Michelle Wander, University of Illinois
This article covers basic information about organic potting mixes for organic farming systems. It includes an overview covering the kinds of media (compost, peat, and manure-based materials) commonly used, potting mix test information, and information on how to make and use organic potting mixes in your organic greenhouse operation. It also summarizes basic organic potting mix recipes and provides tips on how to handle materials.
Soil or potting mixes used to start seed must be very light, have high water- and air-holding capacity and be free of disease and weed seeds. Planting mixes containing soil can be blended with peat, humus, compost, and/or manure with approved additives intended as soil conditioners and/or organic fertilizers. The need for disease- and weed-free media favors the use of soilless media, or potting mixes, that are made primarily from natural materials, such as peat, compost, and/or manures augmented with perlite, vermiculite, peat moss, and organic fertilizers. Soil-based mixes provide adequate water holding and supply some nutrients. Loamy soils are usually preferred for soil based media. A mix made with high quality compost will generally provide adequate amounts of phosphate, potash, and trace elements, but in some situations a mix will be need to be supplemented by adding bone meal, rock phosphate, sul-po-mag, greensand, kelp meal, and/or other approved organic fertilizers.
Figure 1. A commercially supplied soilless germination mix containing peat, compost, and perlite. Photo credit: Michelle Wander.
- purchase premade mixes,
- arrange for a custom blend made by a media supplier, or
- mix their own media.
Consistency and time savings associated with the purchase of premade mixes as well as their comparative ease of handling, make them an attractive option for many. Freight costs might be a consideration, especially where growers can produce their own high quality compost. Purchased and homemade mixes can vary notably. Typically, media pH ranges from 4.2 to 7.5, with soluble salts from 20 to 840 dS and nitrate from trace to over 800 ppm.
The most important first step for growers is to make sure ingredients are approved for organic production. This means they are approved for organic use, and include “OMRI Listed” or “WSDA Approved” on their label. If you have questions, confirm with your certification agent that any potting mix you are about to use, or any ingredient you intend to use, is allowed. Growing media and commercial composts can include fertilizers, wetting agents, and other inorganic materials not approved, so exert caution. Products like peat moss or limestone are sometimes treated with prohibited materials including wetting or anticaking agents. See Can I Use This Input on My Organic Farm? for more information.
If you have time and are unfamiliar with a mix it is a good idea to get a greenhouse soil test (Grubinger, 2007). These typically assess pH, soluble salts (electrical conductivity), and nitrogen in the nitrate and ammonium forms. For the saturated media extract (SME) method, the sample is mixed with distilled water at a standard dilution and then analyzed. When evaluating a mix consider the following (Leonard and Rangarajan, 2007):
- Particle size is appropriate to container
- Density, will it hold up plants?
- Good aeration and water-holding capacity
- 10 to 20% air space and 40 to 60% water when moist
- Nutrients levels: low to moderate
- Salts and EC: 4 to 8 dS/m (saturated paste extraction)
- pH near 7
- No plant pathogens
- Decomposition rate should be low
Need adequate supply of organic nutrients for growth
- Organic sources are often inconsistent
- Compost based media: select a well-cured compost
- Some soluble fertilizer sources, such as fish meals, are available
Release of nutrients from organic sources difficult to predict
- Some will result in ‘composting’
- Others increase salts
Premium potting mixes will have a pH between 5.5 and 7, soluble salts between 1 and 5 mmhos, nitrate N between 10 and 200 ppm, P2O5 greater than 3 ppm, K2O greater than 25 ppm, Ca and Mg present in amounts greater than 30 and 10, respectively with sodium and chloride contents falling below 130 and 200 ppm. Water-holding capacity should be 65%. Consider the results based on the crop you are growing and how long you want to hold it before planting. Compare results from batches of potting mix that performed well with those that underperformed.
