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Calculating Costs for Growing Media

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soilless growing media for greenhouse production

Growing media represents from 4-6% of the costs of production for most potted crops and bedding plants. It is often assumed that costs associated with mixing or purchasing growing media are not worth calculating because growing media is a relatively small expense in relation to larger costs such as labor and energy. In reality, the opposite is true. Many of the managerial and labor problems affecting growers are directly related to mixing and handling growing media. Thus, it is important for growers to determine the costs associated with preparing their own media or purchasing a commercial mix. This handout will discuss the cost factors associated with these two options.

Grower-formulated growing media. Approximately one-third of commercial greenhouse operations in the United States mix their own media. Growers who mix their own growing media have both control and flexibility: control over the selection of ingredients and the quality of the final product, and flexibility to choose from many media recipes tailored to specific crops, container types, etc. Growers usually prepare a mix from basic ingredients such as sphagnum peat, perlite, vermiculite, and field soil. Alternatively, growers may modify a commercial soilless mix by adding 10 to 20% field soil to alter the physical and nutritional properties of the medium, reduce cost, and increase consumer satisfaction after sales.

Costs that should be considered in mixing growing media are:

1) Ingredients. Ingredients are a major cost associated with growing media. The larger grower can often buy ingredients in bulk (truckload) quantities and save considerably on costs. In contrast, the small- or medium-sized grower often pays higher prices for ingredients because of a lack of equipment and/or storage facilities to handle truckload-size quantities. The cost of ingredients can be determined quickly with the aid of a hand calculator:

Example 1. You have decided to prepare a growing medium using sphagnum peat moss and perlite. The growing medium will be composed of 3 parts peat moss and 2 parts perlite (by volume). The peat moss will cost $1.15/ft3 and the perlite will cost $2.50/ft3. How much will this growing medium cost per ft3?

List all the variables to find out what is known and unknown:

  1. 3 parts peat and 2 parts perlite (by volume)
  2. Peat @ $1.15/ft3 ; perlite @ $2.50/ft3
  3. Cost of growing medium is unknown

Perform calculations:

  1. Total # parts = 3 parts peat + 2 parts perlite = 5
  2. Cost = (#parts peat x cost peat) + (#parts perlite x cost perlite) ÷ total # parts
  3. Cost = (3 parts peat x $1.15/ft3) + (2 parts perlite x $2.50/ft3) ÷ 5 parts total
  4. ($3.45 + $5.00) ÷ 5 = 8.45 ÷ 5 = $1.69/ft3 for growing medium

Example 2. You have decided to prepare a growing medium using sphagnum peat moss, perlite and field soil. The growing medium will be composed of 2 parts peat moss, 1 part perlite, and 1 part field soil (by volume). The peat moss will cost $1.10/ft3, the perlite will cost $2.60/ft3, and the field soil will cost $0.50/ft3. How much will this growing medium cost per ft3?

List all the variables to find out what is known and unknown:

  1. 2 parts peat, 1 part perlite, and 1 part field soil (by volume)
  2. Peat @ $1.10/ft3; perlite @ $2.60/ft3; field soil @ $0.50/ft3
  3. Cost of growing medium is unknown

Perform calculations:

  1. Total # parts = 2 parts peat + 1 part perlite + 1 part soil = 4
  2. Cost = (# parts x cost) + (# parts x cost) + (# parts x cost) ÷ total # parts
  3. Cost = (2 x $1.10/ft3) + (1 x $2.60/ft3) + (1 x $0.50/ft3) ÷ 4 parts total
  4. ($2.20 + $2.60 + 0.50) ÷ 4 = 5.30 ÷ 4 or about $1.33/ft3 for growing medium

2) Fertilizers and amendments in the growing medium. Fertilizers, limestone, and/or wetting agents are usually added during the mixing operation and these incur extra costs. These amendments can be easily calculated into the cost of mixing growing media.

