Soil Science Society of America Journal 66:367-373 (2002)
© 2002 Soil Science Society of America
DIVISION S-1—SOIL PHYSICS
Determining Air Entry Value in Peat Substrates
M. R. Nematia,
J. Caron*,b,
O. Bantonc and
P. Tardifb
a Université du Québec, INRS-Eau, 2800 rue Einstein, C.P. 7500, Ste-Foy, QC, Canada, G1V 4C7
b Département des Sols et de Génie Agroalimentaire, FSAA, Université Laval. Québec, QC, Canada, G1K 7P4
c Université du Québec, INRS-Eau, 2800 rue Einstein, C.P. 7500, Sainte-Foy, QC, Canada, G1V 4C7
* Corresponding author (jean.caron{at}sga.ulaval.ca)
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ABSTRACT
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Insufficient aeration in artificial mixes used for nursery and greenhouse production is commonly reported to be a problem. To diagnose aeration problems, gas relative diffusivity estimates can be used. These estimates can be obtained indirectly from measurements of air entry value, saturated hydraulic conductivity and the water desorption curve. In here, a rapid tension chamber method for measuring air entry is proposed. To verify applicability of the method, different potted substrates were inserted into a tension chamber apparatus and water was extracted from the saturated substrate by establishing a contact with an unsaturated body located outside the chamber. The potential within the chamber was monitored over a 5-min period and the point of air entry was calculated thereafter. Statistically equal estimates of air entry were obtained with this technique compared with values derived from the tension table, for a wide range of air entry values in peat-based substrates.
Abbreviations: 
, hydraulic potential • 
, water content
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INTRODUCTION
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INSUFFICIENT AERATION is a common problem in growing media used for nursery and greenhouse production. The problem can occur because of a general tendency to overwater potted plants, because of substrate compaction, or because of the nature of the substrate components. Container geometry can also play a role. Indeed, substrates are used in pots ranging from 20 to 200 mm in height. Following saturation and drainage, shallow perched water tables and capillary fringes are often observed at the bottom of these pots, resulting in an average water potential values in the pot related to container height. Consequently, air-filled porosity is observed to decrease with container depth (White, 1964; Fonteno, 1989; Rivière, 1990), increasing the risk of asphyxia. Hence, researchers have endeavored to better characterize the aeration processes in such media. Allaire et al. (1996), Nkongolo (1996), and Caron et al. (2000) have shown that, in peat substrates, gas relative diffusivity estimates are far superior to air-filled porosity estimates in characterizing the aeration processes and predicting plant performance. These in turn can be estimated from air-entry values, saturated hydraulic conductivity, and water desorption curve.
Because they are very fragile relative to mineral soils, growing media must, whenever possible, be characterized in situ using methods that limit substrate disturbance. Consequently, special methods to determine saturated hydraulic conductivity (Allaire et al., 1994) and water desorption curves (Paquet et al., 1993) in potted substrates have been designed to be used for routine quality control purposes. Resulting values can also indirectly estimate gas diffusivity (Nkongolo, 1996). However, this estimation procedure requires that the value of air entry into the substrate be first determined (Allaire et al., 1996).
The air entry value is normally estimated from the water desorption curve at a high potential. It corresponds to the potential at which air truly enters the substrate after some drainage has occurred. In fragile substrates, estimation of air entry value from water desorption curves faces some limitations. The process is time consuming. It should take into account possible substrate shrinkage, where the early drop in the water content of the water desorption curve may be confused with the increasing bulk density of the material.
In mineral soils, air-entry permeameters (Bouwer, 1966; Topp and Binns, 1976; Fallow and Elrick, 1996) are used for air entry determination. However, they cannot be used in potted substrates, because of the pot's small size and configuration.
Alternatively, simple designs have been suggested for potted substrates in which a tension is applied from underneath the sample in a funnel through a water column, to a point at which the water column breaks (Childs, 1969). The process is relatively rapid, but since no measurements of either water content or air pressure are performed, it is not known if true air entry has occurred and where it has occurred. The problem of the exact location is particularly relevant if determination is carried in situ on substrates potted in 150- or 200-mm high containers, as it then becomes difficult to determine the exact potential applied or when air has entered the system without using additional measuring devices (time domain reflectometry [TDR] or air pressure probes). Another limitation is the need to ensure air tightness in potted substrates while limiting their disturbance.
This paper presents a rapid method for estimating the value of air entry in potted substrates that limits media disturbance, provides reliable estimates of air entry, is fast and readily applicable to potted media. Results are compared with a tension table method of determining air entry values.
