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Orchids Cultivation




Bark or Sphagnum moss, which one is better?


Part II. Physical properties of the Substrates

There is no unique substrate could be claim that it is the best substrate for growing of Phalaenopsis. How to mix some substrates into a mixture? What is the ratio for its substrate? Besides of the trial and errors method, a new technique developed by the Biosystems Engineering Lab., National ChungHsing University was introduced as follows:

a.       Define the physical properties of substrate

b.      Establish the standard operation procedure (SOP) to determine the substrate physical properties.

c.       Measure the physical properties.

d.      Establish the basic information of these substrates.

e.       Mix the substrates according to the predetermination ratio, if necessary.

The definition and the measurement method are adopted the Approximate method.

Source: William C. Fonteno. 1996. Growing media: Types and the physical/chemical properties. In: Water, Media, and Nutrition for Greenhouse Crops (David Wm. Reed, ed). pp.93-123. Ball Publishing, Batavia, IL USA.


 1.     The procedure of the approximate method

1.1          Take a pot (container), sealed the bottom hole with the adhere tapes. The volume of pot is Vm.

1.2          Fill the pot with the substrate to the top level of the pot.
The substrate volume (Vp) is equal to the pot volume (Vm).

1.3          Add water into the pot slowly until the substrate is saturated to the surface. No more water was flow out of the pot.

1.4          Wait for one o’clock. If the water level is lower than the top of pot, add water into substrate until the water reach the pot top.

1.5          Wait for the second period of 15 minutes until the substrate was saturated.

1.6          Record the total weight of added water (Wadd)

1.7          Remove the adhere tape at the bottom of pot and collect the drained water (Wdrop). The required time was usually more than 1 hour.

1.8          Take out and weight the wet sample (W1).

1.9          Put the wet sample into the hot air oven at 80℃ for 36 h, then weight the

1.10      dry sample (W2).


2.     Define and calculate the physical properties

2.1  Total Porosity (Tp)

TP represents the percent total pore spare on a basic of volume. TP value could be used to evaluate the water absorbing ability and the aeration ability.


2.2  Pot capacity ( PC )

PC is the percent water on a volume basic after drainage.


2.3  Moisture content ( MC )

MC represents the percent moisture on a weight


2.4  Air space ( AS )

AS is the percent volume of drainable pore space on a volume basic.


2.5  Water adsorptivity ( WA )

WA is the ratio of the water adsorption percent of substrate.


3.     The interpolation of the substrate physical properties

The five physical properties could be used to express the water holding capacity and air exchange ability.

3.1          Water holding capacity

  1. PC: pot capacity

PC value represents the adsorption water of a unit substrate volume.
The larger value of PC, the more was the water holding capacity.

  1. MC: moisture content

As the substrates have the larger MC value, it could adsorb more amount of the water.

  1. WA: Water adsorptivity

WA value represents the adsorption water of a unit substrate weight. The more of the WA value, the more water was adsorbed.

  1. TP: total porosity

TP could be used to express the amount of holding water in the pore of substrate as substrate is maintained in wet state. The large value of TP, the more water could be hold in substrate.

3.2          Air exchange ability

  1. AP: air space

AP value represents the total void in the substrate. The large the AP value, the more air could be exchanged in the substrate.

  1. TP: total porosity

TP can be used to express the air exchange ability as the substrate is kept in dry state. Larger TP values show the good air exchange ability.


4.     The physical properties of some substrates.

The physical properties of some substrates are listed in Table 1.

The sphagnum moss had the highest values of PC, MC and WA. It indicated that this substrate have the excellent water holding ability. The TP and AS values of the fresh moss sphagnum were 93.5 and 58.1, respectively. Because of the good water holding capacity and air exchange ability, the substrate was popular in Asia region. As the substrates were inserted into the pot too tightly, in other words, the growers provided to too much substrate into the pot, the air exchange ability then reduced significantly. Too much water made the root damage and rot.

The PC and WA values for the Pinus rediata bark were 30.5 and 1.57, respectively and for the Mixture 2 (from the Culture Guide of Anthura B.V.) were 18.8 and 30.1. The TP and AS value for two kinds of substrates were similar.

The effect of the substrate physical properties on the growing of Phalaenopsis was demonstrated and the results are presented in Figure 1 and 2. In this experiment, two kinds of substrates, pinus radiate bark from the New Zealand and Shanghai bark from the China were selected. The variety of Phalaenopsis and management operation was same. Both barks have the similar air exchange ability, however, the water holding capacity of Shanghai bark was significantly lower than that of pinus radiare bark.

The root’s development of the orchids grows in different barks look similar. However, the leaf shape and leaf area were totally different. The leaf development of the Phalaenopsis with Shanghai bark was pretty bad. It also affected the flower quality. The reason could be explained easily. As the water holding capacity was not enough, the nutrient supply from this substrate was back. The development of the Phalaenopsis was in the bad condition.


Table 1. The physical properties of some substrates



Pot Capacity


Moisture Content


Water Adsorb


Total Porosity


Air Space

New moss






Old moss






Pinus radiata bark






Shanghai bark






Mixture 2 bark








Figure 1. The substrates, pinus radiate bark, from the New Zealand


Figure 2. The substrates, Shanghai bark, from China


About the author:

Professor Chiachung Chen, Ph. D.

Biosystems Engineering Laboratory
National ChungHsing University

250 KuoKuang Rd.,

Taichung, Taiwan 40227

Tel: 886-4-22857562, Fax: 886-4-22857135

Web site: http://bse.nchu.edu.tw

Email: ccchen@dragon.nchu.edu.tw