4.3 Use of Soilless Growing Media in Hydroponic Production
4.4 Use of Soilless Growing Media in Tissue Culture
4_Soilless-Plant-Growth-Mediums
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Soilless Plant Growth Mediums
Overview
Title image credit: "Hand Trowel with Soil" by Image Catalog is licensed under CC0 1.0
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Introduction
Learning Objectives
Explain the types and utility for soilless substrata for plant growth.
Identify the different types of soilless cultures.
Discuss the advantages of alternate growth mediums.
Key Terms
aquaponic production - a system of growing plants in the water that has been used to cultivate aquatic organisms
hydroponic production - the production of normally terrestrial, vascular plants in nutrient rich solutions or in an inert, porous, solid matrix bathed in nutrient rich solutions synthetic
substrate - plant growing media made of artificial materials
tissue culture (micropropagation) - a method of propagating a large number of plants from a single plant in a short time on a nutrient culture medium under laboratory conditions
Introduction
Soilless plant growth mediums are crucial to the success of many areas of plant sciences, with practical applications in nursery container production, hydroponic farming, and tissue culture. Soilless production allows the grower to have more control over environmental factors that directly impact plant growth and often include varying proportions of natural or inorganic ingredients for mixtures tailored to meet the crop’s needs.
Use of Soilless Growing Media in the Nursery Container Production
Container plant production (Figure 4.4.1) is a major area of the horticulture industry, and most plants available to consumers are grown in plastic pots. Nearly all container-grown plants are planted in soilless growing mixtures rather than true soils. The benefits of using soilless growing media rather than soil are increased uniformity of the environment for root development, better control over characteristics such as water retention, nutrient availability, and drainage, as well as a decrease in weight for ease of transportation from the nursery to the retailer or customer (McMahon, 2020).
Qualities of a Good Growing Medium
While soilless mixtures are usually tailored to best suit specific crops, there are several basic properties that all good growing mediums share (Acquaah, 2009).
- Good aeration and drainage: There should be a good proportion of air space, and the mixture should drain freely.
- Porosity: The mixture should have good water holding capacity and be easily wetted.
- Durability: The materials should be long-lasting in the pot and resist decomposition.
- Chemical properties: The mixture should have a good Cation Exchange Capacity (CEC) and a pH suited to the crop grown. There should be nutrients in sufficient quantity for healthy plant growth. The growing medium should not produce any toxins. Natural materials that produce growth-inhibiting compounds (such as sawdust, wood chips or bark from certain species) should be fully composted or soaked in water to allow chemicals to leach from the product.
- Functionality: The mixture should flow easily through equipment, such as automated pot and tray-filling machines.
- Sterility: Soilless growing mixtures are usually pasteurized to kill pathogens.
Common Ingredients in Potting Mixtures
Excerpt used with permission from "Growing Media for Greenhouse Production" by E. Will & J.E. Faust, University of Tennessee Extension. Copyright © UT Extension.
Peat
Peat is a main component of most soilless media mixes used today. It is produced by the partial decomposition of plant material under low-oxygen conditions (Figure 4.4.2). Differences in peat are related to the climate under which they are produced and the species of plant from which it is formed. Peats from sphagnum mosses have a spongy, fibrous texture, high porosity and water-holding capacity, and a low pH (Figure 4.4.3). Peats formed from sedges are darker, more decomposed and contain more plant nutrients and higher CEC than peat from sphagnum mosses.
In the US, the American Society of Testing Materials has designed a system of peat classification based on generic origin and fiber content. Under this classification, sphagnum peat moss must contain more than 75 percent sphagnum peat moss fiber
and a minimum of 90 percent organic matter. Hypnum moss peat is composed of Hypnum species with a fiber content of at least 50 percent and an organic matter content of at least 90 percent. Reed-sedge peat must contain at least 33 percent reed,
sedge or grass fibers. Peat humus has a total fiber content of less than 33 percent.
The majority of peat moss used for horticultural purposes in the US is sphagnum peat moss from Canada or the south east US. Peats are classified as light or dark depending on the degree of decomposition. Most peats from Canada sold for use in the U.S. are light peats, having a loose, coarse structure and very little decomposition. More highly decomposed, dark peats have higher CEC and nutrient content. However, the finer structure results in poor media aeration and loss of volume. Media composed of dark peat must be handled carefully to avoid compaction.
