- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6400) Unit Title: HORTICULTURAL SOILS & PLANTING MEDIA ____________________________________________________________________________ (CLF6402) Topic: HORTICULTURAL SOILS Time Year(s) 2 hours 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (G-2) - Describe the soil texture considered ideal for gardening. - Cite four characteristics which can distinguish soils used in ornamental horticulture from soils for general agriculture. - List eight materials used to amend horticultural soils. Special Materials and Equipment: Shallow bowls, sponges, a wide sampling of inorganic and organic soil amendments References: Ball, Vic. (Ed.). (1985). BALL RED BOOK (14th ed.). Reston, VA: Reston Publishing Co. Boodley, James W. (1981). THE COMMERCIAL GREENHOUSE. Albany, NY: Delmar Publishers. Cooper, Elmer L. (1990). AGRISCIENCE: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Hausenbuiller, R. L. (1972). SOIL SCIENCE: PRINCIPLES AND PRACTICES. Dubuque, IA: Wm. C. Brown Company. Editors of Sunset Books and Sunset Magazine. (1988). WESTERN GARDEN BOOK. Menlo Park, CA: Lane Publishing Co. Evaluation: Unit Exam ===================================================================== *** INSTRUCTORS PLEASE NOTE *** The detail of this topic presentation goes beyond the scope required to meet the requirements of the Core Cluster in this area. It will take longer to teach than indicated above if covered entirely. It is included for use as local enrichment as appropriate for the class. ===================================================================== TOPIC PRESENTATION: HORTICULTURAL SOILS A. Soil and Horticulture: The management of soils for gardening and ornamental horticulture is, on the whole, more intensive than that for agriculture in general. 1. Especially in the area of nursery production and the production of high-value specialty plants, the value of the plants justifies higher growing costs. Growers can afford to spend money on, among other things, relatively expensive soil amendments to attain better and faster results. 2. In the landscape, the high value of plants in relation to their size and placement (or their recreational and scenic value, as with turf), justifies the extra inputs necessary to provide and maintain a soil environment conducive to plant vigor. B. The Functions of Soil: Horticultural soils have the same functions as general soils, with a few points emphasized: 1. Support: Besides providing support, the soil for many nursery products is part of the packaging and marketing material. Keeping soils light is important for this reason. 2. Nutrients: Most horticultural products are grown with very high levels of nutrient added from sources other than the soil (fertilizers). A fertile soil is desirable, especially in a landscape situation, but not imperative. Still, soil conditions must be correct for effective nutrient exchange (pH, CEC, etc.). 3. Moisture: Water must be able to move in and out of horticultural soils with ease. 4. Air: Air is extremely important in horticultural soils, especially where fast growth (from high rates of respiration) is desired. C. Composition of Soils: The composition of soils used in ornamental horticulture varies tremendously, from the "average" mineral soil described in the last lesson to "soilless media" composed entirely of amendments and with an 80%+ total porosity. Following are some of the components found in horticultural soils. (NOTE: While we will speak in this lesson of horticultural "soils," this is not really the most accurate term. In horticulture, and especially in nursery production, we often employ "growing media" which have little or no actual soil content. Natural soil, which contains minerals such as sand, silt, and clay, is absent in these mixes which are made entirely of various soil amendments.) 1. Mineral: The mineral proportion of a horticultural soil is often less than for an agricultural soil, especially in intensive container production, or where shipping of plants necessitates a lightweight medium. a. Mineral Soil: While finding less use in container production recently, soil is still the mainstay of most nursery and greenhouse production. Of course, it is also the main component of field soils. It is listed here as a mineral component because the sand, silt, and clay in mineral soil are so influential in a horticultural soil. Some points on mineral soil: 1) High value or fast-growing horticultural crops should be field grown only where the native soil is of a good quality, such as a loam or sandy loam soil. Otherwise, expect to pay high soil management costs. 2) Only high-quality, loamy topsoils should be used in container production. These should be free of herbicide residues or other contaminants. 3) It is necessary to pasteurize or fumigate field soils used for nursery production because soil can harbor fungus organisms harmful to many species of plants. This is also necessary in the case of some landscape plantings, though rarely. 