Water retention characteristics of coarse porous materials to construct purpose-designed plant growing media

Figures & data

Figure 1. Graphic representation of the calculated porosities. The pores are represented in white. Two main pore domains are observed: the pores inside the CPMs, or intra-particle porosity (${\varphi }_{intra}$), as well as the voids among particles, or inter-particle porosity (${\varphi }_{inter}$). The (A) total porosity (${\varphi }_t$) accounts for both and it was calculated by $1-\frac{{\rho }_{bulk}}{{\rho }_p}$. The (B) intra-particle porosity (${\varphi }_{intra}$) excludes the ${\varphi }_{inter}$ and was calculated by $1-\frac{{\rho }_{app}}{{\rho }_p}$.

Table 1. General characteristics of the CPMs and the sand. EC = electric conductivity, ${\rho }_{bulk}$ = bulk density, ${\rho }_{app}$ = apparent density, ${\varphi }_t$ = total porosity and ${\varphi }_{intra}$ = intra-particle porosity (only the porous of the materials, without the surroundings).

	Size		EC	${\rho }_{bulk}$	${\rho }_{app}$	${\rho }_p$	${\varphi }_t$	${\varphi }_{intra}$
Material	(mm)	pH	($\mu$S cm^–1)	(g$\cdot$cm^−3)	(g$\cdot$cm^−3)	(g$\cdot$cm^−3)	(cm${ }^3\cdot$cm^−3)	(cm${ }^3\cdot$cm^−3)
Porlith	2–7	5.7	2.5	0.65	1.24	2.77	0.76	0.55
Expanded shale	2–7	7.4	109.9	0.78	1.42	2.41	0.68	0.42
Expanded clay	4–8	7.7	807.0	0.41	1.24	2.15	0.81	0.42
Tuff	3–8	6.9	81.5	0.88	1.47	2.55	0.66	0.42
Pumice	2–12	7.3	173.9	0.57	0.81	2.12	0.73	0.61
Lava	5–15	7.5	49.5	0.86	1.65	3.05	0.72	0.46
Sand	0.1–0.5	5.2	2.0	1.48	–	2.60	0.43	–

Figure 2. Observed water retention data and fitted retention curves for the sand, composite samples and pure CPMs. Sand: ($\times$) measured, ($\cdot \cdot \cdot \cdot$) fitted, sand mixed with CPM: ($\bullet$) measured, (- -) fitted, CPMs: ($\circ$) measured, (—) fitted (uncertainty is symbolized by - -). The graph of porlith (upper left) shows the measured points for sand. In the graphs can be observed the inflections of the air entry point(s), and the water held in the range of available water content (AWC) from pF 1.8 to pF 4.2.

Table 2. Available water capacity (AWC), field capacity (FC), and parameters of unimodal (applied just for sand) and bimodal Peters–Durner–Iden model (2015) fitted to the data of pure materials and the materials mixed with sand.

	Parameter
Material	$h_{m1}$	${\sigma }_1$	$h_{m2}$	${\sigma }_2$	${\theta }_r$	${\theta }_s$	$w_1$	RMSE	AWC	FC
Porlith	6.99	0.286	284.20	0.205	0.17	0.76	0.50	2.37$\cdot$10^−3	0.37	0.44
Exp. shale	6.25	0.200	3.36	0.830	0.11	0.68	0.66	4.88$\cdot$10^−2	0.08	0.09
Exp. clay	2.69	0.200	120.70	1.091	0.00	0.81	0.18	3.34$\cdot$10^−2	0.09	0.10
Tuff	908.42	0.208	3.54	0.567	0.11	0.66	0.73	1.43$\cdot$10^−2	0.17	0.23
Pumice	2.68	0.355	2.68	0.338	0.15	0.73	0.60	4.12$\cdot$10^−2	0.09	0.11
Lava	3.49	0.499	56.97	0.207	0.07	0.72	0.04	0.84$\cdot$10^−2	0.06	0.07
Sand	20.45	0.691	–	–	0.02	0.43	–	7.03$\cdot$10^−3	0.03	0.03
Porlith + sand	29.38	2.791	29.38	0.200	0.11	0.46	0.80	0.69$\cdot$10^−2	0.08	0.13
Exp. shale + sand	176.51	1.248	27.68	0.307	0.04	0.42	0.86	0.44$\cdot$10^−2	0.06	0.08
Exp. clay + sand	27.68	0.200	27.68	0.201	0.09	0.47	0.75	0.15$\cdot$10^−2	0.06	0.09
Tuff + sand	26.07	0.297	186.78	1.856	0.07	0.43	0.25	0.58$\cdot$10^−2	0.10	0.13
Pumice + sand	26.92	1.678	22.19	0.401	0.02	0.48	0.85	0.23$\cdot$10^−2	0.07	0.09
Lava + sand	121.15	1.170	24.17	0.305	0.01	0.44	0.86	0.33$\cdot$10^−2	0.05	0.05

For simplification, ${\theta }_s$ was set equal to ${\varphi }_t$ and thus not fitted. The parameter $h_0$ was set to 10^6.8 cm, which is the suction at oven dryness for 105°C (Schneider and Goss 2012). $h_i$ and ${\sigma }_i$ are shape parameters. The first can be related to the air entry point and the latter can be related to the inclination. $w$ is a weighing factor.

Figure 3. Volumetric water content of the CPMs intra-particle porosity (${\varphi }_{intra}$) held at matric force of pF 1.8 (considered as field capacity) and AWC (cross hatched columns) when embedded in sand compared when embedded in silt. The pebbles of the CPMs were embedded in silt to ensure capillary contact when measured with ceramic tension plates at matric forces ranging from 1.8 to 3. Therefore, the water content of the pure sand and silt was obtained and subsequently subtracted with Equation (1)(1) ${\theta }_{CPM}=[{\theta }_t-((1-X)\mathrm{x}{\theta }_{s{a}_i})/X]\mathrm{x}(1-{\varphi }_{inter})$(1) . The result is the water stored specifically inside the pebbles.

Schneider M, Goss KU 2012: Prediction of water retention curves for dry soils from an established pedotransfer function: Evaluation of the Webb model. Water Reso. Res., 48(6). doi:10.1029/2011WR011049

 Google Scholar