Bark
Bark, a byproduct of saw mills, is used extensively in the nursery industry and has a role in greenhouse media as well (Figure 4.4.4). It functions to improve aeration and reduce the cost of media. Pine bark is the most widely used bark source, especially in the south- east US where local supplies are plentiful and inexpensive. Bark variability stems from the species and age of tree, method of bark removal and degree of decomposition. Raw bark is screened and wood (tree cambium) is removed before the <0.5 inch fraction is composted.
Bark must be aged or composted before use as a media component to eliminate the presence of phytotoxic compounds. Composting also decomposes the material to a point where further slow decomposition as a media component does not tie up nitrogen needed for plant growth, and does not result in great loss of volume. Bark particles of less than 3/8 inch in size are used in greenhouse media.
In general, nutrient content and pH (3.5 - 6.5) of unprocessed bark are low. However, the Ca content of barks tends to be high, resulting in a gradual increase in pH during composting. Final composted bark CEC is generally low. When using bark as a media component, it is wise to monitor for pH and nutrient changes in the media and be aware of the low water-holding capacity of the material. The presence of bark may also necessitate using higher doses of growth regulator applied as media drenches, since the bark appears to make the growth regulator less available to the plant.
Coir
The media component coir originates from ground-waste coconut husks (Figure 4.4.5). After most of the fibers are removed, the remaining coir, or coir dust, is marketed for media. Chemical and physical properties of the coir are variable, depending largely on the amount of fiber remaining in the material. Particle size ranges from about 0.5 to 2 mm, is greater than 80 percent pore space and air-filled pore space at container capacity is 9 to 13 percent. Coir has a high water- holding capacity, higher than peat in some tests, and is as or more easily rewetted after drying than peat. Coir-based media undergo slightly less settling than peat-based media.
Coir contains relatively low levels of micronutrients, but significant levels of phosphorus and potassium. The pH of coir ranges from 5.5 to 6.5. Since no lime is needed for pH adjustment and coir does not provide these nutrients, supplemental calcium and magnesium may need to be added to the fertilizer program. The EC of coir ranges from 0.4 to 3.4 mmhos/cm.
A concern with coir has been the reported high chloride levels (typically 200 - 300 ppm). Since most recommendations for media are 100 ppm or less chloride, coir may not be a preferred media component if non-leaching subirrigation is used. Coir has been suggested as a low-cost replacement for sphagnum peat moss in media. Production trials with a variety of plants indicate that there is great potential for this alternative media component. Because of the variability in the qualities of coir, it is important to purchase it from a reputable dealer with good quality-control practices.
Perlite
Perlite is a volcanic rock that is crushed and heated rapidly to a high temperature (1,800F). The material expands to form a white, light-weight aggregate with high pore space (Figure 4.4.6). Water-holding capacity is fairly low, as water is retained only on the surface and in the pores between particles. Perlite is added to media to improve drainage. It is chemically inert with almost no CEC or nutrients, and a neutral pH. Perlite may contain levels of fluoride that are injurious to fluoride-sensitive foliage plants. Maintaining a pH above 6 and reducing the use of fluoride-containing superphosphate fertilizer should avoid fluoride toxicity problems. Fine grades of perlite are available for use in plug production. Perlite dust can pose as a health risk; therefore, dust masks must be worn by workers during handling of this product.
Vermiculite
Vermiculite (Figure 4.4.7) is a silicate material that is processed much like perlite. Heating causes tremendous expansion of the particles and results in a highly porous lattice structure with good water-retention properties. Vermiculite is available in a number of grades from fine, for seed germination, to coarser grades for use as media amendments. Although the finer material allows the media to flow more evenly into plug trays when filling, the particles are too small to hold much air or water for the developing roots. It is also susceptible to compaction.
CEC of vermiculite is fairly high (2 - 2.5 meq/100cc) and pH varies from slightly to very alkaline, depending on the source. Most vermiculite mined in the US has a pH be- tween 6.3 and 7.8. Vermiculite provides some Ca, Mg and K. Particles are soft and easily compressed, so must be handled carefully.
