Growing media is a crucial ingredint through which plant roots grow and extract water and nutrients. Selecting a good growing medium is fundamental to good plant care and is the foundation of a healthy root system.
Growing media for use in container and pots is available in two basic forms: soil based and organic based. Compared with soil based media that has field soil as a major component, organic based media (a base of organic materials that may be compost, peat, coconut coir, or other organic materials, mixed with inorganic ingredients) promotes better root development. In temperate areas, nurseries can choose from a wide range of commercial products for their growing media, including peat moss, vermiculite, and perlite, and premixed blends of these ingredients. Most nurseries in the tropics, however, do not have easy and affordable access to these materials, and even nurseries in temperate areas are seeking to replace some of these ingredients with more local and sustainable materials. In the tropics, growers often create their own media using locally available ingredients.
A favourable growing medium consists of two or more ingredients. Growers must be familiar with the positive and negative characteristics of the various ingredients and how they will affect plant growth when creating a suitable growing medium, or even when purchasing a commercial one. This chapter describes the uses, functions, and properties of growing media ingredients. From this information, you can experiment with available materials and find the best combination for your nursery.
What Soil or Growing Media Does
- Physical Support
The growing medium must be porous yet provide physical support. Young plants are fragile and must remain upright so that they can photosynthesize and grow. With larger nursery stock in individual containers, a growing medium must be heavy enough to hold the plant upright against the wind. Bulk density is the responsible factor and will be discussed in the next section.
Plant roots need a steady supply of oxygen to convert the photosynthate from the leaves into energy so that the roots can grow and take up water and mineral nutrients. The byproduct of this respiration is carbon dioxide that must be dispersed into the atmosphere to prevent the build up of toxic concentrations within the root zone. This gas exchange occurs in the large pores (macro pores) or air spaces in the growing medium. Because nursery plants grow rapidly, they need a medium with good porosity—a characteristic termed “aeration” that will be discussed in more detail in the next section.
- Water Supply
Nursery plants use a tremendous amount of water for growth and development, and this water supply must be provided by the growing medium. Growing media are formulated so that they can hold water in the small pores (micro pores) between their particles. Many growing media contain a high percentage of organic matter such as peat moss and compost because these materials have internal spaces that can hold water like a sponge. Therefore, growing media must have adequate porosity to absorb and store the large amounts of water needed by the growing plant.
- Supply of Mineral Nutrients
Most of the essential mineral nutrients that nursery plants need for rapid growth must be obtained through the roots from the growing medium.
Physical Properties of Growing Media
1. Water-Holding Capacity
Micro pores absorb water and hold it against the pull of gravity until plants can use it. The water-holding capacity of a medium is defined as the percentage of total pore space that remains filled with water after gravity drainage. A good growing medium has a high water-holding capacity but also contains enough macropores to allow excess water to drain away and prevent waterlogging. Water-holding capacity varies by the types and sizes of the growing medium ingredients. For example, a peat moss particle will hold much more water than a similarly sized piece of pumice. The degree of compaction is also extremely important. When growing medium particles are damaged during mixing or compacted when the containers are filled, the percentage of macropores is severely reduced. Overmixed or compacted media will hold too much water and roots will suffocate. Finally, the height of the container affects the water-holding capacity; a certain amount of water will always remain in the bottom of the container. When filled with the same medium, short containers will have a higher percentage of waterlogging than taller ones.
The percentage of pore space that remains filled with air after excess water has drained away is known as aeration. As we have already discussed, oxygen for good healthy roots is supplied through the larger macropores, which also allow the carbon dioxide from respiration to dissipate. A good growing medium, especially for rooting cuttings, contains a high percentage of macropores.
The total porosity of a growing medium is the sum of the space in the macropores and micropores; plants need both. A growing medium composed primarily of large particles will have more aeration and less water-holding capacity than a medium of smaller particles, which will have less aeration and more water-holding capacity. Either of these media would restrict plant growth. Plants growing in a medium with all large particles would dry out too quickly, and those growing in a medium with all small particles would suffer from waterlogging. For a single-component medium, the ideal particle range to promote both water-holding capacity and aeration is about 0.03 to 0.24 in (0.8 to 6 mm). In actual practice, however, a good growing medium will contain a mixture of ingredients with different particle sizes and characteristics.
How Its Made
Growing media components are either organic or inorganic. Organic components include peat moss, bark, coconut coir, rice hulls, etc. Inorganic components include perlite, pumice, vermiculite, sand, hydrogel, etc. Some of these components hold water on their surface, others hold water within their structure, while others hold little compared to other components.
