phyzix
Well-Known Member
This is an adaptation of work by Dr. M. Marrush, Plant Science professor at the University of California, Davis. I take no credit for this work, I merely reproduced and formatted it. Dr. M. Marrush has no affiliation to this website or myself. This is merely high quality information about container mediums that are proven effective for germination, transplanting, and general growth of plants.
I. Introduction
During the past 25 years, much has changed in commercial propagation practices. One of these changes is that, except in special cases, such as the propagation of hardwood cuttings and seeds in field beds, soil is seldom used today. Media with soil do not provide the range of options, such as lightness in weight, ease of handling, and freedom from pathogens, pests, weed seed, and contaminants, which are required for modern propagation practices.
Plants are increasingly propagated from seeds and cuttings using soilless media in high-density containerized systems, such as plug trays or cell packs. The use of containers provides advantages, such as flexibility in time of planting, ease of handling and movement of materials, and adaptation to mechanization. Germination and rooting typically take place in modern computer-controlled greenhouses, polyhouses, or other protected environments that provide the needed moisture, temperature, and light. These technologies allow more efficient, cost effective, and labor-saving ways of propagating plants.
II. Functions of a Medium
While media for sowing seeds are generally finer in texture than those in rooting media, the functional characteristics of both are similar and can be considered together. A propagating medium provides anchorage and support and an appropriate air-moisture balance for metabolic processes.
The ideal propagation medium should be loose or friable enough to maintain adequate amounts of air (one with an oxygen content of at least 12%) for germination and cell division. Oxygen is required for respiration to oxidize starches, fats, and other food reserves in the seed or cutting, and its utilization is proportional to the amount of metabolic activity. Good aeration in the medium also allows removal of carbon dioxide, a by-product of respiration, which may cause damage to seedlings when present in excess.
During propagation, it is necessary to prevent moisture loss from the medium and to maintain a humid atmosphere in the immediate surrounding air. The medium surrounding the seed must remain moist. It should have a high moisture-holding capacity, yet allow excess water to drain freely so that the seeds are not drowned in water. Roots may rot if the medium is poorly drained.
III. Properties of Media
It is often said that there are as many media as there are propagators, but there is no one best medium for all plants or all conditions. Factors, such as species, the time of year, use of growth-promoting substances, or the interaction of these and/or other variables, have a considerable influence on propagation success.
Whether for seeds, cuttings, or transplants, the medium at the start must be at least partially disinfested and free of pathogens, harmful pests, weed seeds, and toxic chemicals. Any problems early on, such as the presence of weed seeds, will require extra labor and cost for removal later. As a rule, the medium should contain little or no nutrients. Nutrients, per se, are of little value until roots are formed and can be added with water as needed. If present in excess, nutrients can cause injury to seedlings.
The important features of a medium can be described technically in terms of physical properties, such as bulk density and pore spaces, and chemical properties, such as electrical conductivity and pH. Knowledge of these properties provides the propagator with objective or easily measured criteria to evaluate and compare different lots or batches of the same mixture, and leads to an increased understanding of media dynamics.
IV. Materials and Mixtures
Most propagation media are mixtures of various materials. While media are sometimes made with three or even four different materials, in most cases, media with equivalent properties can be accomplished using two components. These are easier to prepare and perhaps more predictable in their performance or outcome.
Many different materials have been used in propagation media. These can be classed as inorganic or organic and may be used more or less in the natural state, manufactured, or blended. Inorganic materials include the following: sand, perlite, vermiculite, polystyrene, and rock wool (mineral wool). Organic materials include peat, bark, wood by-products, and composts, although many organic or farm-derived waste materials have been used on a more limited scale. As a rule, the mineral component is used to improve drainage and aeration by increasing the proportions of large air-filled pore space.
V. Inorganic Materials
Sand: The use of sand has declined dramatically during the past 25 years, and it has been largely replaced by lighter or more easily-handled materials such as perlite, or by commercial premix. A medium grade (0.02 to 0.8 in; 0/5 to 2 mm) of washed, sharp builder’s sand is recommended, since finer sand reduces pore space and drainage. Sand contains little or no nutrients and is used most often in mixtures with peat, perlite, and/or other organic components, and sometimes as a single-component medium. Seeds or cuttings propagated in sand tend to have coarse root textures.