Rock wool
Rock wool is made from basalt rock, steel mill slag or other minerals that are liquefied at high temperature and spun into fibers. The fibers are formed into cubes or blocks, or granulated into small nodules for use as a component of horticultural media. The granules have high porosity, air space, water-holding capacity and available water. These qualities, along with its ability to rewet rapidly, makes rock wool a good media component for subirrigated crop. Rock wool is slightly alkaline and has almost no cation exchange capacity or nutrients.
Polystyrene Foam
Flakes or beads of expanded polystyrene foam are added to media to improve aeration, drainage and reduce cost. They supply no nutrients, CEC or water-holding capacity and the pH is neutral. Styrofoam® should not be steam heated. The beads can migrate to the top of the media and may become a nuisance if dispersed by water or wind.
Use of Soilless Growing Media in Hydroponic Production
Excerpt used with permission from “Soilless Growing Mediums” by D. Thakulla, B. Dunn, & Bizhen, H., Oklahoma State University Extension. Copyright © OSU Extension.
History
The term “hydroponics” was first introduced by American scientist Dr. William Gericke in 1937 to describe all methods of growing plants in liquid media for commercial purposes. Before 1937, scientist were using soilless cultivation as a tool for plant nutrition studies. In 1860, two scientists, Knop and Sachs, prepared the first standardized nutrient solution by adding various inorganic salts to water, then using them for plant growth. Later, scientists started using an aggregate medium to provide support and aeration to the root system. Quartz sand and gravel were the most popular aggregate mediums used in soilless cultivation at that time. In the late 1960s, Scandinavian and Dutch greenhouse growers tested rockwool plates as a soil substitute, which resulted in revolutionary expansion of rockwool-grown crops in many countries. Today, many alternative porous materials are used as growing media in hydroponics, including organic medias like coconut coir, peat, pine bark and inorganic mediums such as mineral wool, growstone, perlite and sand. For more information about hydroponics see Unit 9, Lesson 2: Soilless and Hydroponic Production.
Aquaponics couples hydroponics with aquaculture, using nutrient-rich water to feed the hydroponically grown plants. Nitrifying bacteria convert the ammonia into nitrates. For more information about aquaponics see Unit 9, Lesson 2: Soilless and Hydroponic Production. The three main live components of aquaponics are plants, fish (or other aquatic creatures) and bacteria. Producers also choose growing media that will provide plant nutrition, support the plants and provide surface area for the growth of bacteria. Clay pebbles, lava rocks and expanded shale are among the most widely used growing media in aquaponics.
Characteristics of Growing Mediums
Selection of a growing medium depends on the type of plant, the pH of irrigation water, cost, shelf life of the product, the type of system that is being used and a grower’s personal preference (Table 4.4.1). A grower should look for specific qualities in choosing media. Soilless media must provide oxygen, water, nutrients and support the plant roots just as soil does.
Grow media | Cost | Lifespan | pH |
Mineral wool | Medium | Renewable | Basic |
Coconut fiber | Low/Medium | Short | Neutral |
Expanded clay | High | Reusable | Neutral |
Perlite | Low | Reusable | Neutral |
Vermiculite | Medium | Reusable | Basic |
Oasis cubes | Low | Short | Neutral |
Sand | Low | Reusable | Neutral |
Peat | Medium | Short | Acidic |
Grow stones | Medium | Reusable | Basic |
Rice hulls | Low | Short | Neutral/Acidic |
Pine bark | Low | Short | Acidic |
Pumice | High | Reusable | Neutral |
Sawdust | Low | Short | Acidic |
Polyurethane foam | Low | Short | Neutral |
Gravel | Low | Reusable | Basic |
Expanded shale | Low/Medium | Reusable | Neutral |
Lava rock | Low | Reusable | Neutral |
An ideal growing medium should have all or some of the following characteristics:
- Good aeration and drainage. While the medium must have good water retention, it also must provide good drainage. Excessively fine materials should be avoided to prevent excessive water retention and lack of aeration within the medium.
- Durability. The medium must be durable over time. Soft aggregates that disintegrate easily should be avoided.
- Porosity. The medium must stay damp from the nutrient flow long enough for plants to absorb all their required nutrients between cycles.
