainage layers were almost universally found to either decrease or have no effect on the water-holding capacity of containers. Thicker drainage layers decreased water retention more than thinner layers, but the most effective drainage substrate to use was dependent on the specific potting medium. Coarser organic potting media showed the greatest reduction in water-holding capacity with the addition of drainage layers, and fine loam-based media the least. For loamless organic media, both models agreed that all drainage layers reduced the water retained in the medium alone. For loam-based media the models differed in their estimates, but the most plausible model based on visual examination showed that all drainage layers reduced water retention of the medium alone.

The present study was conducted using method and material limitations similar to those of home gardeners. This was intended to ensure the results would be valid for practical application in their intended context. The result can be taken to show that warnings about the raised perched water table and increased water retention should be considered inaccurate. Instead, drainage layers can be considered a potentially useful tool for growers and home gardeners to reduce the water retention of a given potting medium. A 60 mm later of coarse sand was the most universally-effective drainage layer with all potting media tested and therefore the best single recommendation for growers wishing to increase drainage. Due to the focus of the research on practical application, the pore and grain size, quantitative water potential, air-entry value, and other characteristics of the potting media and drainage substrates were not measured. Thus the results cannot be modelled in relation to these metrics, and gaining improved understanding of why the present results were obtained would be a valuable focus for future research.

Supporting information

S1 Appendix. ANOVA tables.

(PDF)

pone.0318716.s001.pdf^{(76KB, pdf)}

S1 Data. Raw data from measurements of CPB media trials.

(CSV)

pone.0318716.s002.csv^{(4.1KB, csv)}

S2 Data. Raw data from measurements of CV media trials.

(CSV)

pone.0318716.s003.csv^{(4.1KB, csv)}

S3 Data. Raw data from measurements of JI media trials.

(CSV)

pone.0318716.s004.csv^{(4.1KB, csv)}

S4 Data. Raw data from measurements of drainage layer trials without potting media.

(CSV)

pone.0318716.s005.csv^{(3KB, csv)}

Acknowledgments

The research received no external support.

Data Availability

All relevant data are within the manuscript and its Supporting information files.

Funding Statement

The author(s) received no specific funding for this work.

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PLoS One. 2025 Feb 25;20(2):e0318716. doi: 10.1371/journal.pone.0318716.r001

Author response to Decision Letter 0

2 Aug 2024

PLoS One. doi: 10.1371/journal.pone.0318716.r002

Decision Letter 0

Somayeh Soltani-Gerdefaramarzi

2 Oct 2024

PONE-D-24-32597Effect of drainage layers on water retention of potting media in containersPLOS ONE

Dear Dr. Rowe,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Reviewer #1: This paper presents an interesting work on exploring the best settings for the drainage layer down under the plant pot in consideration of the perched water effect or capillary barrier effect. I recognised the significance of this work's contribution from a very gardening and soil hydrological perspective. I am very keen to assist the author in publishing this work. Nonetheless, in terms of soil physics, the depth of comprehension and discussion is still shallow. To assist the authors in publishing their work at the high quality that PLOS One truly needs, I decided to reach out to a major revision to see if the author can substantially improve their research presentation in this manuscript by following the specific comment below. Also, the author is most welcome to rebut in an acceptable manner if there are any disagreements or differences in understanding saturated and unsaturated soil physics.

1. In order to deepen the knowledge basis and discussion in this paper, you may want to have a careful review of the ink-bottle effect and capillary barrier or clay liner, just like what you described for perched water above any possible aquitard or aquiclude, although you just reassign such an engineering-scale geological condition into the scenario in a plant container or a pot.

2. In addition, you may want to expand your understanding of unsaturated and saturated soil further from a soil hydrological point of view with assistance from a few good references.

Yan, G.; Li, Z.; Galindo Torres, S.A.; Scheuermann, A.; Li, L. Transient Two-Phase Flow in Porous Media: A Literature Review and Engineering Application in Geotechnics. Geotechnics 2022, 2, 32-90. https://doi.org/10.3390/geotechnics2010003

Yan G, Bore T, Schlaeger S, Scheuermann A, Li L. Investigating scale effects in soil water retention curve via spatial time domain reflectometry. Journal of Hydrology. 2022 Sep 1;612:128238.

3. Even though you have presented some data to manifest the research findings you have in the abstract, you have not quantitatively analysed the grain size distributions of coarse, fine, and loamy layers with different settings in drainage layers. It would be much better if you could provide more sieving analysis for different configurations between the soil substrate at the top and the gravel-sand mixing drainage layer at the bottom. Have you checked the packing conditions in your plat pot (e.g., soil density, porosity, void ratio, etc.)? Different packing conditions will lead to different findings, and they ought to be carefully controlled and listed in your experimental settings for statistical analysis.