4) Soil is a fine-textured material which usually requires coarse textured amendments to make it suitable for container crops. 5) Soil is a heavy material, especially after irrigation. This must be considered if crops planted in soil-heavy mediums are to be shipped. b. Amendments: These mineral (inorganic) materials are for use as amendments, primarily in the production of container plants and in special landscape situations such as planter boxes. Their use in the field, except for the last two, is limited. 1) Sand: Sand is an inexpensive and readily available material to supply a fast-draining substrate for horticultural mixes. a) Sand has the same chemical characteristics as the parent material it came from. b) The best sands for horticultural purposes is are uniform sands, i.e., sands composed mainly of one particle size. Coarse or medium sands are best. c) Be careful about sand source. Some river sands can be contaminated with herbicide residues from agricultural run-off. d) Sand is a heavy material. e) WARNING! Do not add sand to a clay soil in the field or landscape thinking it will improve drainage. Unless used in very high quantities (up to 80% of the mineral content), sand will only serve to compact the finer silts and clays. Adding sand to a clay soil usually results in a combination which behaves more like concrete than like soil! 2) Pumice: This is lighter in weight than sand or gravel and has a porous texture which enables it to absorb water in addition to improving drainage and aeration. These desirable characteristics justify a higher cost. It is useful for large-volume container mixes. 3) Perlite: This extremely lightweight material is mined and expanded through baking, and then packaged for horticultural use. Perlite is: a) Superb for improving both water retention (it is porous and absorbent) and aeration (it has a coarse texture). b) Neutral in pH and has no nutrients. c) Sterile as received from the manufacturer, and needs no pasteurization or fumigation. d) Long-lasting. e) Relatively expensive: For greenhouse use, primarily. f) Difficult to handle because of its light weight and the dust it creates. g) Don't use perlite in general landscape plantings. The lightweight particles float away, "pool," and look funny. 4) Vermiculite: Mined and baked like perlite, vermiculite is used to improve water retention in prepared mixes. a) Holds about 500 times its weight in water. b) Improves cation exchange capacity, and helps buffer soils against pH change. c) Supplies some potassium and magnesium to plants. d) Breaks down with tillage. Don't use it in field plantings; it is for greenhouse and container use only. e) Use horticultural grades only. Like perlite, vermiculite is relatively expensive. 5) Styrofoam: Used as a perlite substitute, but not water absorbent. Not recommended due to refusal to break down, floating/blowing problems, and the environmental impact of manufacture. 6) Lime: Added to adjust pH upwards and to neutralize acid media. Can be effective and economical on a large scale, as in a field planting. 7) Gypsum: Used to counter high levels of sodium (Na+) in sodic soils and to improve tilth and water movement in heavy clay soils. 2. Air: Especially in nursery situations and where plants are grown in containers, adequate air porosity is essential. a. In high-value operations, with costly facilities such as greenhouses to amortize, time and speed of production is important. Therefore, well-drained media having at least 10% air porosity should be used. b. High air content is attained through heavy use of coarse amendments, both organic and inorganic in nature. c. In landscape situations, aeration operations help to keep soils well aerated in the root zone of plants. 3. Water: Water porosity (water content) of horticultural soils varies tremendously but should be at least 50%. a. Soil amendments improve water (and nutrient) holding capacity. b. Permeability to water is high in horticultural soils resulting in fast drainage which means rapid air/water exchange. c. Properly amended soil will hold moisture in ways that make water more available to plants, i.e., well-prepared horticultural soils will have a high Field Capacity and a low Permanent Wilting Percentage 4. Organic Material: Organic material is present at relatively high percentages in most horticultural soils, and in the case of much nursery container production, can be the primary component of a soil mix. When added to a field soil, organic amendments should be used in large quantities, to constitute at least 25% of the soil volume. Small additions of organic matter are ineffectual. Humus, the colloidal breakdown product of organic material, is highly desirable in soils as it considerably improves moisture retention and cation exchange capacity. Augmenting humus formation is one of the important reasons for adding organic materials to the soil. Others include improving texture, and improving soil nutrition as well. A wide variety of organic soil amendments are available for incorporation into soils. Some examples follow: a. Nitrified Redwood Sawdust: An economical, widely-used material for nursery and landscape applications. Improves both water retention and drainage/aeration. 