- Sterile. A clean and sterile growing medium will minimize the spread of both diseases and pests. A clean medium does not introduce additional nutrients to the roots. Some media can be reused by pasteurizing at 180 F for 30 minutes or using a 10% bleach soak for 20 minutes followed by multiple rinses of tap water.
- Chemical properties. Neutral pH and good cation-exchange capacity (the ability to hold nutrients).
- Functionality. Lightweight, easy to handle, reusable and durable.
Overview of the Most Popular Hydroponic Growing Mediums
Except used with permission from “Soilless Growing Mediums” by D. Thakulla, B. Dunn, & Bizhen, H., Oklahoma State University Extension. Copyright © OSU Extension.
Mineral Wool
Mineral wool (such as Rockwool) is a sterile, porous, non-degradable medium composed primarily of granite and/or limestone, which is superheated and melted, then spun into small threads and formed into blocks, sheets, cubes, slabs or flocking. It readily absorbs water and has decent drainage properties, which is why it is used widely as a starting medium for seeds, rooting medium for cuttings and for large biomass crops like tomatoes.
Advantages
- It has a large water retention capacity and is 18% to 25% air, which gives the root system ample oxygen as long as the medium is not completely submersed.
- It is available in multiple sizes and shapes for various hydroponic applications. Everything from 1-inch cubes to huge slabs can be found.
- Mineral wool slabs can be reused by steam sterilizing the slabs between crops. Structurally, it does not break down for three to four years.
Disadvantages
- It has a high pH, and nutrient solutions must be adjusted to accommodate for that factor. The initial pH of the commercial material is rather high (7.0 to 8.0), therefore, continuous pH adjustment to a more favorable range (5.5 to 6.0) is required, or the medium must be conditioned by soaking in a low-pH solution before use.
- Mineral wool does not biodegrade, which makes it an environmental nuisance when disposed of. Lately there has been a decline in the use of mineral wool.
- It has a restricted root environment and a low buffering capacity for water and nutrients. The water flow to plant roots may be hindered, even when the water content is apparently high.
- Many people find mineral wool dust irritating to the skin.
Coconut Coir
Coconut coir is also known by trade names like Ultrapeat®, Cocopeat® and Coco-tek®. It is a completely organic medium made from shredded coconut husks. Different sources and production procedures result in a large variability of end products in the market. The most popular is the compressed briquette form, which requires soaking in water before use. During soaking, the coir rehydrates and expands up to six times the size of the original briquette.
Advantages
- Coconut coir is slightly acidic and holds moisture very well, yet still allows for good root aeration.
- There are claims that coir dust enhances rooting due to the presence of root-promoting substances.
- Coir can be used either as a stand-alone medium or as an ingredient in a mix for the cultivation of vegetables and cut flowers. It can also serve as a rooting medium for cuttings under mist and in high humidity chambers.
- It is biodegradable, organic and non-toxic, which makes its disposal easier and environmentally friendly.
- Since it is compactable, it can be bought compressed then expanded at home, which saves money on shipping.
Disadvantages
- If the husks are soaked in salt water during manufacturing and not rinsed with fresh water, then there could be a problem with high salinity.
- Coconut coir is rich in sodium and chlorine and may damage the plants, which is why it must be washed. Usually, calcium and magnesium need to be added to both facilitate sodium removal and provide nutrients.
Expanded Clay Aggregate
Expanded clay pellets are made by heating dry, heavy clay and expanding it to form round porous balls. It is commonly known as lightweight expanded clay aggregate (LECA), grow rocks or Hydroton®. They are heavy enough to provide secure support for the plants, but are still lightweight. Their spherical shape and porosity help to ensure a good oxygen/water balance so as not to overly dry or drown the roots.
Advantages
- Expanded clay pellets release almost no nutrients into the water stream and are neutral with a pH of about 7.0.
- They have high pore space, which results in better flow of solution. They rarely become clogged or blocked, so water drains very effectively, which makes it a great option for ebb and flow systems as well as aquaponic media bed systems.
- After use, the pellets can be washed and sterilized for reuse.
- They are very stable and can last for many years.
Disadvantages
- The clay pellets do not have good water-holding capacity as compared to many other substrates. They drain and dry very fast, which may cause roots to dry out.
- They are fairly expensive.