4. In the methodological section, you may want to insert more information about the measuring errors and uncertainties that you encountered during the tests and write up a good self-defence to strengthen the rigour and prudence of your experimental methods. Besides, you forgot to assign codes and numbers for all equations you left in this section, and neither did you for notations of all mathematical symbols. Also, experimental demos and visual illustrations are lacking in this method section.

5. Regarding the analysis of mean (ANOM) and analysis of variance (ANOVA), you can provide a table to illustrate how it was statistically calculated with all settings in such a design of experiment. Corresponding statistical equations can also be inserted into this section to enrich your mathematical complexity, but please do not forget to number them.

6. If possible, you can give more discussion on air-entry value, capillary infiltration breakthrough water pressure and its corresponding soil moisture storage, porosity (packing conditions), etc. Those will help you professionalise your discussion in addition to the relatively superficial understanding of the experimental data.

7. You can highlight what you intend to demonstrate with a few red boxes in the dashline in Figures 10 and 11.

8. The conclusions are all lumped into a single paragraph in a relatively poor writing structure. The conclusion ought to be in this sequence: Paragraph 1: (i) Concise summary of your research gaps identified through literature review and research objectives; (ii) Briefing your methodology with some very critical experimental settings; Paragraph 2: (iii) Listing your conclusions or research findings point-by-point; Paragraph 3: (iv) Mention the limitations and reflection of your work; (v) Highlight the significance of your research and engineering applications.

Reviewer #2: Dear Editor and Author

I hope this message finds you well. I would like to express my appreciation for the article, which I believe holds significant practical value. However, I have identified several points that should be addressed to enhance the transparency of the content. These are outlined below:

1. In line 130, it is noted that fiberglass mesh is used exclusively for the containers containing the sand drainage layer. Given that the size of the drainage hole at the end of the container is 10 mm, which exceeds the aggregate size of the substrate in the drainage layer, could you clarify this choice?

2. I recommend including a schematic diagram of the container to improve understanding.

3. It is unclear why the soil was not covered during the drainage period and why the duration of drainage was limited to only 3 hours. Typically, in a control pot, water drainage lasts longer. Please provide clarification on this matter.

4. More detailed information on how the containers were filled would be beneficial. Specifically, how was layering of the soil prevented during filling?

5. In line 154, it is stated that moisture levels in the media within containers are not uniform; however, line 139 mentions that the soil was air-dried to achieve uniform moisture. It would be helpful to explain this apparent contradiction for clarity.

6. In laboratory settings, it is common practice to saturate soil by introducing water from the bottom to allow trapped air to escape from the top. Was this method employed in your experiment?

7. How did you ensure that the pots were not blocked, particularly in the control pots that lacked a drainage layer?

8. At the beginning of the Materials and Methods section, media were numbered; however, there appears to be incomplete numbering.

9. In line 129 "The base had a slight domed area raised by 4mm relative to the edges". Why was this container chosen?

Lastly, I noticed several typographical and spelling errors throughout the article, many of which could be easily corrected using text editing software.

Thank you for considering these suggestions. I believe addressing these points will significantly enhance the quality and clarity of your work. Wishing you continued success.

Best regards,

**********

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Reviewer #1: No

Reviewer #2: No

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PLoS One. 2025 Feb 25;20(2):e0318716. doi: 10.1371/journal.pone.0318716.r003

Author response to Decision Letter 1

20 Dec 2024

Reviewer #1

1. I have added a consideration of soil water hysteresis and the ink-bottle effect (lines 302-313). Capillary barriers are discussed in the introduction (lines 27-36).

2. I have included a citation in the discussion of soil water hysteresis and the ink-bottle effect (line 309).

3. Unfortunately these investigations were not conducted at the time of the experiment, and the original media are no longer available to examine. I believe that the research still has practical value, particularly for applied horticulturists who may need to make decisions about containers and growing media without being able to conduct laboratory investigations of their own. I have added an acknowledgement of this limitation to the study in the discussion (lines 314-328).

4. I have specified the precision of the measurements in the method (lines 134 and 143). I have added definitions for the terms in the equations (lines 159-160, 165-166, 170-171), and numbered the equations separately from the text (equations 1-4). Unfortunately no other photographs were taken of the experimental setup, but I have added a schematic diagram (figure 1).

