Fundamentals of Container Media Management: Part I

Successful production and management of high quality container-grown plants requires an understanding of the unique environment found in containers and how it is affected by the physical and chemical properties of the growing media.

Compared with roots of plants growing in the field, those growing in containers are exposed to a more stressful environment. The following examples help to illustrate the condition. During active growth, a plant can extract all the available water in a typical container in as little as one or two days. Following irrigation, the medium is saturated at the bottom of the container. Roots located in the saturate medium exist without air until the plant uses enough water to create air spaces. As the medium dries, the salt concentration in the soil solution can increase to high levels. Key nutrients such as nitrogen and potassium are lost to both plant uptake and leaching and may become rapidly depleted if they are not supplied periodically. Growing medium temperatures can exceed 120°F and can fluctuate by 30°F or more between day and night.

Some stressful conditions can be considered a consequence of the restricted media volume attempting to balance the needs of the plant. The problem is not, however, that the medium fails to meet these needs, but rather the short time period during which the needs can be met. A sound management program is required to adequately meet the needs of plants and minimize stressful conditions.

Container Media Characteristics

A growing medium that optimizes production of high quality container plants is essential. In contrast to field production, the volume of medium available per plant is limited and must have good physical and chemical characteristics that, when coupled with an intelligent management program, support optimum growth (Table 1). Physical properties are often considered more important to a container medium, because they cannot be easily changed after ingredients have been placed in a container. Chemical properties, however, can be altered.

Table 1. Characteristics of a good container medium.

1. Good water-holding capacity	Physical property
2. Good aeration and drainage	Physical property
3. High permiability to water and air	Physical property
4. Light weight	Physical property
5. Good fertility	Chemical properties
6. Pasteurized	Management practice
7. Inexpensive	Costs based on availability and transportation

Physical Properties

An ideal medium should drain rapidly following irrigation and contain 10% (by volume) or more air space after draining (Table 2). Oxygen stress conditions are likely to develop at values lower than 10%. At the same time, on a volume basis, the medium should contain at least 50% water and available water should be at least 30%. Wet weight (or volume weight) is another important physical property, particularly when large plant containers are being handled in a nursery setting. Although it varies considerably, depending on the components used in the mix, a value between 70 and 90 lb./cu. ft. is usually acceptable.

Table 2. Physical properties of an ideal container medium and of some commonly used media.

	Total porosity (E)	Water Content (Pv)	Air Capacity (Ea)	Available Water (PVa)	Wet Weight
Medium	(% by volume)				(lb ft-3)
Ideal medium	60–75	50–65	10–20	> 30	70–90
Peat-Perlite	93	73	20	48	54
Peat-Vermiculite	94	81	13	60	62
UCMix*	73	62	11	44	71

Note: These values were determined after irrigation and drainage (container capacity) for a medium depth of 14–15 cm. *Peat-Sand-Redwood sawdust.

A mix should be designed to maximize both its water and air content, using the guidelines given for a good medium. Because physical properties vary when mixing two or more components, be sure to evaluate the mix after formulation. Although there are some rough methods to estimate these values on site, it is best to have the tests performed by a commercial laboratory. Once the values of physical properties have been determined, component proportions can be adjusted to meet the requirements.

Particle Size of Medium Components

Most media used in ornamental plant production consist of a combination of organic and inorganic components. The two most commonly used organic components are peat moss and composted softwood bark. Other wood products such as sawdust and wood chips are sometimes used along with composted organic matter, composted sludge, manure, straw, rice hulls, and peanut hulls. Some of the more common inorganic materials include sand, vermiculite and perlite. Occasionally, calcined clay or pumice is also used. The use of mineral (field) soil as a component of container media is not recommended for reasons that include heavy weight; non-uniform particle size distribution and small pore size; poor drainage; variable chemical properties; non sterile, potential carrier of insects; weeds and diseases. Soils may also contain chemical residues (pesticides) and undesirable high salt and toxic ion levels.

Adding organic amendments to a growing medium improves its physical properties by increasing the water-holding capacity and aeration while decreasing wet weight. To accomplish this, the mix components should have a desirable particle size: For container media most of the particles in an amendment should be between 0.5 and 4 mm, with less than 20% below 0.5 mm (Table 3).

Table 3. Particle size guidelines for organic and inorganic amendments.

Particle diameter (mm)	Desirable Content (% by weight)
10–2	< 20
2–0.5	> 60 (100% ideal)
< 0.5	> < 20

A sufficient quantity of organic amendment must be used to effect significant changes in physical properties. For most purposes there is little benefit in adding less than 40% of the amendment. In addition to the requirements for particle size distribution and rate of incorporation, an organic amendment should be stable with respect to decomposition. If there is not fully composted material in the media, it will use nitrogen during the growing season. If the nitrogen is not supplemented through fertilization, plant growth will be reduced. As the composting process proceeds, there will also be a reduction in volume of the medium.

Sand is a common component of nursery growing media and is primarily used for ballast weight. This purpose may underestimate the potential impact it may have on the physical properties of the medium. Its use in a mix also requires that particle size distribution be evaluated. Natural sands are composed of particles that range in size from 0.05 mm to 2 mm. For descriptive purposes, sand is subdivided in five particle size groups (Table 4). Larger particulates are considered to be gravel while smaller ones include the silts and clays. Desirable sand for container production contains mainly medium and coarse particles, at least 70% by weight. The use of sands with a wide distribution of particle sizes (nearly equal percentages of each size) is undesirable, as it will likely result in poorly aerated media. Sand use should be restricted to less than one-third of the mix volume, as it significantly increases the volume weight.

The particle size guidelines given here are not to be used as rigid specifications, but assist in evaluating and selecting materials for a container medium. They do not guarantee an ideal mix. Only laboratory evaluation can confirm the quality of a medium.

Table 4. Particle size distribution in a desirable horticultural sand.

Category	Size (mm)	Desirable Content (%)
Gravel	> 2	0
Very Coarse Sand	1–2	0–5
Coarse Sand	0.5–1	70–80*
Medium Sand	0.25–0.5
Fine Sand	0.1–0.25	0–20
Very Fine Sand	0.05–0.1	0–2
Silt and Clay	< 0.05	0

References

The following books are some of the most relevant publications that provide good to extensive coverage in many aspects of container media management and may prove to be useful references for container nursery and greenhouse growers. The book sections dealing with physical properties are indicated in parentheses.

1. Bunt, A.C. 1988. Media and Mixes for Container-grown Plants. Unwin Hyman Ltd., Great Britain, 309 pages (Chapters 2 and 3).
2. Handreck, K.A. and N.D. Black. 1989. Growing Media for Ornamental Plants and Turf. New South Wales Univ. Press, Australia (Very practical, simple, easy-to-read grower-oriented book with very useful information and tables).
3. Nelson, P.V. 1991. Greenhouse Operation and Management. 4th Edition. Prentice Hall, Englewood Cliffs, NJ, USA. 611 p. (Chapters 5 and 6).
4. Whitcomb, C.E. 1984. Plant Production in Containers. Lacebark Publications, Stillwater, OK, USA. 638 p. (Chapters 3 and 4).

November 2014