Bioassays, actually growing plants in the mix, are another way to evaluate your potting mix. Cress, oats, beans, and lettuce are fast-growing crops that can be seeded in the mix before a major planting is initiated. Sensitive crops, like onions, which seem to require a very high quality mix for good germination and growth, are also good to grow. It also makes sense to evaluate crops that you grow as part of your bioassay. Note, seeding depth and adequate water and temperature conditions as well as seed quality can confound results.
Factors influencing choice of mix and handling procedures include:
- The availability and price of high quality inputs. Availability is a concern as growers need reliable and consistent media. The main ingredients of soilless mixes are bulky and lightweight and so transportation costs are an important consideration. It is important for organic growers that inputs be from renewable sources.
- Grower desire and ability to make their own consistent compost and mix. This requires access to bulky C-rich materials to make appropriate compost and the time and skill required to do this reliably.
- Storage and handling facilities. Purchased mixes can be delivered in bagged or bulk form as desired by the grower. Bulk materials should be stored to preserve or improve quality and this will vary with the age and stage of decay. Bagged materials are easier to handle but more costly.
- The number and types of seed starts or transplants to be produced. Germination mixes will have lower nutrient content than media intended to produce larger plants. Small seeds placed at the surface require small particle size and excellent seed-media contact.
- Green house space available to grow starts and target size or age of ‘finished’ plant. Cell sizes and cell shape interact with potting mix particle size to determine the water-holding and nutrient supply capacity. Larger cells can support plants longer but require more space.
- Watering regime. The containers and mix used must work well with the watering technique and quality of water available to the grower. Consider whether water has a high pH, and/or whether foliar feeding should be considered in management.
- Nutritional needs. Nitrogen (N) availability is usually the biggest challenge to organic potting mix formulation because sources are quite variable. Nitrogen is often insufficient when compost is the sole nutrient source. This is why organic potting mixes are often supplemented with alfalfa meal, blood meal, crab meal, etc. when seedlings are to be grown for several weeks. Phosphorus limitations typically show up in early spring, especially in summer crops like tomatoes, when start house temperatures are too low and limit mineralization. The smaller the cell size the more important that the mix contains adequate nutrients. Liquid feeding with fish emulsion or other soluble organic fertilizers can supplement the plants’ nutrient needs, when mixes are exhausted of N. Some growers repot plants in fresh mix, often in larger containers, as a way to keep them growing well.
A variety of organic and inorganic ingredients are commonly used in preparing potting mixes, including:
- Material and Description Compost is derived from the biological decomposition of organic matter accomplished by mixing and piling to promote aerobic and/or anaerobic decay. Composting minimizes pathogens, viable weed seeds, and odors. Composts hold water and provide aeration while providing needed nutrients. The lack of availability of consistent, high-quality compost prevents its widespread use for organic media.
- Characteristics Compost used for potting mixes should have a pH of 6.5–8, less than 0.05 ppm ammonia, 0.2–3 ppm ammonium, less than 1 ppm nitrite-N, less than 300 ppm nitrate-N, and more than 25% organic matter. Finished compost should have soluble salts less than 3 mmohs and moisture content 30–35%. Composts made from manures typically have higher nutrient contents (C:N less than 20) than those made from plant materials where the C:N is higher (20–30).
- Common Usage Compost can be used in place of peats or added as a source of fertility. It commonly makes up 20–70% of media, depending on quality and purpose of use, and often displaces peat in mixes. Typically 75% of mix is made of compost and/or peat with 25% perlite or vermiculite. Quality and proportion varies depending on materials (feedstocks) used to make compost. The proportion of peat to compost should be varied with the qualities of compost. Low C:N materials should be added in smaller proportions.
- Material and Description Dried, pulverized, shredded, composted, or otherwise processed, manipulated, or treated animal manures are the excreta of animals mixed with whatever organic bedding or other materials are needed to follow good dairy barn, feedlot, poultry house, or other livestock practice in order to maintain proper sanitary conditions; to conserve plant food elements in the excreta; and to absorb the liquid portion without the addition of other material.
- Characteristics Ideally, manure should be composted before it is added to mixes. Manures vary greatly in quality; some are very good fertilizers but analysis must be done before you can be sure of this.