5. I have added full tables of the post-hoc comparisons to the text (Tables 1-9), and the ANOVA calculations as a supplementary file (S1 Appendix).

6. Unfortunately these investigations were not conducted at the time of the experiment, and the original media and apparatus are no longer available to examine. I believe that the research still has value as it presents a novel and significant result with practical relevance, and also invites further study to deepen understanding of the mechanisms involved. I have added an acknowledgement of this limitation to the study in the discussion (lines 314-328).

7. I have added annotations to figures 11 and 12.

8. I have restructured and expanded the conclusions section (lines 330-367).

Reviewer #2

1. Although the individual particle sizes were smaller than the drainage hole, the friction of the aggregate pieces together was sufficient to prevent the other substrates escaping from the drainage hole without the addition of the fibreglass mesh. I have explained this in the text (lines 119-120).

2. I have added a schematic diagram of the experimental procedure (figure 1).

3. Drainage took only a short time due to the small size of the containers and the potting media used (which have larger particle sizes than field loams). The final measurements were taken when water was no longer visibly draining from the container. I have explained this in the text (lines 140-142). The containers were left uncovered to match the conditions of a real-world growing container being watered. I have explained this in the text (lines 138-140). Due to the short drainage period, the effect of evaporation is likely to be negligible.

4. The containers were filled by hand. This was again chosen to represent the real-world application of drainage layers in practical gardening. The clear containers allowed observation of the potting media and no evidence of layering was observed during filling, watering, or drainage. I have added this to the text (lines 142-143). I have also added a consideration of these practical limitations to the discussion (lines 323-328).

5. The different potting media were not dried to the same moisture level as each other (the three media were treated as independent experiments and not compared to each other), but every sample of a given medium was dried to the same moisture level (for validity between the trials of each experiment). I have clarified this in the text (lines 129-132).

6. The water was poured from above as it typically would be during real-world watering of container plants (lines 136-137). I have added a consideration of the watering method to the discussion (lines 302-313).

7. No special procedure was employed, but the containers were monitored during drainage and no blockages or inconsistencies were observed. I have mentioned this in the text (lines 142-143).

8. "No. 1" is the name of the specific formulation of John Innes compost, rather than numbering to identify the media in the text. I have added quotation marks to make this clearer in the text (line 98).

9. This was chosen to represent the typical shape of a nursery container, which generally have a raised centre and a lowered border or feet to allow water to escape when placed on a flat surface. I have mentioned this in the text (lines 115-117).

PLoS One. doi: 10.1371/journal.pone.0318716.r004

Decision Letter 1

Somayeh Soltani-Gerdefaramarzi

2 Jan 2025

PONE-D-24-32597R1Effect of drainage layers on water retention of potting media in containersPLOS ONE

Dear Dr. Rowe,

============================================================

Please submit your revised manuscript by Feb 16 2025 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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We look forward to receiving your revised manuscript.

Kind regards,

Somayeh Soltani-Gerdefaramarzi, Ph. D.

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

**********

6. Review Comments to the Author

Reviewer #1: I do believe that the author has made the best effort to improve the academic rigour and quality of the manuscript from the last round of revisions. The author has not only provided more statistical analysis in addition to the previous bar charts but also significantly deepened his/her understanding of unsaturated soil science and capillarity in terms of soil suction, moisture and ink-bottle effects. Therefore, I recommend consideration of publication as it is decent work for in-door planting rather than unsaturated soil mechanics and vadose zone seepage science.

However, prior to formal acceptance, I would like to invite the author to reflect on their experimental limitations and then write up some good proposals for future research at the end of the discussion section. Regarding this limitation and prospect, other works deserve to be reviewed and cited herein.

Diamantopoulos E, Durner W (2012) Dynamic nonequilibrium of water flow in porous media: a review. Vadose Zone J 11(3):vzj2011.0197

Yan, G., Bore, T., Schlaeger, S., Scheuermann, A., & Li, L. (2024). Dynamic effects in soil water retention curves: An experimental exploration by full-scale soil column tests using spatial time-domain reflectometry and tensiometers. Acta Geotechnica, 1-27.

After all, dynamic nonequilibrium soil suction also matters with local heterogeneity, e.g., ink-bottle effects, further inducing concerns of the so-called “snap-off” and “Haine jump” effects at the local microscale. With such a self-reflection section, you can defend yourself by mentioning those limits in your current work and demonstrate your in-depth understanding of unsaturated soil knowledge that will be used to orient your academic pursuits in the future.

