Thane Hill

ESRM 412 – Spring 2005

Plant Growth Media and fertilizer in the Nursery

Choice of plant growth media and fertilizer are important when propagating plants form seed or cuttings in containers.

Media

There are many disadvantages to using garden soil (loam) in potting mixes. Garden soil is heavy, inconsistent in quality and not uniform, and not sterile. Various other materials may be mixed instead to create “soilless” media to be used in container propagation. Good mixes all share some common characteristics.

Properties of good potting mixes:

• Sufficiently firm and dense enough to keep cuttings or seeds in place.
• Able to maintain moisture (to avoid frequent watering)
• Sufficiently porous to allow good drainage and aeration
• Free from pest, weeds, and pathogens
• Low salinity
• High cation exchange capacity (for retention of nutrients and fertilizers)
• Uniform and consistent in quality
• Economical and readily available (1)

“Soilless” media are composed of an organic component (sphagnum peat moss, bark or wood chips) and a coarse mineral component (sand, pumice, perlite, or vermiculite) which are lightweight and help improve drainage and aeration.

Sphagnum Peat Moss

Sphagnum peat moss is the most commonly used soilless medium.It is widely available and relatively inexpensive. (3) Sphagnum peat is formed from sphagnum mosses in very acid bog conditions which preserve most of the plant fiber structure. (4) It is lightweight, relatively pathogen free, may contain anti-fungal properties, and has a high water holding capacity.

Pine Bark

Composted pine bark may be substituted, in part, for peat moss. (3) Bark particles have a relative high cation exchange capacity, while most particles have internal water-holding capacity. The large moisture content of fresh bark makes it heavy. Once bark dries below 35 percent of its total water-holding capacity, it becomes difficult to rewet. (4)

Sand

Sand is the least expensive and most readily available large particle material. (3) Quartz sand is most often used in propagation and should be sterilized by fumigation or steam-pasteurized before use. (1) Sand is the heaviest ingredient used in potting mixes which is good for top-heavy plants that might blow or tip over, but bad for plants that will be shipped or moved a lot. (3)

Pumice

Pumice is a very lightweight volcanic rock that is used sometimes to increase aeration and drainage in potting mixes. It is sometimes used in field plantings or in container cuttings. Pumice has a fair water holding capacity. It is not heat treated and so is not sterile. (1)

Perlite

Perlite is a volcanic rock that is heated and expanded to become a lightweight material. Because it is heated to 1400-1800F, it is sterile. Perlite improves drainage and aeration by creating tiny air tunnels that allow water and air to flow freely to the roots. Perlite can hold 3-4 times its weight in water, yet will not become soggy. (3,4)

Vermiculite

Vermiculite is a mineral that is heated and expanded to become a lightweight material. Because it is heated to 1400-1800F, it is sterile. One of the major shortcomings of vermiculite is its poor physical stability after wetting. If not handled properly, vermiculite compacts and loses its ability to hold air. Vermiculite holds water and fertilizer and contains calcium and magnesium. (3,4)

Compost

Compost is made from a variety of locally available materials, usually a combination of ingredients that are high in nitrogen and carbon. Compost holds water well and provides nutrient. Compost can also provide natural protection against diseases of the seedlings and roots of plants due to beneficial organisms that live in well-made compost. (3)

Table 1. Comparison of Potting mix materials
Material	Cation Exchange	pH values	Sterile (y/n)	Water Holding Capacity	Weight	Cost
Soil	Poor	4.5-6.0	No	Poor	Heavy	Low
Sphagnum peat moss	Fair	3.5-4.0	Yes	High	Lightweight	Medium
Bark	High	4.0-5.0	No	Medium	Medium to Heavy	Low
Sand	Poor	4.5-6.0	Yes	Poor	Heavy	Low
Pumice	Poor to Fair	4.5-6.0	No	Medium	Very Lightweight	High
Perlite	None	6.0-8.0	Yes	Poor	Very Lightweight	High
Vermiculite	High	4.0-5.0	Yes	Good	Very Lightweight	High
Compost	High	4.5-6.0	No	good	Medium	Low

Fertilizers

Container mixes require fertilizer supplements until the seedlings or cuttings become established because of the excessive draining of water and nutrients. Pre-plant fertilizers (gypsum, limestone and microelements) can be added to the soil during the mixing process. Post-plant fertilizers (nitrogen, phosphorus, potassium) can be added to the soil later in liquid form through irrigation or as top dressing of slow-release fertilizer.

Slow-release fertilizers come in three forms:

1) Coated water-soluble pellets or granules,

2) Inorganic materials that are slowly soluble, and

3) Organic materials of low solubility that gradually decompose by biological breakdown. (1)

Gypsum

Gypsum is a mineral soil conditioner consisting primarily of calcium-sulfate and ground finely. It aids in the reclamation of soils high in soluble salts and neutralizes the soils P.H. by leaching of these salts. Direct applications will alleviate and improve water penetration within the soil structure to quickly stimulate root growth. (8) Fairly inexpensive, easy to apply source of calcium and sulfur. (9)

Limestone

Limestone is a mineral soil conditioner composed primarily of calcium-carbonate and ground finely. It is capable of correcting soil acidity and neutralizing soil P.H. Direct soil application enhances the bacteria within the soil to decompose plant residues quicker, releasing valuable nitrogen's and phosphorus for plant growth. (8)

Microelements

Microelements include iron, manganese, zinc, copper, boron, molybdenum, chloride, and nickel. These are not considered to be as important as the macroelements because they are used by the plant in lesser quantities but they are just as essential in plant development.

