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Root Media Principles
Historical Perspective
Amended, field harvested mineral soil , with some proportion of sand, silt and clay, was the most common growing medium for container grown plants until the late 1950s. Anyone learning greenhouse production techniques since 1970 has most likely not worked with anything but soilless media.
One of the first reported uses of a peat and sand based medium for production of potted chrysanthemums was in a research report from The Ohio State University around 1930. The 15-year period from 1955 to 1970 is recognized as the time when concentrated efforts in Europe and the United States were made to replace container media consisting primarily of amended mineral soils with porous, blended media that could be consistently reproduced from batch to batch, location to location and year to year.
The decreased availability of mineral soils, the increased use of herbicides in field crop production, the need for more rapid plant growth to shorten cropping time, and the need to lower production costs were just some of the reasons motivating researchers and growers to look for alternative media.
Root Media
A variety of materials can be mixed to provide root media for container grown greenhouse crops. Root media selection is easiest with a clear understanding of the properties and characteristics desired.
Defining the properties and characteristics of root media for greenhouse crops from plugs to hanging baskets is best done by considering the physical, chemical and biological aspects of the root zone environment. These factors control water and nutrient availability and can be used to minimize runoff.
Physical Properties
The physical components of root media include air, water and solid proportions.
The typical percentage of these three components in a six-inch pot of soilless medium are 10 to 20 percent solid, 20 to 30 percent air and 50 to 70 percent water. These levels exist after the medium has been well watered and allowed to drain.
This contrasts with mineral soils in the field which are more like 50 percent solid, 25 percent air and 25 percent water after saturation and drainage.
Not all the water is available to the plant. In mineral soils the water not available to the plant is less than 5 percent. In a soilless peat-based medium 5 to as much as 15 percent of the water is unavailable to the plant.
Container size determines the actual proportion of air and water.
The amount of air and water held in a given root medium is a function of the height of the column of medium. The shorter the column is, the greater amount of water and the less amount of air. The taller the column is, the smaller the ratio of water to air spaces.
A given root medium cannot have 20 percent air space in a six-inch pot and in a plug tray. For example, a medium in a six-inch pot may have 20 percent air space after watering. That same medium in a plug tray may only have two to four percent air. The amount of solid and pore space remains constant but the amount of water versus air space changes.
The 20 percent air space for the six-inch pot is the average amount of air space in the middle of the container, not the amount at the top or the bottom. After saturation and drainage, the top layers of root medium have more air (25 to 35 percent) and the lower layers have less (5 to 10 percent).
This is why adding gravel or coarse drainage material to a growing container with drainage holes does not provide better drainage. The gravel only shortens the height of the column of root medium and decreases the amount of drainage.
Available Water Holding Capacity (AWHC) describes how much water a plant can use. It is measured as the water available after irrigation and up to wilting and can be done on a scale.
The difference between the saturated weight (2) and the wilted weight (1) could be called the usable container capacity of the volume of water available to the plant.
Effect of Container Size
Medium = 1 Peat : 1 Vermiculite
1 gal 6-inch 4-inch BPC
Air Water Solid
0%
20%
40%
60%
80%
100%
From Bilderback and Fonteno
Root rot pathogens - Prior to the use of peat-based media, greenhouse soils were treated with heat or chemicals to reduce the presence of root rot organisms and weed seeds. While peat can be a source of Pythium , Rhizoctonia , fungus gnats and other organisms, current practices are not to heat or chemically treat the peat. By not treating, it is assumed that a balanced, competing populations of organisms will develop, preventing dominance of any one pathogenic organism that might occur in a sterile environment. Selective chemicals are used if needed to treat pest populations that develop above acceptable levels.
Beneficial organisms - Inoculation of root media with beneficial organisms is not yet a commercial practice but may be in the near future. Trichoderma and mychroriza are examples of fungi that can be added to root media to compete with pathogenic organisms or to increase the uptake of certain nutrients.
Should you purchase blended media or mix your own on site? It is generally recommended that operations with less than 100,000 square feet of greenhouse purchase blended media rather than mix their own. When purchasing blended media, it is also logical to consider purchasing prefilled containers.
Product uniformity over time, from batch to batch, is probably the greatest desired trait in container root medium. Stability of a batch over time, under production conditions, watering for example, is also very important.
Components
Components are mixed to make one medium (singular) or different media (plural). There is not one best blend of components. The goal is to get the desired characteristics.
Nitrification
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NH 4 +
2H+
NO 2 -^ NO 3 -
Bacteria 1 Bacteria 2 Ammonium Nitrite Nitrate
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Having a greater number of components does not make the mix inherently better. Two components may be all that are needed.