There are also components, such as perlite, that hold very little water, if any. Keep in mind that a specific type of ingredient can vary in its water holding capacity and physical structure, depending on its origin and how it is processed. For example, bark can vary greatly in its source and structure depending on how it is processed, aged, composted and screened. This is also true for peat moss. Light brown, fibrous peat moss has a porous structure and can hold up to 16 times its weight in water.
However, if this same peat moss is processed into fine particles, the available water can be cut in half and the air porosity decreases dramatically. If you blend your own growing medium, your source materials should be consistent to produce a quality and predictable growing medium. It is important to know the structure as well as the chemical and physical properties of the ingredients you use to be sure that the growing medium blend you produce is the same, batch after batch.
Common organic ingredients include compost, coconut coir, peat moss, bark, rice hulls, sawdust or any other appropriate, locally available material. These materials are lightweight, have high water-holding capacity and CEC, and some contain minor amounts of mineral nutrients. Some of these organic ingredients require screening or composting of local raw materials before use. The nursery may choose to do the processing, or a local supplier may specialize in the composting or processing local materials to sell to the nursery at a reasonable cost.
Sphagnum peat moss is currently the most common organic component of growing media in temperate zone nurseries. Although types of peat moss may appear similar, they can have very different physical and chemical properties. The horticultural properties of Sphagnum peat moss and the fact that it has uniform quality make it the only peat moss choice for plant nurseries that use peat moss. Most peat moss comes from Canada, some comes from New Zealand, and the one known tropical source is Indonesia. Therefore it is expensive and problematic to import peat for most tropical nurseries. In addition, extraction and transportation of peat moss on a large scale is a sustainability concern, and even temperate nurseries are considering alternatives. For tropical areas where peat moss is reasonably affordable and available, some nurseries use it in limited amounts for germinant mixes or for learning to grow unfamiliar native species. Some nurseries may use peat as a transition component, comparing peat’s properties to local materials such as composts or coir to develop local alternatives for growing media while moving forward with plant production.
Because of the risks of using soils and the expense of importing peat moss, many tropical nurseries prefer organic compost as a green alternative to peat moss. For example, in Florida, a variety of native plants grown in biosolid yard waste compost were as large or larger than those grown in a peat-based growing medium. Composts are an excellent sustainable organic component for any growing medium and significantly enhance the medium’s physical and chemical characteristics by improving water retention, aeration porosity, and fertility. Some composts have also been found to suppress seedborne and soilborne pathogens. Compost quality can vary considerably between different source materials and even from batch to batch so growers need to always test new materials before general use.
Composting is the physical and chemical decomposition of organic materials caused by the digestive activities of insects, fungi, and bacteria. Raw materials for compost include any plant wastes such as vegetable or fruit scraps, leaves, weeds, or byproducts, such as cacao pods, coffee pulp, sugarcane bagasse, orchard prunings, and rice hulls; aquatic wastes such as aquatic weeds (such as the noxious weed water hyacinth) or fish parts from fish processing; animal wastes such as manures, feathers, and bedding; and wood wastes such as bark or sawdust. Sometimes these products are considered waste materials and are burned or disposed of at a cost to the producer—composting turns them into a valuable resource. Organic nursery wastes may also be recycled through composting; including used growing media and culled seedlings. Sustainability and renewability of the compost source is important to consider. For example, seaweeds were once considered a good compost material, until people began to understand the importance of seaweeds to fish breeding and the damage that comes from depleting wild seaweed during fish-breeding season. Some nurseries grow part of their own compost ingredients using nitrogen-fixing trees or fast-growing plants such as comfrey, which can be cut back year after year. Growers anywhere should be able to find a sustainable source of organic matter that can be composted and used as a growing media component.
A byproduct of processing coconut husks is known as coir dust, coco peat, or simply coir. This material has proven to be an excellent organic component for container growing media and is readily available in some tropical locales. Coconut coir has many desirable qualities: high water-holding capacity; excellent drainage; absence of weeds and pathogens; physical resiliency (withstands compression of baling better than Sphagnum peat); slow decomposition; easy wettability; and acceptable levels of pH, cation exchange capacity, and electrical conductivity.
Coir is very similar to peat in appearance and structure, and, like peat, physical and chemical properties of coir can vary widely from source to source. Coir is low in nitrogen, calcium, and magnesium but can be relatively high in phosphorus and potassium.