1) Available in coarse and fine grades; bulk or in bales. 2) Material requires no sterilization. 3) Breaks down slowly; long lasting in the soil. 4) Be sure sawdust has been nitrified, or nitrogen tie-up will occur (described below). Also, material improves if given time to decompose to some degree. Clues that material is adequately treated: a) Dark, rich color b) "Ammonia" smell c) Material may be steaming or feel warm to the touch due to microbial activity. 5) Inexpensive, compared to peat moss and most other materials. b. Peat moss: More correctly "moss peat," this soil amendment has been a mainstay of the horticulture industry since its beginnings. Some soilless mixes are based completely on peat moss, with a few other amendments added for a balance of qualities. Most often, it is used as an amendment. Important points on the use of peat moss: 1) Also called sphagnum, peat moss is the partially decomposed remains of dead plant materials that built up in bogs over a period of hundreds or even thousands of years. 2) Peat moss is imported from Canada and Europe, and is handled in (usually) 4 cu. ft. bales. These expand to 6 cu. ft. 3) Peat moss varies in quality, but is porous (around 90%+ air space by volume!) and very water retentive (holds about 15 times its weight in water). 4) The pH of peat moss is low, between 4.0 and 4.5. This must be adjusted for in peat-based mixes by the use of lime. 5) Peat moss is free of weeds and other harmful organisms as received from the shipper. 6) Dry peat moss repels water. It should be pre-wet or treated with a surfactant before being used to containerize plant material. 7) Once moistened, peat moss will absorb water readily, but if allowed to dry out completely it will be difficult to re-wet. 8) Peat moss has little nutrient value for plants. 9) Peat moss is expensive and used primarily for greenhouse and nursery crops. It has limited use in the landscape. c. Manures: Often mixed with bedding materials, manures provide nutrient-rich amendments for horticulture, improving soil structure, moisture retention, and nutrition status. Some care must be taken in their use, however: 1) They must be well decomposed or risk "burning" plant roots. 2) Overused, they can introduce too many soluble salts into the soil. 3) They should be pasteurized or fumigated to eliminate weed seeds and pathogens. 4) Availability is often variable. d. Sludge: Processed sewage material can be mixed with other organics such as wood chips and ground bark, decomposed, and used as a nutrient-rich, high quality amendment. 1) Higher cost material than, for instance, redwood sawdust but more nutrient dense. 2) Not approved for some food crops; this is not a concern for ornamental use. e. Ground Fir Bark: Widely used as a primary component in soilless mixes, and as an amendment. 1) Available in many grades from almost powdery (for use as a substrate in soilless mixes), to very coarse (for use as a medium to grow epiphytes). 2) Low nutrient value; requires the addition of nitrogen. 3) Inexpensive. f. Leaf Mold: Decomposed leaves. Excellent textural qualities, but of variable quality and availability. Oak leaf mold is the best. g. Rice Hulls: Uniform, long-lasting amendment to speed drainage and improve soil structure. Relatively inexpensive when available. Not available in all areas. h. Cocoa Bean Hulls: Similar characteristics to rice hulls. Low in nutrient value, pleasant smelling. NOTE: When adding organic materials (composed largely of carbon) to the soil it is important to insure that there are enough nutrients present (especially nitrogen) to satisfy the needs of both the plants and the microorganisms that are present in the soil. A carbon-nitrogen imbalance will mean a lack of available nitrogen for plants and will result in poor growth and chlorosis. __________________________________________________________ ACTIVITY: 1. Have students collect samples of a variety of soil amendments, learning the name and uses of each, and whether they are of organic or inorganic origin. 2. Using their collections for review, students should be able, presented with an array of soil amendments, to recognize and name the various materials. __________________________________________________________ 5. Other Components: In ornamental horticulture materials are sometimes added to soil to improve soil-water relationships, or other management considerations. These are usually added in small quantities, composing a very small percentage of soil composition. Examples follow: a. Surfactants: These break down resistance to water absorption, improving water infiltration and reducing runoff. b. Polymer gels: These materials hold many hundred times their weight in water, and are used to improve water retention in a mix. They are used to reduce frequency of irrigation. D. Textural Characteristics 1. Field Soils: As mentioned above, the field production of ornamentals requires a high quality, loamy soil. a. Sandy loams are preferred because of their ease of tillage and fast-draining qualities. These are desirable for high volume production. b. Soils for landscaping are often of poor quality to begin with, but can be amended to compensate for poor texture if necessary. c. Organic amendments (such as nitrified redwood sawdust), added in sufficient quantity, will help clay soils drain more effectively and sandy soils retain moisture longer. 