Greenhouse Potting Media

J. Raymond Kessler, Jr.

 

      Choosing a rooting media for growing greenhouse crops in containers seems a monumental task when one considers all the different components and recipes available to choose from. Flower producers often spend a lot of time and energy on media formulations, mixing, and management. Others simple purchase commercially available bag mixes and grow good crops. Sometimes blaming poor crop performance on the growing substrate is an over-simplified supposition. However, no amount of cultural adjustment can overcome the limitations of a poor medium. Many growers find a great challenge in trying to develop a custom blend that outperforms all the others available. The purpose of this discussion is to approach root-zone management logically.

 

Functions of a Root Medium

      A root medium must provide four basic functions to support good plant growth:

1.   Media must provide anchorage or support for the plant. Individual roots grow among soil particles and provide a firm foundation for physically supporting the stem as it grows.

2.   Media must serve as a reservoir for plant nutrients. With the exception of carbon and oxygen, plants obtain all essential elements from the growing medium. These elements must be in an available form, in sufficient quantities, and in proper balance for adequate growth. Nutrient elements must not only be present in the medium but also available for root uptake.

3.   Media must hold and provide available water. A container medium must hold sufficient quantities of water to provide plant needs for one irrigation to the next.

4.   Media must provide adequate gas exchange between the roots and the atmosphere. Respiration is required by roots to provide the energy for uptake of water and nutrients and root growth. The substrate must provide sufficient oxygen and remove carbon dioxide for metabolic processes to occur.

      Individual media components can provide some or all four of the functions of a medium but not at the required levels of each.

 

Properties of a Root Medium

Cultural problems relating to growing media are related to the fact that plants in greenhouse production are grown in containers with a shallow depth and of limited volume. Plants growing in the ground outdoor have an essentially unlimited soil volume and depth in which to grow. The area for drainage, root penetration, and aeration is usually very large. However, in a container, the area for root penetration is restricted. Therefore, the reservoir for water and nutrients is limited. This reservoir must be replenished frequently through irrigation and fertilization. The medium in a pot is only open to the air at the top and through drainage holes in the bottom. Thus, aeration and drainage are restricted in a container. These are the primary reasons that field soils perform very poorly in a container.

      A container medium consists of three parts: 1) a solid phase, 2) a liquid phase, and 3) a gasses phase. About one-half of a good container medium is solid phase. The solid phase may be characterized by its texture. Texture refers to the distribution of particle sizes comprising the medium. The structure of a medium refers to the extent to which soil particles stick together to form aggregates. The remaining voids between particles in the medium is occupied by water or air. These interlocking channels between particles are the pore space. Both the total volume of pores (called porosity) and pore size distribution are important. The terms macropores and micropores are often used to refer to pore size. Macropores are larger pores that usually contains air while micropores are small pores that usually contains water.

      Media texture and structure relates to water retention because water is attracted to soil particles by capillary action. Capillary action is a result of cohesion (an electrostatic attraction between water and charged surfaces on soil particles). Therefore, water exists as a film covering particles within the medium. Capillary action tends to pull water into and hold water within the medium.

      When water is added to a contained of medium, it moves downward through the pore space because of capillary suction and the pull of gravity. Water moves like a front, pushing air out of the pore space ahead and pull air into the pore space behind. Assuming that the drainage holes do not impede the downward movement, water will drain from the container until the capillary forces in the medium match the downward gravitational forces. When these two forces equalize, movement of water out of the container stops. The greater the depth of the soil volume, the stronger the force of gravity and the weaker capillary action. Conversely, the shallower the soil volume, the weaker the force of gravity and the stronger the capillary action.

      Further, water toward the top of the soil column is more affected by gravity and water toward the bottom of the column is more affected by capillary action. As a consequence, less water and more air is retained toward the top of the soil column and more water and less air is retained toward the bottom of the soil column.

      One solution to increasing drainage and aeration is to use media components with large particle sizes to increase the pore space and make the texture courser. However, increasing pore space size also reduces the capacity of the medium to hold water. This is why media for greenhouse often use consists of a highly absorbent organic material (peat to hold water) and a course aggregate material (perlite for drainage and aeration). This is a compromise between the need for drainage and aeration and water retention.

      The weight of a growing medium per unit volume in the bulk density. Bulk density of a greenhouse medium is important to moving plants in the greenhouse and shipping plants to market. Many of the components of container media are light weight to reduce the labor and cost of handling both media components and the media itself. Light weight media also help reduce freight cost in shipping. Bulk density also relates to how tightly packed a medium is. High bulk density can reduce aeration and water movement and reduce root growth into the medium.