A small amount, for example less than 10 percent by volume, of some component may not have any significant effect.
Variability due to particle size or the source of any one of the components may lead to greater changes in media characteristics than changing the component used.
When selecting components, the most important criterion is probably uniformity in a batch and over time.
Media components are evaluated for
- the texture (particle size such as fine, medium and coarse)
- the cation exchange capacity (the ability to hold nutrients)
- the pH (acidic or basic) and pH buffering capacity (resistance to change in pH)
- the bulk density (the weight per unit of volume)
The bulk density is important for providing anchorage and stability to the plant and considerations related to moving and shipping.
Components should also be biologically stable.
The cost of the component and the transportation cost are perhaps the factors often given the most weight. The cost of medium to fill a six-inch pot ranges from 3 cents to 9 cents. While six cents is not a big part of the total unit (plant) production cost of perhaps 2 to 3 dollars, it can be a big part of the profit per pot!
For the following descriptions, the components used in common greenhouse media have been classified as either organic, inorganic or synthetic. Using these categories is just one of many possible ways of approaching this topic.
Let’s look at some of the more common components of greenhouse root media and compare their physical, chemical and biological properties.
Organic Components
The most widely used organic components are:
- Peat
- Pine bark and other barks
- Coconut coir
Peat is the most common component used. There are several kinds of peat. The main type used in growing media is sphagnum peat. Hypnum and reed sedge peat are used to a lesser extent.
Chemically it contains significant amounts of potassium, sodium and sometimes chloride. Compared to peat media, less potassium and more calcium and magnesium need to be incorporated prior to planting. It requires less lime at the start which may lead to less calcium and magnesium being present later in the crop as well as potential for falling pH under acidic fertilizer conditions. This is discussed further in the article “Component comparisons: coconut coir” reprinted from Grower Talks, February 1996, which is found in this booklet.
Inorganic Components
Inorganic coarse components are typically added to peat or bark to provide greater porosity and drainage. These components include:
- Vermiculite
- Perlite
- Rockwool
- Calcined clay
- Sand
Vermiculite is a raw clay mica that is mined from the earth. Particle size of vermiculite can vary from fine (#4) to coarse (#2). The most common size is #3. Physically it has good aeration and water holding characteristics, but this can break down when the medium is compressed or mixed too long. Effectiveness will depend on particle size distribution. Chemically, vermiculite has cation exchange capacity and can hold nutrients. It contains magnesium and potassium, which are very slowly available to the plant. They are not supplied in sufficient quantities to alter fertilizer practices. Its pH is neutral. It is not biologically active.
Perlite is manufactured from an aluminum, silica volcanic rock that is mined from the earth. The mined product is crushed and then heated to 1800 degrees F. Physically perlite is good for aeration and has a low water holding capacity. Chemically it has low cation exchange capacity and nutritional value. Its pH is neutral. Biologically it has a low value except for helping to provide air.
Rockwool is manufactured by heating a mix of minerals to a molten or liquid state at very high temperatures. Special equipment is then used to spin fine fibers and strands that are gathered together to make horticultural and insulation grades of rockwool. The horticultural quality of rockwool depends on the manufacturer. Physically, rockwool displays acceptable air and water capacity. It absorbs water quickly and the water is readily available. From a chemical standpoint, rockwool has a low nutrient holding capacity and tends to have a pH greater than 7.0. As a result it may reduce the lime requirement. It is biologically inert.
Calcined clay is mined clay that has been fired at high temperatures. From a physical perspective, calcined clay may influence water absorption. Chemically clays vary. They provide good cation exchange capacity but they do not add much at the rates often used, 10 percent by volume. Clays are biologically inert. Another consideration is that clays are
more expensive than most components. They add weight, which is most likely what can justify the additional cost. They are generally not used.
Sand, like clay, provides weight. Types of sand can vary dramatically. Physically sand is not as good for aeration as many presume. It may increase the rate of water absorption. Sand is chemically inert although, depending on the source, it may contain contaminants. Sand with a limestone or marl component will influence pH much like lime. It is biologically inert, although, again, it may contain contaminants. Sand is generally not used.
Synthetic Components
Synthetic components include
- Polystyrene flakes or beads, more commonly know by the trade name Styrofoam
- Urea formaldehyde foams like Oasis Foams, Hygromull or Floramull
Key Points to Remember:
- Watering practices probably have the greatest impact on media air and moisture content. Other factors include: container size, component particle size distribution, stability of the components and media handling practices.
- Consider the variability of peat, pine bark, coconut coir, vermiculite, perlite and rockwool.
- Product uniformity over time is probably the most desired trait in a container root medium.