**********

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Reviewer #1: No

**********

PLoS One. 2025 Feb 25;20(2):e0318716. doi: 10.1371/journal.pone.0318716.r005

Author response to Decision Letter 2

7 Jan 2025

Dear Reviewer,

Thank you again for your valuable feedback on my submission. I attach a revised manuscript and a tracked changes manuscript.

I have restructured and expanded the discussion. It now includes further references, and more detailed proposals for future research.

I hope these improvements meet with your approval, and I am grateful for your time and advice.

Yours sincerely,

Avery Rowe

PLoS One. doi: 10.1371/journal.pone.0318716.r006

Decision Letter 2

Somayeh Soltani-Gerdefaramarzi

21 Jan 2025

Effect of drainage layers on water retention of potting media in containers

PONE-D-24-32597R2

Dear Dr. Rowe,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Somayeh Soltani-Gerdefaramarzi, Ph. D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

Reviewer #1: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

Reviewer #1: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

Reviewer #1: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

Reviewer #1: Yes

**********

6. Review Comments to the Author

Reviewer #1: The author has fully addressed the comment I left before. This manuscript can be accepted as is after two rounds of revisions.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

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PLoS One. doi: 10.1371/journal.pone.0318716.r007

Acceptance letter

Somayeh Soltani-Gerdefaramarzi

PONE-D-24-32597R2

PLOS ONE

Dear Dr. Rowe,

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

S1 Appendix. ANOVA tables.

(PDF)

pone.0318716.s001.pdf^{(76KB, pdf)}

S1 Data. Raw data from measurements of CPB media trials.

(CSV)

pone.0318716.s002.csv^{(4.1KB, csv)}

S2 Data. Raw data from measurements of CV media trials.

(CSV)

pone.0318716.s003.csv^{(4.1KB, csv)}

S3 Data. Raw data from measurements of JI media trials.

(CSV)

pone.0318716.s004.csv^{(4.1KB, csv)}

S4 Data. Raw data from measurements of drainage layer trials without potting media.

(CSV)

pone.0318716.s005.csv^{(3KB, csv)}

Data Availability Statement

All relevant data are within the manuscript and its Supporting information files.

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PERMALINK

Effect of drainage layers on water retention of potting media in containers

Avery Rowe

Roles

Abstract

Introduction

Materials and methods

Fig 1. Experimental procedure.

Results

Coir-perlite-bark medium

Fig 2. Mean and standard deviation water-holding capacity (WHC) of the entire container of CPB medium with different drainage layer types.

Table 1. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer types.

Table 2. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer substrates.

Table 3. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CPB medium with different drainage layer depths.

Fig 3. Estimated mean water-holding capacity (WHC) of the CPB medium alone with different 30 mm drainage layers, according to models A and B.

Fig 4. Estimated mean water-holding capacity (WHC) of the CPB medium alone with different 60 mm drainage layers, according to models A and B.

Coir-vermiculite medium

Fig 5. Mean and standard deviation water-holding capacity (WHC) of the entire container of CV medium with different drainage layers.

Table 4. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer types.

Table 5. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer substrates.

Table 6. Post-hoc Tukey comparisons of water-holding capacity of the entire container of CV medium with different drainage layer depths.

Fig 6. Estimated mean water-holding capacity (WHC) of the CV medium alone with different 30 mm drainage layers, according to models A and B.

Fig 7. Estimated mean water-holding capacity (WHC) of the CV medium alone with different 60 mm drainage layers, according to models A and B.

John Innes medium

Fig 8. Mean and standard deviation water-holding capacity (WHC) of the entire container of JI medium with different drainage layers.

Table 7. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer types.

Table 8. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer substrates.

Table 9. Post-hoc Tukey comparisons of water-holding capacity of the entire container of JI medium with different drainage layer depths.

Fig 9. Estimated mean water-holding capacity (WHC) of the JI medium alone with different 30 mm drainage layers, according to models A and B.

Fig 10. Estimated mean water-holding capacity (WHC) of the JI medium alone with different 60 mm drainage layers, according to models A and B.

Discussion

Fig 11. Comparison of CPB medium with 60 mm layer of grit (left) to CPB medium alone to a depth of 60 mm below the top (right).

Fig 12. Comparison of JI medium with 60 mm layer of grit (left) to JI medium alone to a depth of 60 mm below the top (right).

Conclusions

Supporting information

Acknowledgments

Data Availability

Funding Statement

References

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Somayeh Soltani-Gerdefaramarzi

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Somayeh Soltani-Gerdefaramarzi

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Somayeh Soltani-Gerdefaramarzi

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Somayeh Soltani-Gerdefaramarzi

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