Know your components

Successful production of container-grown plants is largely dependent on the chemical and physical properties of the growing media. Therefore, it’s critical to carefully choose the best components that fit your needs. Below are some organic and inorganic amendments and their properties.

Organic amendments

Peat

Peat is a common component in both nursery and greenhouse mixes. Peat is usually included in a mix to increase the water-holding capacity or to decrease the weight. Peats used in horticulture are usually classified into three types: moss peat, reed-sedge and peat humus. Moss peat, more often called peat moss, is the most common form used in the industry and is derived mostly from sphagnum moss.

Peat moss is the least decomposed form of the peat types. It’s lightweight, high in moisture holding capacity and very acid (pH 3.8 to 4.3). A significant problem with peat moss is “wetting up.” Peat moss is inherently hydrophobic (repels water). To address this situation, some suppliers offer a product with a wetting agent already included. Before using peat moss, you should conduct a simple test to see how difficult the product will be to wet. If the product does not include a wetting agent, you can incorporate one or use hot water to speed up the wetting process. Most recipes call for peat moss on a volume basis (e.g. 50% peat moss:50% perlite vol:vol). Reed-sedge peat is formed principally from reeds, sedges, marsh grasses, cattails and other associated swamp plants. Peat humus is usually derived from reed-sedge or hypnum moss peat and represents an advanced stage of decomposition. This type of peat is usually dark brown to black and has a low moisture-retention capacity.

Coir (coconut fiber)

Coir is primarily used in the greenhouse industry. It's derived from the husk of the coconut fruit and originates from several countries including Sri Lanka, India, Philippines, Mexico and Costa Rica. Because it originates from such diverse geographic locations it is difficult to characterize specific chemical and physical properties. Pay attention to the total soluble salts (electrical conductivity) and sodium and chloride levels in any coir product. The typical pH range for coir is 5.5 to 6.8. It contains significant amounts of phosphorus (6 to 60 ppm) and potassium (170 to 600 ppm), and can hold up to nine times its weight in water. Since coir contains more lignin and less cellulose than peat, it is more resistant to microbial breakdown and therefore may shrink less. Coir is easier to re-wet after drying than peat moss.

Softwood bark

Bark is the primary component (80% to 100% by volume) in most outdoor container nursery mixes. For many years bark was viewed as a forest waste product but today the availability for container use is limited in some markets due to alternative demands (e.g. landscape mulch fuel) and reduced timber production. Pine bark is preferred over hardwood bark since it resists decomposition and contains fewer leachable organic acids. Pine bark is usually stripped from the trees milled and then screened into various sizes. A good potting medium usually consists of 70% to 80% (by volume) of the particles in the ¹/42- to ³/8-inch range with the remaining particles less than ¹/42 inch. Bark is described as either fresh, aged or composted. Many growers use fresh bark but typically add 1 lb N/yd3 to compensate for the potential nitrogen draft that occurs in the pot. Composting bark involves moistening the bark, adding 1 to 2 pounds N/yd3 from either calcium nitrate or ammonium nitrate, forming a pile and then turning the pile every 2 to 4 weeks to ensure proper aeration. Composting bark typically takes 5 to 7 weeks. Aging is a cheaper process, but aged bark has less humus and a greater nitrogen draw-down in the container than composted bark.

Hardwood bark

The chemical properties of hardwood bark are significantly different from pine bark. The pH of fresh hardwood bark is usually less acid (pH 5 to 5.5) than peat moss or pine bark. Composted bark may be rather alkaline (pH 7 to 8.5). Hardwood bark typically contains toxic compounds, and for this reason should be composted before use. According to the University of Illinois, for each cubic yard of bark, a grower should add 2 to 3 pounds of actual nitrogen, 5 pounds of superphosphate, 1 pound of elemental sulfur and 1 pound of iron sulfate. These amendments should be blended into the bark and some water added to the blended pile. Turning the pile three to five times during the 60-day process is recommended to get a uniform product. The temperature in the pile should approach 150° F to eliminate most pathogens.

Wood substrates

Wood-based amendments have been successfully tested as a renewable alternative for pine bark in the nursery industry and peat moss and perlite in the greenhouse industry. Research on the nursery side has focused on ‘debarked loblolly pine logs’ (majority wood; sometimes referred to as ‘pine tree substrates’ PTS) and ‘whole-tree’ (containing all shoot proportions of the tree and thus consisting of approximately 80% wood fiber; sometimes referred to as ‘wholetree substrates’ WT and ‘clean chip residual’ CCR) processed into container substrates. Many studies have shown that the growth of numerous woody and herbaceous plants using wood-based substrates is comparable to 100% pine bark (PB). However, the percent of wood product, particle sizing, fertilizer rate and type of crop (i.e. short or long term) will need to be strongly considered to achieve favorable results. Research has shown that when pine wood chips are used as a substitute for perlite at rates up to 30% in a peat substrate, there is no need to change cultural production practices. No differences were found in nitrogen use liming requirement or plant growth regulator efficacy. These results should not be applied or assumed for other wood components. Current efforts are centered on using wood fiber as an amendment to peat moss in the range of 10%-40%.

Compost and animal manures

A large variety of compost or animal manure products is available in the marketplace. Disadvantages of manures include possible high salts, fine particle size and weed seeds. The advantages include the nutrient contribution and potential improvement in media physical properties. A primary consideration when evaluating a sludge is the potential for elevated heavy metals including cadmium, lead, zinc, copper and mercury. In some areas, plant-based compost provides a low-cost media amendment. Consider the availability and consistency of the product and the particle size. Particle sizes for plant-based compost can be either too large or too fine depending on the source material and composting process.

Inorganic amendments

Perlite

Perlite is most commonly used as a component in greenhouse growing media or nursery propagation applications. It is produced by heating igneous rock under high temperatures (1,100 to 1,600° F). Perlite differs from vermiculite in that the finished product is a “closed cell” that does not absorb or hold water. For this reason, it is usually included in a mix to improve the drainage or increase the percentage of aeration. Perlite is lightweight (6 to 8 lb/ft3) chemically inert pH neutral sterile and odorless.

Vermiculite

Vermiculite is similar to perlite in that they both originate as mined minerals that are heated to produce a finished product. Perlite and vermiculite differ in the rationale for including them in a mix. Perlite is usually included in a mix to increase drainage, but does not increase the retention of nutrients. In contrast, vermiculite (with its plate-like structure) holds large quantities of water and positive-charged nutrients like potassium, magnesium and calcium. Vermiculite is sterile and lightweight (5 to 8 lbs/ft3). The pH of vermiculite will vary depending on where it is mined. Most U.S. sources are neutral to slightly alkaline, whereas vermiculite from Africa can be quite alkaline. It is usually sold in four size grades: #1 is the coarsest and #4 the smallest. The finer grades are used extensively for seed germination or to topdress seed flats. Expanded vermiculite should not be pressed or compacted, especially when wet as this will destroy the desirable physical properties. Vermiculite has been the focus of news attention several times over the past 30 years with regard to the issue of potential contamination with asbestos related fibers from a related mica mineral called tremolite. Because of this concern and attention most mines monitor this issue closely to avoid problems with commercially available product. As is the case with dry peat moss, handlers should wear a dust mask to avoid inhaling these materials.

Source: James Robbins, University of Arkansas