- They often bind tightly around roots in Dutch bucket systems and can be hard to separate.
- Because clay pellets float for the first few months until they’re saturated, the pebbles can get sucked into filters or drain lines and cause blockages.
Perlite
Perlite is a natural volcanic mineral that expands when subjected to very high heat, and becomes very lightweight, porous and absorbent. It is produced in various grades, the most common being 0 to 2 mm and 1.5 to 3 mm in diameter. Perlite can be used by itself or mixed with other types of growing media.
Advantages
- It has one of the best oxygen retention levels of all growing mediums.
- It is very porous and has a strong capillary action. It can hold three to four times its weight of water.
- Its sterility makes it highly suitable for starting seeds. There is little risk of root rot or damping off.
- It is comparatively inexpensive and is reusable. After use, it can be steam pasteurized.
- Its stability is not greatly affected by acids or microorganisms.
Disadvantages
- Since it is very lightweight, it easily washes away. This drawback makes perlite an inappropriate medium in the flood-and-flush type of hydroponic systems.
- When used alone in hydroponic systems like drip systems, it does not retain water very well.
- Perlite dust can create respiratory problems and eye irritation, necessitating precautions such as wearing goggles and a mask to reduce dust exposure when working with it. When dry, fans can blow it around the greenhouse.
- Perlite is prone to algae growth that can lead to irrigation and fungus gnat problems.
Vermiculite
It is a micaceous mineral that is heated at temperatures near 2,000 F until it expands into pebbles. It is considered an excellent rooting medium. It is often used in combination with other types of media like coconut coir or peat moss to start seedlings. It is produced in various grades, the most common being 0 to 2 mm, 2 to 4 mm and 4 to 8 mm in diameter.
Advantages
- It has a relatively high cation exchange capacity and holds nutrients for later use.
- It is very porous, has a strong capillary action and has excellent water-holding capacity.
Disadvantages
- When used alone, it can retain too much moisture, which can result in waterlogged conditions, inviting bacterial and fungal growth.
- It cannot be steam sterilized as it disintegrates during heating.
- It is comparatively expensive and can contain a small amount of asbestos.
Oasis Cubes
Oasis cubes are a brand of medium manufactured from water-absorbent phenolic foam, also known as floral foam. It is a grow medium designed for both seeds and cuttings and is mostly used for plant propagation. Oasis cubes are most used for rapid germination of crops such as lettuce and cole crops (cabbage, collards and kale), onions and alliums, herbs and sometimes tomato and eggplant seedings.
Advantages
- It has a neutral pH and a great water-retention capacity.
- It is pretty versatile and can be transplanted into many different types of hydroponic systems and grow mediums.
- It is inexpensive and no pre-soaking is required.
- It comes in several different sizes.
Disadvantages
- It does not have any buffering capacity, cation exchange capacity or initial nutrient charge.
- Beyond seed germination and propagation, it is of limited value.
- The foam can break off and clog pump filters.
Sand
Sand is inarguably the oldest hydroponic medium and is very common. It is commonly mixed with other substrates like vermiculite, perlite and coconut coir. When using sand as a growing medium, growers often prefer coarse sand, as it helps to increase aeration to the roots by increasing the size of the air pockets between the grains of sand.
Advantages
- It is comparatively inexpensive and is readily available in most locations.
- The finer sand particles allow lateral movement of water through capillary action, which makes the solution applied at each plant evenly distributed throughout the root zone.
- When mixed with vermiculite, perlite and/or coconut coir, it helps aerate the mix for roots.
- Sand is very durable because it is neither chemically nor biologically affected.
- It can be easily steam-sterilized for reuse.
Disadvantages
- It has very low water- and nutrient-holding capacity and can exacerbate deficiencies quickly.
- Salt buildup may occur in the sand during the growing period. This can be corrected by flushing the medium periodically with pure water.
- It is very heavy.
Peat
Peat consists of partially decomposed marsh plants, including sedges, grasses and mosses. Sphagnum peat moss, hypnum peat moss, and reed and sedge peat moss are three types of peat in horticultural classification. Sphagnum peat moss is the most desirable and popular type, as it has higher moisture-holding capacity and does not break down as rapidly as other types of peat.
Advantages
- Peat moss has a high moisture-holding capacity and can hold up to 10 times its dry weight of water.