2. Container Soils: Soils to be used in containers must have a very coarse texture. a. Because containers are shallow (there is no "reservoir" of soil below the root zone in a container to help draw moisture down through the soil), water tends to "cling" to the soil. b. This must be compensated for by formulating container mixes with a high degree of coarse texture or coarse pores. c. The use of straight mineral soils from the field, even very sandy loams, is out of the question. Coarse amendments such as peat moss, fir bark, perlite, and pumice must be used to improve drainage and aeration. d. Materials used to amend the texture of container soils should be as light as possible if shipping is a concern. __________________________________________________________ ACTIVITY: 1. To demonstrate how soil depth affects drainage, have students do the following exercise with a sponge and water. The sponge acts much like soil, allowing water to be absorbed and then drain out. 2. Soak a rectangular-shaped sponge in a bowl of water. 3. Without squeezing, bring the sponge out of the water with the flat side of the sponge parallel to the table. Water will drain out, then stop. The sponge in this position represents a shallow soil, holding onto a large proportion of water. 4. Now, again without squeezing, turn the sponge up so it is turned up on its end in relation to the table. Water will start to flow again from the bottom of the sponge. The "deeper soil" (which has not changed volume) holds less water. 5. Notice that the top layer of the sponge (analogous to the root zone in a field planting) is drier than the lower part of the sponge, (analogous to the saturated lower horizons where roots don't grow). 6. Discuss with students the difference between a soil in the field, and same soil in a shallow container, and the implications for soil preparation and irrigation. __________________________________________________________ E. Chemical Characteristics: The chemical characteristics of horticultural soils require monitoring. The use of a wide variety of soil amendments can affect factors like pH and salinity. The low percentage or absence of mineral soils can affect cation exchange capacity and nutrient availability. Routine testing and correction of poor chemical conditions is essential for productive horticultural soils. 1. pH: This can be fine-tuned for the particular crop if necessary, but, in general, ornamentals prefer a medium with a slightly acid pH, 5.0 to 7.5. a. pH is easily tested for (with a pH meter--forget the litmus paper!) and should be monitored ON A REGULAR BASIS in the nursery. pH can change as the materials used to mix soil change. b. In the landscape pH should be adjusted for at installation, and checked occasionally, especially in heavily fertilized locations such as turf. (Fertilizers can alter pH over time.) 2. Nutrient balance: In production horticulture, less fertility is provided by the growing medium (especially in soilless mixes) and there is a greater reliance on fertilizers. Soil should be tested on a regular basis to determine which nutrients need replacing with fertilizers. 3. Salinity: This is a critical characteristic of horticultural soils. a. Many nursery crops are very sensitive to salts in the soil, and will not grow under salty conditions. b. Nursery crops and landscapes are highly fertilized which poses the danger of salt build-up. c. Irrigation water can contain salts, which, if not managed correctly, can accumulate in soil. d. Salts can be tested for easily with an electrical-conductivity (EC) meter. Salt levels are expressed in millimhos per centimeter or mmho/cm. e. Soil with an EC reading of 5 mmho/cm or more can stunt plant growth. Flushing soils with clear water can reduce salts. F. Special Considerations: With horticultural soils there is less concern with the soil formation than with the soil formulation. Because we much can be done to alter the soil composition and chemistry, it is important that several things be remembered besides how the soil affects plant growth: 1. The cost of the ingredients. The use or application of the medium must justify its costs of formulation. 2. Reproducibility: If a particular soil mix is being depended on to produce certain plants from season to season, it is important that the grower is able to reproduce the mix used. Use consistent preparation methods, use the same suppliers, and follow the recipe exactly. 3. Availability: The materials used for mixing or amending soil must be available. Not all amendments are available at all times. 12/13/90 MH/sg #%&C