- Stability of a batch over time is also very important.
- We still have more to learn about biological properties.
Nutritional Additives and Amendments
Nutritional additives and amendments include chemicals and compounds that are present at less than 10 percent of the volume of the medium. These may include:
- Preplant fertilizer
- Liming materials
- Wetting agents
- Water absorbing gels
- Postplant fertilizer
Preplant Fertilizer
The macro and micronutrient fertilizer mixed into the root medium at formulation is called the base charge or the base nutrient charge. There is not one recommended level of base charge for all crops or situations. The recommended rate depends on the amount of time from mixing to planting, whether the medium will be used for propagation or growing on, and the fertilizer tolerance of the crop to be planted in the medium.
Lime varies not only by the chemical type but also by how finely it is ground. Coarser ground limestone is commonly used in field agriculture, but finer lime is desired for container media. General terms for lime particle size in increasing degree of fineness are pulverized, superfine and microfine. By definition, pulverized lime has at least 65 percent passing a 100 mesh screen. Superfine has at least 65 percent passing a 200 mesh screen. For microfine lime, 99 percent will pass a 325 mesh screen. Superfine or pulverized lime are recommended.
The source of the lime, where it is mined for example, can also influence the solubility of the lime. In other words, not all limes of a particular particle size can be expected to react the same.
The key is to find a consistent, high quality product and not to use just any old lime available.
Wetting Agents
Wetting agents are usually necessary to get superior wetting of peat and bark based growing media.
Proprietary wetting agents are one of the amendments that set apart commercially available media. The most commonly used nonproprietary wetting agent is Aquagro 2000, which is available as a granular or a liquid concentrate. The granular consists of fine vermiculite as a carrier for the wetting agent. It is stable and can be mixed with fertilizers and lime being added to the medium. The liquid would typically be diluted with water and added to the medium during blending.
You can test to see if a wetting agent is needed later in the crop by applying it to a few pots and using a scale to determine if water uptake is increased.
Water Absorbing Gels
Water absorbing gels were originally developed as a potential method of water purification. The project was not successful since, as the gels absorbed water, salts and ions were also absorbed. Today, horticultural gels are marketed to increase available water holding capacity (AWHC) and extend the time between watering.
Water quality, fertilizer type and concentration, and irrigation frequency affect hydration. The hydration of water absorbing gels is reduced by increasing concentrations of fertilizer.
There are several different manufacturers of water absorbing gels suitable for incorporation in peat-based medium.
The cost is generally not justified by benefits in water holding capacity, although some extension of the moisture release time is possible. Incorporating a water absorbing gel
such as Supersorb C adds 2 to 3 cents to the cost of the root medium per six-inch standard pot. To make economical sense, the product must save money by reducing watering labor in the greenhouse or improving the quality or perceived consumer value of the plant.
Complete Premix Additives
Complete premix additives can be purchased. Macronutrients, micronutrients, wetting agent and lime can be ground, mixed and even adjusted with filler to provide a uniform product that is easier to incorporate uniformly. Lime or hydrated lime can be added to the medium nutrient charge to get the desired initial pH based on the crop and growing conditions.
Postplant Fertilizer
A postplant water soluble fertilizer “fix” can substitute for a preplant medium charge. When medium nutrient levels are low, water soluble fertilizer can be applied in solution after the crop is planted.
Example Root Media Amendments in Pounds per Cubic Yard for Several Fertility Levels
Amendments No charge “Half” charge (1.5 lb)
“Full” charge (3 lbs) Dolomitic lime^1 6 – 15 6 - 15 6 – 15
Hydrated lime^2 0.5 – 1 0.5 - 1 0.5 – 1
Gypsum (CaSO 4 ) 0 0.5 1
Superphosphate (0-46-0)^3 0 0.5 1
KNO 3 0 0.5 1
Trace elements * * *
Wetting agent * * *
(^1) Dolomite lime varies with the grind or type of product incorporated. More lime is usually added
with low alkalinity water (less than 150 ppm CaCO3) sources and less lime with high alkalinity (greater than 250 ppm CaCO3) water sources.
(^2) Hydrated lime is added when a rapid rise in pH is necessary, for example the starting pH is
desired at planting as opposed to 7 to 14 days after planting.
(^3) Two pounds of single superphosphate (0-20-0) can be used in place of 1 pound gypsum and 1
pound 0-40-0. These rates for gypsum and superphosphate can safely be doubled, but higher rates will likely have minimal beneficial impact and may contribute to leaching of fertilizers and high medium EC levels.
*Incorporation rates for trace elements and wetting agents vary greatly. Follow manufacturer’s recommendations.