- Most peat mosses are acidic with pH of 3.8 to 4.5, which can be an advantage for some acid-loving plants.
- Even though peat moss retains water incredibly well, it can drain freely. Excess water quickly moves through the material to drain out.
- Disposal of used peat moss does not pose any environmental problem.
Disadvantages
- It is generally considered as a substrate conducive to numerous soil-borne diseases. Although peat can be sterilized, it does not alleviate the problem, as sterilization leaves a biological vacuum that can be easily filled by pathogenic fungi.
- In some cases, its acidic property may be a disadvantage for some crops, so lime or dolomite is usually added to increase the pH.
- It is not sustainable. Peat moss extraction from bogs is a destructive process that removes layers that took centuries to develop.
Growstones
Growstones are made from recycled glass. They are light weight, unevenly shaped, porous and reusable. They have good wicking ability and can wick water up to 4 inches above the water line. It is important to have good drainage to prevent stems from rotting.
Advantages
- Since growstone is inert, it does not supply plants with any additional inputs or elements that could interfere with the nutrient solution in the system.
- It is highly porous and provides a lot of aeration to the roots.
- Because it is made from glass, it is non-toxic and guaranteed to be free of contaminants like pathogens.
- Growstones can be reused or further recycled.
Disadvantages
- Sometimes growstones can cause root damage because they tend to grip the plant roots too much. This also makes it difficult to move the plants from one medium or grow area to another.
- Growstones come coated with a fine dust of silica, which needs to be carefully washed off. This is best done outdoors or in a well-ventilated space as the dust can clog drains and is dangerous to inhale.
Rice Hulls
Rice hulls are a byproduct of the rice industry. Even though it is an organic plant material, it breaks down very slowly like coconut coir, making it suitable as a growing medium for hydroponics. It is often used as part of a mix of growing media such as 30% to 40% rice hulls and pine bark mix. Rice hulls are referred to as either fresh, aged, composted, parboiled or carbonized. Parboiled hulls have been shown to be superior to other hulls as a medium amendment.
Advantages
- The overall pH of parboiled and composted rice hulls range from 5.7 to 6.5, which is right in the optimal pH range for most hydroponically-grown plants.
- They are comparable to perlite in water-holding capacity per weight but have a greater air-porosity ratio and can hold more oxygen in the root zone.
- They drain well and retain little water in general.
Disadvantages
- Fresh and composted rice hulls often have high amounts of manganese. If pH is not maintained properly, manganese toxicity is a potential problem.
- Rice hulls work well when mixed with peat or coir, but not as well when used as a standalone medium.
- It has a low cation-exchange capacity.
Pine Bark
Composted and aged pine bark was one of the first growing media used in hydroponics. It was generally considered a waste product, but has found uses as a ground mulch, as well as substrate for hydroponically grown crops.
Advantages
- Compared to other types of tree bark, pine resists decomposition better and has fewer organic acids that can leach into the nutrient solution.
- A naturally biodegradable material, used bark can be recycled in many ways, including as mulch.
- Because of its fibrous structure with pockets of many sizes, it holds nutrient solution and air well.
Disadvantages
- It absorbs water easily, which may result in water-logged conditions. A layer of rocks at the bottom will aid drainage greatly.
- Pine bark floats and may pose problems with an ebb and flow system. It is more suitable for a drip or a wick system.
- The pH of pine bark is acidic and might be a disadvantage.
Pumice
Pumice is a siliceous material of volcanic origin. It is graded and kiln dried to 80 F, making it sterile and ready to use. It can be mixed with other types of growing media, such as vermiculite or coir to improve aeration and drainage.
Advantages
- It breaks down slowly and is very lightweight.
- Its light-colored appearance makes it an ideal media for summer growing as it does not absorb heat.
- It has a high oxygen-retention level.
Disadvantages
- It has essentially the same properties as perlite but does not absorb water as readily.
- It can be too lightweight for some hydroponics systems, if bought as small pieces.
Sawdust
There are many variables that determine how well sawdust will work, predominantly the kind of wood used and the purity of it. Sawdust from Douglas fir and western hemlock have been found to give best results, while western red cedar is toxic and should never be used. A moderately fine sawdust or one with a good proportion of planer shavings is preferred, because water spreads better laterally through these than in coarse sawdust.