Perlite: Originally used as an insulating material, perlite is one of the most widely used materials in propagation media, sometimes as a single component, but more often in mixtures, such as 2:1, 1:1, and 1:2, by volume ratio with sphagnum peat. Perlite is mined from crushed aluminum silicate volcanic rock that expands and extrudes when heated rapidly to 1000°C. Its primary uses are for increasing or maintaining pore spaces and as a lighter substitute for sand. It is sterile, does not decay drains freely, and can be reused if redisinfested. The pH of perlite is usually close to neutral, but many vary between 6.5 and 8.4. A course (horticultural) grade is usually recommended for use in propagation.
Vermiculite: Another heat-treated mineral, vermiculite is mined from a mica-like ore (aluminum-iron-magnesium silicate). When heated to 760°C, it expands into plate-like layers that have a grey-tan color and silvery sheen. It is sterile and light like perlite and has pH usually ranging between 7.0 and 7.5 but can range from 6.0 to 9.5, depending on the processing and the source. It behaves like an organic material, having a high capacity for holding water and nutrients. It contains relatively high amounts of Ca, Mg, and K. It is easily compressed when moistened and does not re-expand. Vermiculite is considered to be a good “seed starter” material and used commonly in germination mixes with peat, bark, and composts.
Rock Wool: Another inert, sterile, heat-treated material, rock wool has a very high total pore space volume of 98% and a high aeration porosity of about 50%. It will not hold water or nutrients in its structure and drains very freely, for this reason; it is an ideal medium for rooting cuttings. It is typically manufactured into various performed shapes, such as cylinders, blocks, and wedges, and prepackaged to be used as a single-component medium. Rock wool is durable, and its properties are reliable and consistent.
VI. Organic Materials
Peat: Found in swampy areas, peat is the partially decomposed residue of mosses, reeds, and sedges. It occurs abundantly in cool climatic areas of Canada, Northern Europe, and Russia. Types, grades, and quality of peat vary considerably. Sphagnum peat is the type preferred for use in propagation media. It contains over 75% of sphagnum sp. (acid bog moss) and a minimum of 90% organic matter. Young sphagnum peat, harvested from the top of bogs, is blond, and has very acidic pH somewhere between 3.5 and 4.0. It is light like perlite, has aeration porosity and about 30%, is relatively free of infesting organisms, and contains little or no nutrients. Older, darker peats are more decomposed, usually denser (lower aeration porosity), and have higher pH values. Sphagnum peat absorbs between 10-20 times its weight in water and decomposes slowly.
Bark and Wood By-Products: A by-product of the lumber industry, pine bark and composted pine bark are used widely in container growing media and are increasingly being used in propagating media by some nurseries. It is relatively light (two to three times the weight of peat), typically low in salts and known to have disease-suppressing properties. Depending on the source, the pH of pine bark varies from 3.8 to 6.5, although sources with values between 5.0-5.5 are more often used. The best materials are screened through ¼ to 3/8 in. (0.6 to 1.0 cm) mesh, and typically have aeration porosity between 30% and 40%. It is a good substitute for peat, but has a lower water-holding capacity. Related by-products, such as sawdust, wood shavings, and wood chips, have been used in the industry, but their use has been more limited. Many of these products decompose while in use and can sometimes deplete the nitrogen in the medium to the detriment of the plants.
Coconut Husk: During the past 10 years, a waste remaining after coconut husks are processed, has made a rapid impact in the floricultural industry and is now commonly used in growing substrates. Some growers claim that it is better than peat as a growing material. It has a very open structure, yet is quite resistant to compaction and slow to decompose. It has a very high water-holding capacity, but diseases are less likely to occur with it than with peat. Depending on the source, the soluble salts content varies widely, as does the pH.
Composts and Organic Wastes: Attention is increasingly being placed on use of composts, leaf molds, coconut husks, paper mill sludges, and other waste-derived materials as media. In the past, there has been little use of composts and related materials in North America. Increasing concern for the environment and a movement toward “reduce, reuse, recycle” may portend a growing trend for their use in propagation. Major deterrents for use of these materials in propagation include the following: inconsistent quality, potential phytotoxicity because of high salt levels or unsatisfactory pH, differences in species response, and lack of scientific information. Examination of various paper mill sludges and composts indicate that these are three to five times heavier, and their porosity and aeration characteristics are comparable to, or better than, those of peat. Thus, from this standpoint, these waste-derived materials are ideal substitutes for peat in propagation media.