      Plant nutrients are present in the soil solution in ionic form either as cations, with a positive charge, or as anions, with a negative charge. Root-media components like clay, peat, and vermiculite have negatively charged surfaces that electrostatically attract and hold cations. Cation exchange capacity (CEC) is a measure of the ability of the media to hold and buffer cations with the soil solution. Opposing forces (e.g., thermal motion) tend to dissociate cations from these surfaces. Therefore, cations are constantly exchanged between charged sites on the soil particles and the soil solution resulting in a changing equilibrium and concentration gradient. The degree to which one cation can replace another depends on the nature of the cation (e.g., ionic charge), its concentration, and the activities of other cations. Cations may be listed in order of binding strength as follows:

 

H+ > Ca+2 > Mg+2 > K+ = NH+ > Na+

 

Cations on the left are bound to soil particles more tightly than those on the right. As the concentration of cations decrease in the soil solution, forces opposing cation binding cause a release of bound nutrients into the soil solution for root uptake. Conversely, as the concentration of cations increases in the soil solution, most of the exchange sites become saturated. Weakly adsorbed cations (such as Na+) are readily replaced by stronger ones (like Ca+2) so are more subject to leaching.

      One last major property of media that has a major impact on the root’s ability to acquire and utilize nutrient is the pH. Media pH is a measure of the hydrogen ion concentration, and is measured on a logarithmic scale ranging from 0 to 14. On this scale, pH 7 is neural and the concentration of hydrogen ions (H+) equals hydroxyl ions (OH-) in solution. Measurement of soil pH determines pH of the soil solution which, like nutrients, would be subject to rapid change if not for the buffering effect (CEC) of solid components in the soil. Because H+ is a major cation adsorbed to cation exchange sites, the solid component has a potential acidity that tends to stabilize pH of the soil solution over time.

      The importance of soil pH to roots is its effect on nutrient uptake and the availability of nutrients in the soil solution. Plants vary widely in their pH optimum for growth and each species can grow within a pH range. Therefore, plants are able to cope, to varying degrees, with changes in soil solution pH, but extremes can have a profound effect on growth. Extremes in pH can affect both the availability and solubility of individual nutrients in the soil solution. Most greenhouse crops grow in a pH range of 5.0 to 7.0. In greenhouse media composed largely of peat, limestone is added to raise pH within acceptable ranges for growth.

 

Root-Media Components

Organic Components

      Peats are the major organic matter source used in greenhouse media today. It is the product of sphagnum peat moss in varying degrees of decomposition.

 

Peat moss is the most common form of peat and can hold about 60% its own weight in water. It has a pH of 3.0 to 4.0 so must be amended with limestone to raise the pH to acceptable levels for plant growth. Peat moss for greenhouse comes in 3 to 5 cu.ft. compressed bales that must be "fluffed-up" before use. One drawback of peat is that it is difficult to re-wet once it gets dry.

 

Bark may be the composted bark from many different tree species (pine, fir, redwood) processed by the lumber industry. Fresh bark requires nutrients, mainly nitrogen, to decompose. In a contained medium, fresh bark can suppress plant growth which may be related to tieing-up nitrogen or possible the presents of toxic compounds. Composting for about 30 days reduces the need for nitrogen and may detoxify the bark. Bark is often used in greenhouse media because it is less expensive than peat. However, few media contain more than 20%

 

Others

      Reed-sedge peat is composed of swamp plants, mostly reeds, sedges, and grasses.

      Hypnum peat is highly decompose hypnum peat moss or reed-sedge products.

      Rice hulls are the composted outer hull from rice processing.

      Composted yard wast from municipal wast collection.

      Coconut Coir is the ground inner husk from coconut processing.

      Manures from various animals are composted to use in media.

 

Inert Components

      These components primarily contribute to the porosity and pore space distribution of the media to increase drainage and aeration.

 

Vermiculite is a mica-like silicon mineral that is mined in Montana and South Carolina in the U.S. The ore is placed in ovens at 1400°F where it expands to particles with a plate-like structure. Vermiculite has a high capacity for holding water and a very high cation exchange capacity. It also has large amounts of magnesium and potassium that are available for plants. It is sterile and light weight (about 7-10 pounds / cu.ft.). Vermiculite is not very durable and will compress if handled wet. The pH of vermiculite is 7.0 to 7.5. Be sure to use the horticultural grades, industrial grades are for insulation purposes.