Advantages
- The best thing about sawdust is that it is very cheap or usually free.
- It retains a lot of moisture, so care must be taken while watering.
Disadvantages
- Sawdust might acquire salt levels toxic to plants. Therefore, the sodium chloride content of the samples should be tested before using. If any significant amount of sodium chloride is found (greater than 10 ppm), sawdust should be thoroughly leached with fresh water.
- Growers need to ensure their sawdust is not contaminated with soil and pathogens or chemicals from wood-processing facilities or undesirable tree species.
Polyurethane Grow Slab/Cubes
Polyurethane grow slabs and cubes are an uncommon hydroponics medium used as an alternative to oasis cubes or rockwool for starter cubes. It can be found as poly foam at hobby or fabric stores. It comes in rolls or sheets of different thickness and sizes. Starter cubes can be self-made by just cutting 1- to 2-inch-thick poly foam sheets/rolls.
Advantages
- It is a comparatively cheaper alternative to rockwool or oasis cubes for starting seeds.
- It is easy to find.
Disadvantages
- It may contain harmful chemicals.
- It is not likely to have predetermined holes for seed germination.
Gravel
Gravel has been used with great success, especially in ebb and flow systems. It is a fragmented media from rocks like sandstone, limestone or basalt and has large spaces between each particle. This helps give a plentiful supply of air to the roots, however, the medium does not hold water well, which can cause roots to dry out quickly.
Advantages
- Gravel is usually fairly cheap, works well as a starter medium and is typically easy to find.
- It is durable and reusable as long as it is washed and sterilized between crops.
- It does not break down in structure and can be reused.
Disadvantages
- Its heavy weight makes it difficult to handle.
- Gravel is not suitable for heavy plant roots.
Expanded Shale
Expanded shale is created when quarried shale is heated to temperatures above 2,000 F. The process renders the shale chemically and biologically inert. The heated shale loses its water, which causes the shale to expand. It is considered one of the best aquaponics grow media. It is lightweight and works well in aquaponic grow beds. Each stone has a large surface area for supporting the bacteria necessary to convert ammonia into nitrates.
Advantages
- The free draining quality of this medium aids in the necessary oxygenation of roots.
- Expanded shale holds up to 40% of its weight in water, allowing for better water retention around plants.
Disadvantages
- Expanded shale has a slightly polished surface area, but edges can be sharp, which can harm the root system of plants.
- Its heavy weight makes it difficult to handle.
Lava Rock
Lava rock is a lower cost alternative to expanded clay or expanded shale. These types of rock form when hot lava rapidly cools down. They contain air pockets inside, which gives an additional surface area for beneficial bacteria.
Advantages
- They are lightweight, porous and provide beneficial drainage, aeration, water retention and even trace elements to the system.
Disadvantages
- A notable disadvantage is their jagged texture. The sharp edges of lava rocks have the potential to cut your hands as well as damage the root system of plants.
Use of Soilless Growing Media in Tissue Culture
Micropropagation—also called plant tissue culture—is a method of propagating a large number of plants from a single plant in a short time under laboratory conditions (Figure 4.4.16). This method allows propagation of rare, endangered species that may be difficult to grow under natural conditions, are economically important, or are in demand as disease-free plants.
To start plant tissue culture, a part of the plant, such as a stem, leaf, embryo, anther, or seed, can be used. The plant material is thoroughly sterilized using a combination of chemical treatments standardized for that species. Under sterile conditions, the plant material is placed on a plant tissue culture medium that contains all the minerals, vitamins, and hormones required by the plant. The plant part often gives rise to an undifferentiated mass known as callus, from which individual plantlets begin to grow after a period of time. These can be separated and are first grown under greenhouse conditions before they are moved to field conditions.
Soilless Substrates for Micropropagation
Plant material grown in tissue culture is usually placed in a growing medium that has been tailored to meet that specific plant’s needs. The most common substrate used as a base in micropropagation is agar, which is a gelatinous product of some species of red algae (Figure 4.4.17). Agar will be mixed with ingredients—such as plant growth regulators—to initiate root or shoot development, mineral salts, sugar, vitamins, and even organic components like banana puree, coconut milk, or yeast extract (McMahon, 2020).