I. Introduction
During the past 25 years, much has changed in commercial propagation practices. One of these changes is that, except in special cases, such as the propagation of hardwood cuttings and seeds in field beds, soil is seldom used today. Media with soil do not provide the range of options, such as lightness in weight, ease of handling, and freedom from pathogens, pests, weed seed, and contaminants, which are required for modern propagation practices.
Plants are increasingly propagated from seeds and cuttings using soilless media in high-density containerized systems, such as plug trays or cell packs. The use of containers provides advantages, such as flexibility in time of planting, ease of handling and movement of materials, and adaptation to mechanization. Germination and rooting typically take place in modern computer-controlled greenhouses, polyhouses, or other protected environments that provide the needed moisture, temperature, and light. These technologies allow more efficient, cost effective, and labor-saving ways of propagating plants.
II. Functions of a Medium
While media for sowing seeds are generally finer in texture than those in rooting media, the functional characteristics of both are similar and can be considered together. A propagating medium provides anchorage and support and an appropriate air-moisture balance for metabolic processes.
The ideal propagation medium should be loose or friable enough to maintain adequate amounts of air (one with an oxygen content of at least 12%) for germination and cell division. Oxygen is required for respiration to oxidize starches, fats, and other food reserves in the seed or cutting, and its utilization is proportional to the amount of metabolic activity. Good aeration in the medium also allows removal of carbon dioxide, a by-product of respiration, which may cause damage to seedlings when present in excess.
During propagation, it is necessary to prevent moisture loss from the medium and to maintain a humid atmosphere in the immediate surrounding air. The medium surrounding the seed must remain moist. It should have a high moisture-holding capacity, yet allow excess water to drain freely so that the seeds are not drowned in water. Roots may rot if the medium is poorly drained.
III. Properties of Media
It is often said that there are as many media as there are propagators, but there is no one best medium for all plants or all conditions. Factors, such as species, the time of year, use of growth-promoting substances, or the interaction of these and/or other variables, have a considerable influence on propagation success.
Whether for seeds, cuttings, or transplants, the medium at the start must be at least partially disinfested and free of pathogens, harmful pests, weed seeds, and toxic chemicals. Any problems early on, such as the presence of weed seeds, will require extra labor and cost for removal later. As a rule, the medium should contain little or no nutrients. Nutrients, per se, are of little value until roots are formed and can be added with water as needed. If present in excess, nutrients can cause injury to seedlings.
The important features of a medium can be described technically in terms of physical properties, such as bulk density and pore spaces, and chemical properties, such as electrical conductivity and pH. Knowledge of these properties provides the propagator with objective or easily measured criteria to evaluate and compare different lots or batches of the same mixture, and leads to an increased understanding of media dynamics.
IV. Materials and Mixtures
Most propagation media are mixtures of various materials. While media are sometimes made with three or even four different materials, in most cases, media with equivalent properties can be accomplished using two components. These are easier to prepare and perhaps more predictable in their performance or outcome.
Many different materials have been used in propagation media. These can be classed as inorganic or organic and may be used more or less in the natural state, manufactured, or blended. Inorganic materials include the following: sand, perlite, vermiculite, polystyrene, and rock wool (mineral wool). Organic materials include peat, bark, wood by-products, and composts, although many organic or farm-derived waste materials have been used on a more limited scale. As a rule, the mineral component is used to improve drainage and aeration by increasing the proportions of large air-filled pore space.
V. Inorganic Materials
Sand: The use of sand has declined dramatically during the past 25 years, and it has been largely replaced by lighter or more easily-handled materials such as perlite, or by commercial premix. A medium grade (0.02 to 0.8 in; 0/5 to 2 mm) of washed, sharp builder’s sand is recommended, since finer sand reduces pore space and drainage. Sand contains little or no nutrients and is used most often in mixtures with peat, perlite, and/or other organic components, and sometimes as a single-component medium. Seeds or cuttings propagated in sand tend to have coarse root textures.
