Plants & Human Affairs

Plants & Human Affairs (BIOL106) - Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

A Comparison of Plant Growth (Propagation) Media

Introduction:
A good plant growth medium should retain sufficient moisture, provide adequate aeration (pore space), be firm enough to support the plant, not contain detrimental substances including toxins or diseases, be able to be sterilized, and have a uniform consistency. Some common growth media include peat, perlite, vermiculite, soil and sand.

Peat is the partially decomposed remains of bog plants, especially sphagnum moss. This medium is acidic (pH 3.8 - 4.5). Peat is slow to initially absorb water; hot water works fastest. Soil is a mixture of particles of rock, organic materials, liquids and gases. Sand, loam and clay refers to soils with progressively smaller particles. Particle size affects the water-holding ability and amount of pore space available in the soil.

Vermiculite is essentially heated (about 1100� C), popped mica chips. It has a high cation exchange capacity (i.e., can remove positively charged ions from the soil and slowly release them) and contains magnesium and potassium. Vermiculite can retain up to 16 times its weight in water and provides plenty of air spaces. Horticulture grade vermiculite is mined largely in Georgia and is near neutral in pH. Building or insulation grade vermiculite is alkaline, coarser and often treated with chemicals; it should not be used to grow plants. Vermiculite is an excellent seed starting medium. You should avoid breathing the dust from vermiculite because it can contain silica.

Perlite is crushed, heated and expanded lava (actually volcanic sand) that has no mineral nutrients or buffering capacity. Like popcorn, moisture trapped in the lava particles causes them to expand when heated to about 760 - 870 �C. Perlite is less collapsible than vermiculite and has a neutral pH (6 - 8). It is especially good for starting cuttings.

A good all-purpose medium contains a mixture of sand:loam:peat (1:1:1). A good rooting mixture contains a mixture of sand:vermiculite:peat (1:1:1). Soil-less mixtures, those that lack soil such as Jiffy Mix, are especially good for starting seeds. One common formulation is equal mixtures of peat and vermiculite.

The purpose of this exercise is to give you a hands-on opportunity to work with growing plants and especially factors involved in choosing a planting medium.

Exercise:
In this exercise we will examine some physical properties of various plant growth media. Density refers to the weight (in grams) per unit volume (mL). Total pore space refers to the total volume (mL) of space between particles in a dry medium. It can be measured by determining the amount of water it takes to saturate the medium. A medium that contains all the water it can possibly hold after the excess (termed "free water") has drained off is said to be at field capacity. This is obviously the maximum amount of water that is available to a plant for growth. Since water occupies some of the total pore space in a moist medium, it is necessary to calculate the amount of pore space at field capacity.

Questions:

Which medium (peat, perlite, vermiculite, or sand) is densest?

Which medium (peat, perlite, vermiculite, or sand) has the greatest pore space?

Which medium (peat, perlite, vermiculite, or sand) has the greatest water holding capacity?

Which medium (peat, perlite, vermiculite, or sand) has the great air space at field capacity?

If these media are mixed, will the resultant mixture have an average of their properties?

How will pore space, air space and the water holding capacity of the medium change with density?

Hypotheses:

The densest medium will be ___________ because.....

The medium with the greatest pore space will likely be _________ because....

The medium with the greatest water holding capacity will likely be _______ because....

The medium with the greatest air space at field capacity will likely be _________ because....

Mixing these media will result in ........

As the density of the medium increases, the total pore space: ______ (increases, decreases, is not affected).

As the density of the medium increases, the water held at field capacity:________ (increases, decreases, is not affected)

As the density of the medium increases, the air space at field capacity:_________ (increases, decrease, is not affected).

Protocol:

Number and then weigh six paper cups. Record the weight of each in the data table (column #1).
Using a graduate cylinder, pour 200 mL of tap water into one cup and mark the level on the cup. Pour the water into the next cup and mark the level. Repeat for all cups then discard the water.
Fill each cup with the appropriate medium to the mark.
Reweigh the cup and medium. Record your data (column #2).
Calculate the density of the medium (#4) by dividing the medium weight (#3) by its volume (200 mL).
Fill a graduate cylinder to the top mark with water. Then carefully pour water over the medium until all the pore spaces are filled, i.e., the medium is saturated with water. Do this slowly and stir while you pour. You may need to tap the bottom of the cup on bench top to help settle the medium. Record the volume (in column #5) of water used (remember to subtract the final volume from the original). This volume equals the total pore space of the medium.
Carefully invert the paper cup and contents over a screen and let all the excess (free water) drain.
After thoroughly draining, re-weigh the cup and medium and record (#6). The medium is now at "field capacity."
Calculate the water held at field capacity (#6 � #2) and record these data in column #7.
Calculate the air space at field capacity (#5 � #7) and record these data in column #8.
Plot (on the same graph): total pore space (y axis) versus density (x axis); water held at field capacity vs. density; and air space at field capacity vs. density

Table 1: Data Collection Table (The values in the columns marked with an asterik (*) are obtained by measurement. The others are determined by calculations. If any calculations are negative, record it as zero)
Medium Type	1. Wt. Cup (g)*	2. Wt Cup + Medium (g)*	3. Medium Wt. (g; #2 - #1)	4. Density (g/mL; #3/200)	5. Total Pore space (mL)*	6. Cup + Medium + water drained (g)*	7. Water held at Field Capacity (mL; #6 - #2)	8. Air space at field capacity (mL; #5 - #7)

1. sand
2. soil
3. peat
4. vermiculite
5. perlite
6. soil : peat : vermiculite (1:1:1)

Questions:

What do you conclude about your hypotheses above?
Vermiculite is a good medium for starting seeds. Explain why with direct reference to your data.
Perlite is a good medium for propagating cuttings of plants. Explain why with direct reference to your data.
What is a possible disadvantage of a medium with too much air space?
Vermiculite is reported to be able to retain 16 times its weight in water. Do your data support this statement? (i.e., perform this calculation for your vermiculite sample. Then briefly discuss your results). Show your work.
Copy from Table 1 the values for "water held at field capacity" into the table below. Then calculate the average value.

Medium	Water held at field capacity (mL)
soil
peat
vermiculite
average

Now, copy from your data table the value for "water held at field capacity for the the soil:vermiculite"peat mixture ________. How does the average value from the three individual components compare to the value for the mixture? Should they be the same? different?
Explain why the greenhouse uses a mixture of soil, peat and vermiculite for routine plant growth.
Suggest a reason why sand is generally a major component of media for growing cacti and other succulents.

Bonus:
Describe how we could determine the amount of "free water" in each medium when it is saturated.