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MATERIALS AND METHODS
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Tension Table Method
The following procedure was used to determine the air entry value using the tension table apparatus (Fig. 1)
. Measurements were performed on substrates potted in polyvinyl Cl (PVC) cylinders. The wall of each cylinder was perforated before sampling with two tapped holes: one for the TDR probe to monitor water content (30 mm from the bottom), and the other for the tensiometer to monitor matric potential (40 mm from the bottom). The TDR probe consisted of three 145-mm long stainless steel rods, 2 mm in diam. and spaced 15 mm apart, forming a plane. The tensiometer measured 80 mm in length and 8 mm in diam. Before starting the measurements, the zone of influence of the TDR probe was evaluated. It was determined that with this configuration, any changes in water content occurring 10 mm above the probe can be accurately detected. The tension corresponding to that position was estimated by inserting the tensiometer at that precise location.
Following the verification of the probe sampling zone, the potting soil substrates were prepared, poured into the cylinders, and saturated from the bottom. The TDR probe and tensiometer were installed and the cylinder was placed on a tension table with a +0.2 kPa water potential (i.e., with a 20-mm height of water on the surface of the tension table). The tension table was made of 30-µm glass beads and had an air-entry value of about -15 kPa. Additional details on the tension table construction can be found in Topp and Zebchuk (1979).
The cylinders were covered to restrict evaporation, but small holes were made to allow air to enter at the surface. The water content and matric potential of the substrates were monitored daily while the water level was decreased at a rate of 10 mm d-1. The matric potential data were adjusted for the height of the water column in the tensiometer as well as the distance between the tensiometer and the TDR probe (10 mm). Water content was plotted as a function of water potential (–) resulting in a curve with at least two distinct zones: an initial zone showing a nearly constant water content () with decreasing water potential (), and a second zone showing a sharp decrease in water content with water potential, (in some cases followed by a third zone of lesser decrease). The air-entry value was estimated from the intersection for the fitted lines of the first two zones.
Tension Chamber Apparatus Method
Two circular PVC plates, 10 mm thick and 300 mm in diam., were used to construct a tension chamber apparatus (Fig. 2)
. The plates were perforated with six 7-mm diam. tapped holes, equally spaced along the perimeter of the plate. A rubber membrane, 1 mm thick, was placed on the inner side of each plate to ensure air tightness of the cell during it use. The rubber membrane was fixed to each plate and perforated to fit the plate's holes. The center of the upper plate was perforated with one 10-mm diam. tapped hole. Metallic tubing, 10 mm in diam. and 30 mm long, was fitted to the hole and connected to a pressure transducer with plastic tubing. All connections were sealed to prevent air from entering the cell. The center of the bottom plate was also perforated with one 60-mm diam. tapped hole and a rubber plug was used to close the hole when needed. The PVC cylinders (5-mm wall thickness, 210 mm high, and 150-mm i.d.) were filled to the top with peat substrates. Their bottoms were covered with 1.5-mm nylon mesh. The internal surfaces of the plates were greased and six metallic screws, 300 mm long and 6 mm in diam., were inserted into the holes to tighten the PVC plates over and under the substrate-filled cylinders. The cell was then placed in a water bath and the water level was raised gradually to saturate the substrates. The cells cores were maintained at saturation for 
24 h. The rubber stopper was then pushed into the bottom 60-mm diam. hole and the resulting 1 to 2 ml of water pushed into the top tube was removed to allow the insertion of the pressure transducer tip into the tubing. The screws were tightened to make the system completely air tight, with the pressure transducer set at atmospheric pressure. The cells were removed from the water bath and put on the adjustable stand. Hydraulic potential readings were taken every second with a pressure transducer and the data recorded with a CR10 data logger (Campbell Scientific, Logan, UT). After 60 s of hydraulic potential measurements, the rubber plug from the bottom plate was removed. This step constituted an air tightness check, since in an air tight system, only a few drops of water would drain from the cell and the pressure transducer at the top would indicate a potential equal to the substrate height. At this time, another cylinder filled with the same substrate equilibrated beforehand to about -5.0 kPa was moved in contact with the bottom end of the saturated substrate through the open 60-mm hole. This cylinder was 200 mm high with 50-mm i.d. The bottom end was covered with a nylon cloth (15-µm mesh) held in place with two rubber bands. To check the lag between the water potentials measured at the top plate with the pressure transducer and those measured at the bottom of the cylinder, close to the interface between the unsaturated (-5.0 kPa) and the saturated substrate, a minitensiometer (small high flow porous cup 25 mm long and an 8-mm diam.) was placed at the bottom of some of the saturated cylinders.
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Fig. 2. A schematic diagram of the tension chamber apparatus used in the determination of air entry value. Two wing bolts are hidden in the figure.