Perlite: Originally used as an insulating material, perlite is one of the most widely used materials in propagation media, sometimes as a single component, but more often in mixtures, such as 2:1, 1:1, and 1:2, by volume ratio with sphagnum peat. Perlite is mined from crushed aluminum silicate volcanic rock that expands and extrudes when heated rapidly to 1000°C. Its primary uses are for increasing or maintaining pore spaces and as a lighter substitute for sand. It is sterile, does not decay drains freely, and can be reused if redisinfested. The pH of perlite is usually close to neutral, but many vary between 6.5 and 8.4. A course (horticultural) grade is usually recommended for use in propagation.
Vermiculite: Another heat-treated mineral, vermiculite is mined from a mica-like ore (aluminum-iron-magnesium silicate). When heated to 760°C, it expands into plate-like layers that have a grey-tan color and silvery sheen. It is sterile and light like perlite and has pH usually ranging between 7.0 and 7.5 but can range from 6.0 to 9.5, depending on the processing and the source. It behaves like an organic material, having a high capacity for holding water and nutrients. It contains relatively high amounts of Ca, Mg, and K. It is easily compressed when moistened and does not re-expand. Vermiculite is considered to be a good “seed starter” material and used commonly in germination mixes with peat, bark, and composts.
Rock Wool: Another inert, sterile, heat-treated material, rock wool has a very high total pore space volume of 98% and a high aeration porosity of about 50%. It will not hold water or nutrients in its structure and drains very freely, for this reason; it is an ideal medium for rooting cuttings. It is typically manufactured into various performed shapes, such as cylinders, blocks, and wedges, and prepackaged to be used as a single-component medium. Rock wool is durable, and its properties are reliable and consistent.
VI. Organic Materials
Peat: Found in swampy areas, peat is the partially decomposed residue of mosses, reeds, and sedges. It occurs abundantly in cool climatic areas of Canada, Northern Europe, and Russia. Types, grades, and quality of peat vary considerably. Sphagnum peat is the type preferred for use in propagation media. It contains over 75% of sphagnum sp. (acid bog moss) and a minimum of 90% organic matter. Young sphagnum peat, harvested from the top of bogs, is blond, and has very acidic pH somewhere between 3.5 and 4.0. It is light like perlite, has aeration porosity and about 30%, is relatively free of infesting organisms, and contains little or no nutrients. Older, darker peats are more decomposed, usually denser (lower aeration porosity), and have higher pH values. Sphagnum peat absorbs between 10-20 times its weight in water and decomposes slowly.
Bark and Wood By-Products: A by-product of the lumber industry, pine bark and composted pine bark are used widely in container growing media and are increasingly being used in propagating media by some nurseries. It is relatively light (two to three times the weight of peat), typically low in salts and known to have disease-suppressing properties. Depending on the source, the pH of pine bark varies from 3.8 to 6.5, although sources with values between 5.0-5.5 are more often used. The best materials are screened through ¼ to 3/8 in. (0.6 to 1.0 cm) mesh, and typically have aeration porosity between 30% and 40%. It is a good substitute for peat, but has a lower water-holding capacity. Related by-products, such as sawdust, wood shavings, and wood chips, have been used in the industry, but their use has been more limited. Many of these products decompose while in use and can sometimes deplete the nitrogen in the medium to the detriment of the plants.
Coconut Husk: During the past 10 years, a waste remaining after coconut husks are processed, has made a rapid impact in the floricultural industry and is now commonly used in growing substrates. Some growers claim that it is better than peat as a growing material. It has a very open structure, yet is quite resistant to compaction and slow to decompose. It has a very high water-holding capacity, but diseases are less likely to occur with it than with peat. Depending on the source, the soluble salts content varies widely, as does the pH.
Composts and Organic Wastes: Attention is increasingly being placed on use of composts, leaf molds, coconut husks, paper mill sludges, and other waste-derived materials as media. In the past, there has been little use of composts and related materials in North America. Increasing concern for the environment and a movement toward “reduce, reuse, recycle” may portend a growing trend for their use in propagation. Major deterrents for use of these materials in propagation include the following: inconsistent quality, potential phytotoxicity because of high salt levels or unsatisfactory pH, differences in species response, and lack of scientific information. Examination of various paper mill sludges and composts indicate that these are three to five times heavier, and their porosity and aeration characteristics are comparable to, or better than, those of peat. Thus, from this standpoint, these waste-derived materials are ideal substitutes for peat in propagation media.
