Growing Media

The production of greenhouse crops involves a number of cultural inputs. Among these, perhaps the most important is the type of growing medium used. Due to the relatively shallow depth and limited volume of a container, growing media must be amended to provide the appropriate physical and chemical properties necessary for plant growth.

Field soils are generally unsatisfactory for the production of plants in containers. This is primarily because soils do not provide the aeration, drainage and water holding capacity required. To improve this situation several “soilless” growing media have been developed. The following is a description of some of the most commonly used amendments for the production of greenhouse crops.

Peat and Peat-Like Materials

Peat moss is formed by the accumulation of plant materials in poorly drained areas. The type of plant material and degree of decomposition largely determine its value for use in a growing medium. Although the composition of different peat deposits vary widely, four distinct categories may be identified:

Hypnaceous 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.

Wood Residues

Wood residues constitute a significant source of soilless growing media. These materials are generally bi-products of the lumber industry and are readily available in large quantities. Nitrogen depletion by soil microorganisms, during the decomposition process, is one of the primary problems associated with these materials. However, supplemental applications of N to the growing media can make most wood residues valuable amendments.

Leaf Mold – 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.

Sawdust – the species of tree from which sawdust is derived largely determines its quality and value for use in a growing media. Several sawdusts, such as walnut and non-composted redwood, are known to have direct phytotoxic effects. However, the C:N of sawdust is such that it is not readily decomposed. The high cellulose and lignin content along with insufficient N supplies creates depletion problems which can severely restrict plant growth. However supplemental appli-cations of nitrogen can reduce this problem.

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.

Bagasse

Bagasse is a waste bi-product of the sugar industry. It may be shredded and/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, (usually transportation), its use is limited.

Rice Hulls

Rice hulls are a biproduct of the rice milling industry. Although they are extremely light in weight, rice hulls are very effective at improving drainage. The particle size and resistance to decomposition of rice hulls and sawdust are very similar. However N depletion is not as serious of a problem in media amended with rice hulls.

Several other organic materials are suitable for use with container media. Included are: manures; corn cobs; straw; peanut and pecan shells. However these do not constitute major commercial sources of organic amendments.

Sand

Sand, a basic component of soil, ranges in particle size from 0.05mm to 2.0mm in diameter. Fine sands (0.05mm – 0.25mm) do little to improve the physical properties of a growing media and may result in reduced drainage and aeration. Medium and coarse sand particles are those which provide optimum adjustments in media texture. Although sand is generally the least expensive of all inorganic amendments it is also the heaviest. This may result in prohibitive transportation costs. Sand is a valuable amendment for both potting and propagation media.

Perlite

Perlite is a silicous mineral of volcanic origin. The grades used in container media are first crushed and then heated until the vaporization of combined water expands it to a light powdery substance. Lightness and uniformity make perlite very useful for increasing aeration and drainage.

Perlite is very dusty when dry and has a tendency to float to the top of a container during irrigation. It has also been shown that perlite contains potentially toxic levels of fluorine. Although costs are moderate, perlite is an effective amendment for growing media.

Vermiculite

Vermiculite is a micacious mineral produced by heating to approximately 745oC. The expanded, plate-like particles which are formed have a very high water holding capacity and aid in aeration and drainage. Vermiculite has excellent ex-change and buffering capacities as well as the ability to supply potassium and magnesium. Although vermiculite is less du-rable than sand and perlite, its chemical and physical properties are very desirable for container media.

Calcined Clays

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.

Expanded Polystyrene

Polystyrene flakes, a bi-product of polystyrene processing, are highly resistant to decomposition, increase aeration and drainage,and decrease bulk density. Polystyrene may be broken down by high temperatures and by certain chemical disinfecting agents.

Urea Formaldehydes

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.

Preparing Soilless Growing Media

Although amendment combinations may vary, basic objectives in the preparation of a growing media are alike. An effective program should produce a growing media that is:

  1. porus and well drained, yet retentive of sufficient moisture to meet the water requirements of plants between irrigations;
  2. relatively low in soluble salts, but with an adequate exchange capacity to retain and supply the elements necessary for plant growth;
  3. standardized and uniform with each batch to permit the use of standardized fertilization and irrigation programs for each successive crop;
  4. free from harmful soil pests; pathogenic organisms, soil insects, nematodes and weed seeds
  5. biologically and chemically stable following pasteurization; primarily free from organic matter that releases ammonia when it is subjected to heat or chemical treatments.

Since innumerable amendment combinations can produce a growing medium with these characteristics, it is important to consider both the economic as well as cultural optimums. Factors that determine the cost of a growing medium include: transportation, labor, equipment, materials and handling. In many cases the cost of mixing a “custom” growing medium exceeds that of the commercially prepared materials. These factors should be studied carefully before making a decision.

Recommended Growing Media

The composition of a growing medium should be largely determined by the crop being produced. However there are some media formulations which may be used as a base. The following is a list of several of the most commonly used soilless mixtures:

Table 1. Commonly used soilless mixtures for greenhouse crops.
Volume/Volume Ratio Components
2:1 Peat, Perlite1
2:1:1 Peat, Perlite, Vermiculite
2:1 Peat, Sand 3:1 Peat, Sand
3:1:1 Peat, Perlite, Vermiculite
2:1:1 Peat, Bark, Sand
2:1:1 Peat, Bark, Perlite
3:1:1 Peat, Bark, Sand
1. Foam beads may be used in place of perlite.

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