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The principle behind the approach is that the saturated material begins to unsaturate at the bottom as water is withdrawn by contact with unsaturated material and finally reaches the point air enters into the previously substrate through the unsaturated core material. The sensor at the top determines the point of air entry. The advantages in using an unsaturated medium are several.- Since the potential sensor is located at the top while air enters from the bottom through contact with the unsaturated medium, this implies that air has to travel all the way through the sample, if fully saturated, to get to the top.
- The unsaturated medium contains air that can enter the substrate, a small porous plate placed underneath the sample would not, since it would be fully saturated.
- By making air enter from underneath, the location of the wetting front is known, a real advantage in potted media, since the front location is needed for calculation of air entry potential.
- Since excessively fast drainage rate may affect the shrinkage of fragile substrates and hence, the point of air entry, using the same unsaturated medium as the one tested limits drainage rate to that normally expected during infiltration.
Method Comparison
An experiment was conducted with four different types of peat substrates to compare the proposed chamber method with the tension table measurements. The different types of peat substrates were made from a basic commercial peat substrate (Promix BX, Premier Horticulture, QC, Canada) amended with different sizes of perlite material. Commercial perlite was sieved into three different particle-size fractions: 0.6- to 2-, 2- to 4-, and 4- to 6-mm for use in Substrates 2, 3 and 4. The peat mixes were then prepared as follows: Substrate 1 (PS), a peat substrate (Table 1); Substrate 2 (PS-FP), 1 part peat substrate 1 (vol/vol): 1 part fine perlite (0.6- to 2-mm size class); Substrate 3 (PS-MP), 1 part peat substrate 1:1 part medium perlite (2- to 4-mm size class); and Substrate 4 (PS-CP), 1 part peat substrate: 1 part coarse perlite (4- to 6-mm size class).
Each substrate was poured into the PVC cylinders. The substrates were compacted by dropping the cylinders on the table three times from a height of 100-mm. More substrate was added to obtain the desired volume. Four replications were made per treatment, for a total of 16 cylinders. The cylinders were placed in a basin and 1000 ml of distilled water were added to the top of each three times. The cylinders were then set on sieves placed upside down and distilled water was added from underneath in small increments (raising the water level at a rate of 50 mm d-1) until the cores were saturated. The cores were maintained at the saturation point for 24 h.
After saturation, two cores from each treatment were selected randomly and subjected to air-entry measurements, first using the tension table method and then the tension chamber method. The remaining two cores from each treatment were subjected to the same measurements in the inverse order (i.e., first using the tension chamber apparatus and then using the tension table method).
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RESULTS AND DISCUSSION
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Tension Table Apparatus
Measurements of and for the different substrates showed two distinct zones for all four treatments: a small decrease, probably because of substrate shrinkage by retreating menisci and increasing overburden pressure increasing the density of the material, was followed by a sharp decrease zone (Fig. 3a–d)
. For sieved perlite-amended substrates, the intersection of the fitted lines on Fig. 3b to 3d showed the lowest values for the point of air entry with the finest perlite fraction, and the highest values with the coarse and medium perlite, results which were consistent with largest pores sizes created by coarse perlite allowing air entry at higher potential values (Table 2).
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Fig. 3. Determination of air entry values using the tension table method for four different peat mixes in one replicate. The dotted line represents the air entry value.
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Tension Chamber Apparatus
Figure 4
is a typical figure of the recorded data with the tension chamber apparatus. Hydraulic potential was plotted as a function of time, resulting in a curve with five distinct zones:- An initial zone, showing a constant with time before removing the rubber plug.
- A second zone, showing a sharp decrease in with time, immediately after the removal of the rubber plug and corresponding to the drainage of several drops of water.
- The third zone, indicating the time when the system reaches equilibrium and becomes stable.
- The fourth zone, indicating a decrease in after connecting the unsaturated porous body to the tension chamber apparatus at time zero (t = 0)
- The point of rupture where the decrease in ceases when air enters the saturated substrate through the largest pores and begins to increase.
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Fig. 4. Example of the evolution of the potential in the tension chamber apparatus for the peat substrate treatment using the tensiometer located on the upper plate (upper tensiometer, both corrected and uncorrected), and the tensiometer located at the bottom of the cylinder (lower tensiometer) showing five drainage response curve. The data from the upper tensiometer was corrected by adding a positive pressure value corresponding to the suction associated with the column height to eliminate the effect because of this hanging water column on the upper tensiometer reading.
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The difference between the hydraulic potentials measured at the equilibration point (value read in the third zone) and at the point of rupture (value read in the fifth zone) represents the air-entry value (a).