Excess salinity and phenolic compounds in coir can be a problem in areas with inadequate quality control. In addition, some coir sources have reportedly contained chlorides at levels toxic to many plants. Thus, it is very important that salts and other compounds are thoroughly leached with fresh water before shipment and use. Compared with Asia, little coir production occurs in tropical America, and, currently, supplies of coir are limited in some areas. Nurseries must locate a quality, consistent source and then add coir to media on trial basis first, testing effects on a species-by-species basis.
Maple, oak, and sycamore are among the principle leaf types suitable for the preparation of leaf mold. Layers of leaves and soil are composted together with small amounts of nitrogenous compounds for approximately 12 to 18 months. The use of leaf mold can effectively improve the aeration, drainage and water holding properties of a growing media. Although these materials are readily available at low cost, leaf mold is not extensively used in container production.
Barks are primarily a bi-product of the pulp, paper and plywood industries. Suitable particle size is obtained by hammer milling and screening. This produces a material which is suitable for use in container media. Physical properties obtained from tree barks are similar to those of Sphagnum moss.
This type of peat consists of the partially decomposed remains of hyprum, polytrichum and other mosses of the Hypanaceae family. Although it decomposes more rapidly than some other peat types, it is suitable for media use. Many of the peat deposits in the Northern United States are Hypnaceous.
Reed and Sedge – are peats derived from the moderately decomposed remains of rushes, coarse grasses, sedges, reeds and similar plants. These fine textured materials are generally less acid and contain relatively few fibrous particles. The rapid rate of decomposition, fine particle size and insufficient fiber content make reed and sedge peats unsatisfactory for media use.
Humus or Muck
Consists of the decomposed debris of finely divided plant materials of unknown origin. Humus often contains large quantities of silt and clay particles, and when mixed with soil does not improve drainage or aeration. Due to its rapid rate of decomposition and particle size, humus is considered to be undesirable for growing media use.
Sphagnum moss is the dehydrated remains of acid-bog plants from the genus Sphagnum (i.e. Spapillosum). It is light in weight and has the ability to absorb 10 to 20 times its weight in water. This is attributed to the large groups of water holding cells, characteristic of the genus. Sphagnum moss contains specific fungistatic substances which accounts for its ability to inhibit damping-off of seedlings. Sphagnum moss is perhaps the most desirable form of organic matter for the preparation of growing media. Drainage and aeration are improved in heavier soils while moisture and nutrient retention are increased in lighter soils. Germany, Canada and Ireland are the principle regions of Sphagnum moss production.
Bagasse is a waste bi-product of the sugar industry. It may be shredded or composted to produce a material which can increase the aeration and drainage properties of container media. Because of its high sugar content, rapid microbial activity results after the incorporation of bagasse into a media. This decreases the durability and longevity of bagasse and influences N levels. Although bagasse is readily available at low cost, its use is limited.
Raw sawdust, with its high C:N, can negatively affect nutrient availability, especially nitrogen but its properties can be improved with composting. Also, because of inherent differences in chemical properties between different woods, the suitability of sawdust as an organic growing media component is extremely variable. Some species produce sawdust with phytotoxic effects. Only consider using sawdust from sawmills because other wood residues, such as from treated boards, may contain preservatives or harmful chemicals. Sawdust from coastal sawmills can contain high levels of salts, so all potential sources need to be tested before general use in the nursery.
Rice hulls are the sheaths of rice grains, a waste product of rice processing. Rice hulls or husks have been used as a component of potting medium with locally obtained peat for many years in Indonesia. Several nurseries have used composted, screened, and hammer-milled rice hulls in place of composted bark.
Other Possible Organic Ingredients
Nearly any other organic material that is locally available has the potential to be an important addition to nursery growing media. Composted material takes longer to produce, but has a more reliable texture and nutrient content than raw material. For example, composted manure from livestock pens and other organic waste from agricultural operations are excellent candidates and are frequently available for free if you can haul them. All “homemade” materials will take effort to process and fine-tune to create a consistent product. The final product will be worth the effort because you will be developing your own specialized growing media with low-cost local ingredients that do not have to be shipped.
Inorganic materials are added to growing media to produce and maintain a structural system of macropores that improves aeration and drainage. Many inorganic ingredients have a very low CEC and provide a chemically inert base for the growing medium. Inorganic materials with high bulk densities provide stability to large, freestanding containers. Several materials are routinely used as inorganic ingredients in growing media in native plant nurseries, including gravel, sand, vermiculite, perlite, pumice, and polystyrene beads.