 

Perlite is produced for aluminum-silica volcanic rock heated in ovens to 1800°F. The ore expands like popcorn to form white, light-weight (about 8 pounds / cu.ft.) particles with little or no water holding capacity. Perlite is sterile, has a pH of 7.0-7.5, is chemically inert, and has no cation exchange capacity. The material is dusty and particle masks should be used during handling.

 

Calcined Clay consists of clay particles heated to high temperature in ovens to form hardened particles that weigh about 60% more (38 pounds / cu.ft.) than perlite or vermiculite. They are porous, hold large amounts of water, and have a high cation exchange capacity, little nutrition. Calcined clay is not currently used extensively in greenhouse media.

 

Sand is excellent for adding drainage, but the grade and particle size distribution is critical. Purchase only concrete-grade sand (sharp, course sand) that has been washed to remove silt and clay. Sand is an inexpensive material but is also heavy (100 pounds / cu.ft.). It is low in nutrients, low in water holding capacity, and chemically inert.

 

Polystyrene Foam is composed of white, synthetic plastic beads that has many of the characteristics of perlite. It is extremely light-weight (1.5 pounds / cu.ft.) and inert.

 

Soil-less Medium

      Up until the 1950's, most greenhouse operators used media composed of sterilized field soil combined with organic matter (often peat) and an inert component, often sand of perlite. The problem with these mixes was that it was difficult to standardize because field soils are inconsistent, difficult to sterilize, heavy to ship, and might contain residual herbicides.

      The first attempts to standardize greenhouse mixes in the U.S. were but researchers at the University of California in the 1950's. "The UC mixes" were composed largely of different combination of sand and peat moss. A few growers had problems with these mixes because of inconsistent sand supplies. The first truly standardized "soil-less" mixes were the "peat-lite" mixes developed at Cornell University in the 1960's. These media consisted of various combinations of peat and perlite or vermiculite and these are still in use today. Cornell Mix A is half peat and half vermiculite while Cornell B is half peat and half perlite.

      Most of the soil-less media in use today are variation on the Cornell mixes. For example, bark may substitute for a small percentage of peat or a different inert component combination may be use. Eight frequently used mixes are presented in the following table:

2 vermiculite

2 peat moss

 

1 perlite

1 vermiculite

1 pine bark

 

 

2 vermiculite

2 pine bark

 

1 perlite

2 vermiculite

1 peat moss

1 pine bark

1 perlite

 

1 peat moss

3 pine bark

1 sand

 

1 peat moss

3 hardwood bark

1 sand

1 rock wool

1 peat moss

 

 

3 rock wool

7 peat moss

 

 

 

      Soil-less media have limestone added to correct the pH. Dolomitic lime is frequently use because it also supplies the calcium and magnesium needed by plants. Phosphorus is usually incorporated in the mix as either superphosphate (0-20-0) or treble phosphate (0-42-0). Superphosphate is more common because it also contains calcium and sulfur as gypsum. Micronutrients are also included from commercial formulations composed of chelated or fritted sources. A starter charge of nitrogen and potassium may be included for early plant growth. Because peat based mixes are difficult to re-wet once dry, a wetting agent may be added. Care should be taken to purchase a wetting agent specifically for potting media. Some commercial wetting agents have been found to be toxic to some crops. Soil-less media have the following advantages:

1.   Components are readily available in standard, uniform quality.

2.   Consistent chemical and physical properties for plant growth.

3.   Generally, do not require pasteurization.

4.   Cost of preparation and handling is competitive.

5.   Base fertilizer is low but adequate to begin plant growth.

6.   Preparation is easily mechanized.

7.   Mixes are consistent from batch to batch.

8.   Mixes are light-weight.

 

Economics

The first decision involving growing media is often whether to purchase medium or mix a your own at the greenhouse operation. On face value, bagged mixes seem expensive but once all the costs are considered, mixing your own formulation may be out of the question for most small growers. Like many economic decisions in the greenhouse, carefully consider all the costs of both approaches. For a bag mix consider the cost per bag and shipping costs. For mixing your own formulation, consider each component's cost, management time, labor, office expenses, equipment costs and depreciation, and costs of special structures for the mixing facility and component storage. Many large growers mix their own formulation because the cost of commercial bag mixes would be prohibitive. One other option is to purchase bulk mixes from local formulators. These companies will mix a medium to your specification and delivered by truck at a lower cost than bag mixes.