For these peat substrates, air entry occurred in <100 s, except for a few samples, where there was a high proportion of micropores (Fig. 5)
. Consistent with the data from the tension table, the air-entry values obtained from the tension chamber apparatus decreased as the particle size of the peat decreased as shown in Fig. 5 and in Table 2.
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Fig. 5. Determination of air entry values using the tension chamber apparatus for four different peat mixes in one replicate.
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Because the pressure transducer was located at the top of the cylinder, slight delay was possible between the air entry occurring at the position of the wetting front and that detected at the top of the cylinders. However, pressure transducer readings showed that whatever the position sampled, there was no detectable delay between the two locations (Fig. 4). Both signals, after correction, also showed a nearly identical behavior in Zone 4. This indicated that for these substrates, the tensiometer location was not important and the zone of measurement could be restricted to the top surface only.
The results in Table 2 indicate that both methods showed significant differences in the substrate air entry-value (F value significant). Therefore, we were able to create a gradation in air entry between substrates by adding different size perlite. Both the method and method x substrate terms were nonsignificant. The nonsignificant F value for the method indicated that both methods gave the same mean air entry estimates. The nonsignificant method x substrate interaction term means that this applied to all the peat substrates, that is no differences in air entry were found, whatever the substrate. The relationship between the air-entry values obtained from both methods shows that these values were closely correlated (r2 = 0.83***, and significant at P = 0.0001) for all peat mixes (Fig. 6)
. All points, except for one outlier, appeared close to the 1:1 relationship. The presence of this outlier suggests that for routine characterization, it may be better to perform estimation of air entry values on means, rather than on a single sample. This would avoid the possibility of bias and diminish the influence of outliers. The results of this experiment show that the tension chamber apparatus is a valid alternative method to determine the air entry value in peat mixes. The apparatus proved to be easy to operate, providing rapid results with limited substrate disturbance and would be easy to adapt for use with potted substrates. In our experience substrate-filled rigid nursery pots, when sufficiently air tight, could easily replace PVC cylinders. However, four pots with no center hole or too narrow a center hole, an effort should be made to make, or to cautiously enlarge the hole with a sharpened knife to allow good contact with the porous body.
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Fig. 6. Relationship between air entry values obtained from the two different methods. ***, significant at P  0.001.
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The possibility of measuring air entry in pots represents obvious advantages of limited substrate disturbance, and a characterization performed on samples obtained directly from a greenhouse, or nursery. Such samples integrate the many factors (potting, time, root growth, etc.) affecting substrate physical properties during handling and cultivation.
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ACKNOWLEDGMENTS
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The authors wish to thank Entreprises Premier CDN Ltd. and the CORPAQ (Conseil des recherches en pêche et en agroalimentaire du Québec) for their financial support. We are also grateful to Dr. A. Rogowski and three anonymous reviewers for helpful comments, as well as to P. Drouin and J. Grandmaison for technical help.
Received for publication November 27, 2000.
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REFERENCES
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- Allaire, S.E., J. Caron, and J. Gallichand. 1994. Measuring the saturated hydraulic conductivity of peat substrates in nursery containers. Can J. Soil Sci. 74:431–437.
- Bouwer, H. 1966. Rapid field measurement of air entry value and hydraulic conductivity of soil as significant parameters in flow system analysis. Water Resour. Res. 2:729–738.
- Caron, J., P. Morel, and L.M. Rivière. 2000. Aeration in growing media containing large particle sizes. Acta Hort. 548:229–234.
- Childs, E.C. 1969. The physical basis of soil water phenomena. Wiley Interscience, London.
- Fallow, D.J., and D.E. Elrick. 1996. Field measurement of air-entry and water-entry soil water pressure heads. Soil Sci. Soc. Am. J. 60:1036–1039.[Abstract/Free Full Text]
- Fonteno, W.C. 1989. An approach to modeling air and water status of horticultural substrates. Acta Hort. 26:37–44.
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- Paquet, J.M., J. Caron, and O. Banton. 1993. In situ determination of the water desorption characteristics of peat substrates. Can. J. Soil Sci. 73:329–339.
- Rivière, L.M. 1990. La notion de capacité en bac dans la conduite de l'irrigation des cultures en pots et conteneurs. (In French.) Inf. Tech. Bull. 444:415–424.
- Topp, G.C., and M.R. Binns. 1976. Field measurement of hydraulic conductivity with a modified air-entry permeameter. Can. J. Soil Sci. 56:139–147.
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- White, J.M. 1964. The concept of container capacity and its application to soil moisture fertility regimes in the production of container grown crops. Ph.D. diss., Penn State University, University Park, PA.
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ABSTRACT
INTRODUCTION