Vermiculite is a common component and is a hydrated aluminium-iron-magnesium silicate material that has an accordion-like structure. Vermiculite has a very low bulk density and an extremely high water-holding capacity, approximately five times its weight. This material also has a neutral pH, a high CEC, and contains small amounts of potassium and magnesium. Vermiculite is produced in four grades based on particle size, which determines the relative proportion of aeration and water-holding porosity. Grades 2 and 3 are most commonly used in growing media; grade 2 is preferred when more aeration porosity is desired, whereas grade 3 produces more water-holding capacity. A 1:1 mixture of peat moss and coarse vermiculite is a common growing medium mix in many temperate nurseries.
Perlite is a siliceous material of volcanic origin. Perlite particles have a unique closed-cell structure so that water adheres only to their surface; they do not absorb water as peat moss or vermiculite do. Therefore, growing media containing perlite are well drained and lightweight. Perlite is also rigid and does not compress easily, which promotes good porosity. Because of the high temperatures at which it is processed, perlite is completely sterile. It is essentially infertile, has a minimal CEC, and has a neutral pH. Perlite is typically included to increase aeration, and commercial mixes contain no more than 10 to 30 percent perlite. Perlite grades are not standardized, but grades 6, 8, or “propagation grade” are normally used in growing media. Perlite grades often contain a range of particle sizes, depending on the sieve sizes used during manufacturing. One safety concern is that perlite can contain considerable amounts of very fine dust that causes eye and lung irritation during mixing. Wetting the material while mixing and wearing dust masks and goggles can reduce this risk. Pumice is a type of volcanic rock consisting of mostly silicon dioxide and aluminium oxide with small amounts of iron, calcium, magnesium, and sodium. The porous nature of pumice particles improves aeration porosity but also retains water within the pores. Pumice is the most durable of the inorganic ingredients and so resists compaction. Cinder (often called scoria) is another type of volcanic rock and a common growing media component in volcanic areas such as Hawai‘i, where growers may sift the cinder rocks to obtain the desired sizes for their containers.
Sand is one of the most readily available materials and is relatively inexpensive. The composition of sand varies widely. When considering if local sand is a suitable component, the type of sand and sand particle sizes must be considered. For example, some silty river sands with small particle size can have a serious negative effect on growing media by making them excessively heavy and not contributing to improved aeration or drainage.
Nurseries with access to siliceous (granite or schist derived) sands may be able to use local sand as an inorganic component. Sands derived from calcareous sources (such as coral or limestone) are high in calcium carbonate (CaCO3), however, and can have dangerously high pH values. Growers can test sands by adding a drop of dilute acid or even strong vinegar—a fizzing reaction indicates the presence of CaCO3. It is better not to use coral-based soils or sands if at all possible but, if necessary, add lots of organic matter to help buffer the effects. Some plants grown in your nursery may be adapted to local calcareous soil conditions and may not suffer from the increased pH if the sand is used sparingly.
Sand is used to increase porosity, but small sand particles can lodge in existing pore spaces and reduce aeration and drainage. In general, sizes of 0.002 to 0.010 in (0.05 to 0.25 mm) are too small and will block drainage holes and reduce aeration. Larger (medium to course) particles are more suited to increase porosity. The general recommendation is to wash sand (flushing out salt content if present) and sterilize or pasteurize it before incorporating it in the growing medium. Perhaps the more serious drawback of using sand in growing media is its weight, which causes problems with handling and increases the cost of shipping.
Polystyrene Beads or Flakes
Polystyrene is more commonly known by its trademarked name Styrofoam. Beads or flakes of polystyrene are a processing byproduct. Polystyrene increases aeration and drainage, decreases bulk density, and is highly resistant to decomposition. New polystyrene is unlikely to be a locally available material and many people are phasing out the use of polystyrene for sustainability concerns. It may be possible to recycle polystyrene and use pieces in growing media although it is not biodegradable and is often considered undesirable to outplant on project sites.
Calcined clays are formed by heating montmorrillonitic clay minerals to aproximately 690oC. The pottery-like particles formed are six times as heavy as perlite. Calcined clays have a relatively high cation exchange as well as water holding capacity. This material is a very durable and useful amendment.
These inorganic soil amendments are generally utilized to increase the number of large pores, decrease water holding capacity and improve drainage and aeration. Other materials such as: pumice; cinders; and pea-gravel are also suitable for this use.
Several synthetic soil amendments are bi-products of various plastic manufacturing companies. Others are designed specifically for use in container media. These materials are frequently used in place of sand and perlite and have much the same influence on media properties.
This material is prepared by mixing air with a liquid resin and allowing to cool. Urea formaldehyde foams have a greater water holding capacity than polystyrene but are similar in their influence on aeration and drainage. Raw materials are easily transported and are very effective amendments.