Macroelements

The Macroelements are often sold together in packages. The numbers on the fertilizer packages refer to how much of each macroelement the fertilizer contains. These are in the order: Nitrogen-Phosphorus-Potassium. Nitrogen is necessary for plant growth. It has the most control on vegetative shoot growth. Phosphorus gives the plants health and fruit. It is important in root development, plant energy reactions and photosynthesis. Potassium assists in good root development. It is important for plant water relations and enhanced drought resistance. (1)

Mixing Media and Fertilizers

The Mixture

The mixture and proportions used to create potting mixes are determined by:

• Container Size - Container depth directly affects the percent of the growth medium that is filled with air at container capacity.
• Growth Environment - A growth medium for plants grown in a greenhouse, where control of the moisture level is possible, can have a greater water-holding capacity than a medium for plants exposed to natural rainfall distribution.
• The Plant's Requirements
• The Propagators Experience (5)

Many ‘recipes’ exist which can be used by a nursery propagator, but trial and error is the only real method for determining which mixture will work best for the situation.

These are a few examples of mixture ‘recipes.’

Peat-Lite Mix (to make 1 cubic yard) (1)

• 1.2 ft^3 Sphagnum peat moss (shredded)
• 1.2 ft^3 Vermiculite (fine)
• 4 tbsp. Ammonium nitrate
• 2 tbsp. Superphophate (powdered)
• 10 tbsp. Dolomitic Limestone (finely ground)

ATTRA's Organic Potting Mixes. Recipe #1 (3)

• 50 to 75 % Sphagnum peat moss
• 25 to 50 % Vermiculite
• 5 lbs. ground limestone per cu. yd. of mix

Bedding plant recipe (3)

• 25% Compost
• 50% Peat moss
• 25% Perlite or Vermiculite

Mixing

There are several methods for mixing the media together. A shovel could be used to manually stir the media, but it is extremely important that mixtures be uniform and consistent in quality so that plant growth results are also uniform.

A good system for mixing medium components in a nursery utilizes a rotary-type mixer, such as a cement mixer or a drum and paddle type mixer. There appears to be less breakage of the component particles when rotary-type mixers are used, but difficulties include loading the mixer and retrieving the mixture. Drum and paddle-type mixers can be used effectively if the mixing duration is carefully monitored. Systems are now available that allow the components to be placed in large bins from which they drop onto conveyer belts in layers or directly into the mixer at the proper ratio. Fertilizers and other chemical amendments can also be applied in this manner. Shredder-mixers are also used to prepare media. Such a system can greatly reduce particle size and is unsatisfactory for blending fertilizers, especially controlled-release fertilizers, into the container medium. (5)

Commercially Prepared Mixes

Many Mixes come pre-prepared and so do not require on-site mixing.

There are a number of reasons why growers choose to purchase a commercial mix rather than prepare their own growing medium:

1) Uniformity and Consistency of commercial mixes
2) To save Time and Labor
3) To save Money on Equipment and raw materials
4) The Convenience of purchasing a finished, ready-to-use mix

Costs that should be considered when deciding to mix the growing media on-site are:

1) Ingredients.

2) Fertilizers and amendments in the growing medium.

3) Equipment used for mixing, sterilizing, and handling the growing medium.

Item	Cost (2003 prices)
Soil mixer (2 cu. yd. or 54 cubic ft.)	$10,900.00
Steam aerator	4,800.00
Steam generator	6,995.00
Hoses (for steam generator to aerator and aerator to soil mixer)	674.00
Incline conveyer (for adding ingredients to soil mixer)	5,520.00
Installation (electrical, etc.)	2,500.00
Total	$31,389.00

(2)

4) Labor required for preparing and mixing the ingredients and handling the final product.

The decision of whether to mix media or purchase a commercially-formulated mix should be based on a thorough analysis of the economic, managerial, and production issues facing each operation. (2) Often, it might be more economical for a small to medium size nursery to purchase media ready for use because of the high cost of effective mixing equipment. (5)

Sources

(1) Kester, Dale E. Hartmann and Kester’s plant propagation: principles and practices, 7^th edition. Prentice Hall. 2002.

(2) Boyle, Thomas. Plant and Soil Sciences.University of Massachusetts, Amherst

11/03. Calculating Costs for Growing Media. Available at <http://www.umass.edu/umext/floriculture/fact_sheets/business_management/media_costs.htm>

(3) Kuepper, George. Potting Mixes for Certified Organic Production. September 2004. Available at <http://www.attra.org/attra-pub/potmix.html>

(4) Meche, Michelle. Hort 202, General Horticulture Lab 7. Copyright D.W. Reed, TAMU. Available at <http://generalhorticulture.tamu.edu/h202/labs/lab7/organic.html>

(5) Ingram, Dewayne L. Bulletin 241, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: August 1991. Revised: May 1993. Available at http://edis.ifas.ufl.edu/CN004

(6) Spokane Community College. Greenhouse Nursery Management. Ag 108. Available at http://ol.scc.spokane.edu/PShaw/courses/ghouse/lectures/Media/pumice.htm

(7) Wikipedia, the free encyclopedia. Compost. Available at <http://en.wikipedia.org/wiki/Compost>

(8) Superior Soil Supplements. Ammendments. Available at http://www.superiorsoil.com/amendenments.html

(9) Our Garden Gang. Organic Ade: Beyond Manure. Available at < http://ourgardengang.tripod.com/whbeyondmanure.htm>