3) Equipment used for mixing, sterilizing, and handling the growing medium. What type of equipment is needed to adequately mix growing media? Thorough mixing is required for preparing a uniform growing medium, and this operation becomes critical when a few ounces of trace elements are mixed in a cubic yard of growing medium. A complete soil mixing operation, suitable for a greenhouse operation that is one acre or larger, may include some or all of the following equipment:

Item Cost (2003 prices)
Soil mixer (2 cu. yd. or 54 cu.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

The total cost of this equipment is $31,389. Assuming that this equipment would last for 10 years before replacement, the annual cost of the above setup is $3139. Typical annual usage for a 1- to 1½-acre operation would involve mixing approximately 70 batches of growing media (equivalent to 3780 ft3). Equipment costs (on a per ft3 basis) would thus be $3139 ÷ 3780 ft3, or $0.83 per ft3. Equipment costs would rise if less growing media is mixed (fewer batches of growing media produced per year). Other costs to consider are the amount of labor and parts necessary to keep the equipment working properly.

If field soil or sand are used as ingredients, then the growing medium should be steam pasteurized after mixing to eliminate disease organisms, pests, and weed seeds. If steam is used for heating the greenhouses and boilers are already available for media pasteurization, then the fuel costs for pasteurization can be estimated at about $0.05/ft3 based on a No. 2 fuel oil cost of $1.00/gallon. With a steam generator, about 4.5 gallons of No. 2 fuel oil would be needed to pasteurize 54 ft3 of growing medium, so the fuel cost would be about $0.08/ft3 if No. 2 fuel oil costs $1.00/gallon.

4) Labor required for preparing and mixing the ingredients and handling the final product. This is another major cost associated with mixing a growing medium. The actual number of hours required to prepare the ingredients (shred field soil, screen peat, etc.) and mix them properly is easily calculated. More mechanized operations can mix 1 ft3 of growing medium in considerably less time than less mechanized operations. For example, limited hand labor would be needed if the equipment listed above is used for mixing and pasteurization. The two main tasks would be loading the ingredients into the the mixer and monitoring the pasteurization/mixing process.

Example 3. You have a greenhouse business that has all of the equipment listed above. Assume that an average wage of $8.50 per hour is paid to laborers for preparation of ingredients and mixing growing media, and that 54 ft3 of growing media can be prepared in 1 hour by 2 persons. What are the labor costs ($/ft3) associated with preparing this growing medium?

Labor costs = (# persons x hourly wage x # hours) ÷ # ft3 mixed
Labor costs = (2 persons x $8.50/hr x 1 hour) ÷ 54-ft3]
Labor costs = $17.00 divided by 54-ft3
Labor costs about $0.31 per ft3 of medium

Example 4. You have a greenhouse business that has some of the equipment listed above. Assume that an average wage of $9.00 per hour is paid to laborers for preparation of ingredients and mixing growing media, and that 35 ft3 of growing media can be prepared in 1½ hours by 2 persons. What are the labor costs ($/ft3) associated with preparing this growing medium?

Labor costs = (# persons x hourly wage x # hours) ÷ # ft3 mixed
Labor costs = (2 persons x $9.00/hr x 1.5 hours) ÷ 35-ft3]
Labor costs = $27.00 ÷ 35-ft3
Labor costs about $0.77 per ft3 of medium

There are additional labor costs associated with handling the medium, i.e., moving and storing the final product, and these costs should also be included in the total costs.

Summary of costs for grower-formulated growing media. Once the above costs have been identified and calculated, a reliable cost figure can be approximated. Assuming a per-ft3 cost of $1.38 for ingredients, $0.83 for equipment, $0.08 for fuel, and $0.31 for labor, then the total cost would be $2.60 per ft3. Thus, filling a 4½" azalea pot with this growing medium would cost nearly six cents (Table 1). Note that this figure does not include several other costs such as electricity consumption, repairs, and interest payments on the financed mixing equipment. These costs will vary and should be added to the cost of mixing for a more accurate figure. If the grower is also the manager, then a cost for supervision should be included.

COST OF GROWING MEDIUM (PER SQ.FT) 2½" SQUARE POT 4½" AZALEA POT 20-ROW SEEDLING TRAY 806 CELL PACKS IN A 1020 TRAY
$1.80 $0.008 $0.041 $0.164 $0.300
2.00 0.009 0.045 0.182 0.333
2.20 0.010 0.050 0.200 0.367
2.40 0.011 0.055 0.218 0.400
2.60 0.012 0.059 0.236 0.433
2.80 0.013 0.064 0.255 0.467
3.00 0.013 0.068 0.273 0.500
3.20 0.014 0.073 0.291 0.533
3.40 0.015 0.077 0.309 0.567
3.60 0.015 0.082 0.327 0.600