Benefits and Drawbacks of Tissue Culture Propagation
Micropropagation allows growers who have limited material from a parent plant to create many new plants in a short amount of time. When exposed to the right ingredients in the substrate, a small, sterile section collected from any part of the plant can eventually grow into an independent plant that is ready to transition to the greenhouse.
Tissue culture has become an important way to cultivate a uniform crop for the cut flower industry, orchids, and other houseplant production, as well as for fruit tree propagation. Micropropagation has also proven to be critical for preserving germplasm from rare and threatened species, including many species of orchid.
The use of tissue culture is critical to the practice of “embryo rescue”, where a developing embryo that is unlikely to grow by normal seed reproduction is removed from the seed and allowed to develop in vitro. Embryo rescue is a useful tool to plant breeders who have crossed two genetically distant parents (Aquaah, 2009).
Micropropagation is also an important tool in the field of biotechnology and is often used to grow genetically engineered plants. The grower will use a gene gun or a specially modified bacteria to insert genes or pieces of DNA into the plant material. If the process was successful, the host plant can be divided many times and its material grown on through tissue culture (McMahon, 2020).
The sterile nature of micropropagation allows growers to produce guaranteed disease and pest-free material. A large number of plants can be produced by fewer people in a much smaller amount of space when compared to other forms of plant production.
While micropropagation has revolutionized the field of plant sciences, there are also several drawbacks. Tissue culture requires expensive equipment, a sterile environment, highly trained staff, and high energy inputs. Young plants must be carefully transitioned to life outside of the sterile lab environment and will be extremely susceptible to “transplant shock” caused by changes in light, temperature, soil moisture, and other organisms. Plants grown in tissue culture often lack a functional cuticle and responsive stomates. Maintaining high humidity is critical while young plants acclimate to their new environment (Lineberger, n.d.)
Dig Deeper
Open Source Ecology: Hydroponics
Plant Growth Experiments in Zeoponic Substrates: Applications for Advanced Life Support Systems
Tissue Culture: Micropropagation, Conservation, and Export of Potato Germplasm
Explore a peat bog with Arit Anderson of BBC Gardeners World: Arit Anderson visits a peat bog in Cumbria looking at the subject of peat, its place in horticulture and its role in our environment. Natural England Senior Reserve Manager, Glen Swainson, explains how peat is formed, the habitat that peatlands provide and its role in the carbon cycle. To learn more, follow this link to watch the video on the BBC's website.
Discover how the United Kingdom’s horticulture industry is becoming peat-free: Arit continues to find out how the horticultural industry is adapting to reducing its use of peat and talks to gardeners about how the changes will impact them. To learn more, click this link to watch the video on the BBC's website.
Attributions and References
Attributions
"Growing Media for Greenhouse Production" by E. Will & J.E. Faust, University of Tennessee Extension. Copyright © UT Extension. Used with permission.
OpenStax Biology 2e by Mary Ann Clark, Matthew Douglas, and Jung Choi is licensed under CC BY 4.0.
"Soilless Growing Mediums" by D. Thakulla, B. Dunn, & Bizhen, H., Oklahoma State University Extension. Copyright © OSU Extension. Used with permission.
Title image credit: "Hand Trowel with Soil" by Image Catalog is licensed under CC0 1.0
References
Acquaah, G. (2009). Horticulture principles and practices (Fourth edition). Pearson Education, Inc.
Hydroponic Production of Vegetables and Ornamentals. Embryo Publications, Greece.
Lineberger, R.D. (n.d.). Care and handling of micropropagated plants. Texas A&M University. Retrieved February 8, 2022 from https://aggie-horticulture.tamu.edu/tisscult/Microprop/micropro.html
McMahon, M. (2020). Plant science: Growth, development, and utilization of cultivated plants (Sixth edition). Pearson Education, Inc.
Resh, H.M. (1978). Hydroponic Food Production. 5th ed. Woodbridge Press Publishing
Company, Santa Barbara, CA.
Roberto, K. (2004). How-to hydroponics. 4th ed. Electron Alchemy, Inc. Massapequa, NY.
Savvas, D. (2002). General introduction, 1-2. In: D. Savvas and H. Passam (eds.).