Commercially-formulated growing media. There are many commercial soilless mixes marketed in North America. Most of the commercial preparations are based on the Peat-Lite mixes developed by Drs. J.W. Boodley and R. Sheldrake at Cornell University in the early 1960s. Approximately 75% of greenhouse operations in the United States use a commercially-prepared growing media. There are a number of reasons why growers choose to purchase a commercial mix rather than prepare their own growing medium:

  1. batch-to-batch consistency of commercial mixes
  2. management and labor have more time to focus on production issues
  3. less money is tied up in equipment and raw materials for media preparation
  4. the convenience of purchasing a finished, ready-to-use mix

These are important considerations for many greenhouse operations.

The cost of using a commercial mix is easy to calculate with the aid of a greenhouse supply catalog and a hand calculator. For example, 50 3-ft3 loose-fill bags of Pro-Mix BX cost $8.40 per bag or $2.80 per ft3. Thus, to fill a a 4½" azalea pot using this commercial medium costs about 6.4 cents (Table 1).

Conclusions. The decision of whether to mix media or purchase commercially-formulated mix should be based on a thorough analysis of the economic, managerial, and production issues facing each operation. Using the above guidelines for calculating costs, growers can make a sound economic decision on whether to buy a prepared medium or to mix their own.

Calculating the amount of growing medium needed for greenhouse crops. Before seed sowing or transplanting, it is necessary to calculate the amount of growing medium needed to produce the crop. These calculations require conversion tables that provide information on the number of pots, bedding plant flats, or hanging baskets that are filled with 1 ft3 of growing medium. Examples of calculations are provided below:

Example 5. You are the manager of a new greenhouse. You will be producing pot chrysanthemums year-round, and planting 500 - 6" standard pots and 300 - 4" standard pots each week. The growing medium will be composed of 2 parts sphagnum peat moss, 1 part perlite, and 1 part field soil (by volume). How much peat moss, perlite, and field soil (in ft3) will be needed each week for growing the pot mums? Round up fractions to the nearest whole ft3.

List all the variables to find out what is known and unknown

  1. 500 - 6" standard pots and 300 - 4" standard pots per week
  2. The growing medium is 2 peat moss:1 perlite:1 field soil
  3. Total # parts = 2 parts peat + 1 part perlite + 1 part soil = 4
  4. The growing medium is 2/4 peat moss:1/4 perlite:1/4 field soil or 0.50 peat moss:0.25 perlite:0.25 field soil

Use the Conversion Tables (see handout) for additional information.

  1. # 6" standard pots filled for each ft3 of growing medium = 16
  2. # 4" standard pots filled for each ft3 of growing medium = 65

Perform the calculations:

  1. 500 pots 16 pots/ft3 = 31.25 ft3 of medium
  2. 300 pots 65 pots/ft3 = 4.62 ft3 of medium
  3. 31.25 + 4.62 = 35.87 ft3 per week for the mum crop
  4. 35.87 x 0.5 peat moss about 18 ft3 peat moss per week
  5. 35.87 x 0.25 perlite about 9 ft3 perlite per week
  6. 35.87 x 0.25 field soil about 9 ft3 field soil per week

Example 6. You will be growing bedding plants in a 24' x 96' greenhouse. Your calculations show that 1,130 bedding plant flats can fit into the greenhouse if they are grown flat-to-flat on the floor. You decide to use 804 cell-pack inserts for the flats. You will use a commercially-formulated growing medium that is sold in 3.8 ft3 compressed bales that yield 7 ft3 of medium when fluffed up. How many bales will be needed to fill the flats?

List all the variables to find out what is known and unknown

  1. 1,130 bedding plant flats with 804 inserts
  2. The commercially-formulated growing medium comes in 3.8 ft3 bales that yield 7 ft3 of medium/bale

Use the Conversion Tables (see handout) for additional information

  1. # 804 inserts are filled for each ft3 of growing medium = 6.3

Perform the calculations:

  1. 1,130 flats 6.3 flats/ft3 = 179.4 180 ft3 of medium
  2. 180 ft3 7 ft3/bale = 25.7 = 26 bales of growing medium needed
11/03
Topics: 
Commercial Horticulture
Commercial Horticulture topics: 
Business Management
Greenhouse Management