- Common Usage Manure may be added in small amounts as a source of fertility. Variability in quality makes its optimal use difficult unless very recent soil tests have been made. In most cases manure will be nutritionally imbalanced and require that additional sources of N be added. If the harvested part of the crop has the potential to come in contact with the manure, soil, or soil particles (for example, by soil splash), manure may not be used less that 120 days prior to harvest for human consumption.
Sphagnum peat moss
- Material and Description Peat is a common ingredient in soilless mixes because it is widely available, relatively inexpensive, and has desirable physical characteristics. It holds a lot of water and air and decomposes very slowly. Most peat used in the U.S. comes from Canada and is harvested at rates that are renewable. Barks—even aged barks—should be avoided as seedling mix components because they immobilize nitrogen.
- Characteristics Peat is quite acidic (pH of 3.5–4.0); limestone is usually added to the mix to balance the pH. Add about 5–15 lb lime per cubic yard for mixes containing 70% peat. Peat is not a source of plant nutrients. Color varies with extent of decay; lighter-colored peats (grower or professional grade) provide more aeration. Finer, darker peats (retail grade) are used as a soil amendment.
- Common Usage Often makes up 30–80% of the mixes. Blending equal parts peat and compost results in desirable pH range.
- Material and Description Is a silica-based volcanic rock that has been heated using natural gas, causing it to expand and become less dense. It improves aeration and water holding capacity of mixes.
- Characteristics Perlite is sterile and has a neutral pH.
- Common Usage Typically makes up 30–50% of mixes when combined with peat or high C:N compost.
- Material and Description Is a micaceous mineral expanded in a furnace using natural gas.
- Characteristics Handled roughly, it can lose its air holding capacity. Vermiculite can supply some K, Mg, and other trace minerals.
- Common Usage Typically makes up 30–50% of mixes when combined with peat or high C:N compost.
- Material and Description Is a by-product of coconut fiber industry that is renewable but requires transportation from long distances. Coir lasts longer than peat but usually is more expensive due to transportation costs.
- Characteristics Coir has higher pH (5.5&endash;6.8), electrical conductivity and thus, soluble salts than peat. It is easier to wet than peat. Depending on fertilization practices, coir can become acidic.
- Common Usage Can be blended to high proportion of mixes (up to 80% reported in the literature with success). Coir is commonly blended with perlite and compost.
- Material and Description Produced by red worms as they breakdown organic wastes. Inconsistent quality makes custom mixes necessary.
- Characteristics Castings are nutrient rich and claimed to have growth promoting attributes. The amount of available P and N varies with bedding materials.
- Common Usage Often makes up 10–40% of mixes; the proportion varies with composition in the same way as other composts or manures.
- Material and Description This is the waste product of a fibrous plant grown in the southern U.S. Waste of core fibers of kenaf is suitable for use as potting media.
- Characteristics Ideally is composted before use so it does not immobilize nitrogen.
- Common Usage Blended at up to 50% of mixes, it is advocated as a possible peat replacement where available.
|Source and Material||Amount (lbs/cu.yd.)|
|Lime (calcium carbonate)||5–10|
|Calcium sulfate (gypsum)||1–2|
|Calcium phosphate (rock phosphate)||5–10|
|Potassium magnesium sulfate||1–2|
|Basalt rock powder|
|Granite rock powder|
|Cotton seed meal (can acidify; contamination is an issue, check with certifier)|
|Kelp and seaweed|
|Dry greens, such as nettle, comfrey, and yarrow (1–3 % N). Add in higher amounts to compost||1–2; 20–40 in compost|
For a potting mix example from an experienced farmer, watch this video of Steve Pincus of Tipi Produce.
For another potting mix example, see gardener Bob Strawn’s Potting Mix article describing the mixes he uses in his container gardening.
The following recipes are cited in Kuepper (2004) unless otherwise specified.
Seedling mixes for starting transplants
Seed mix (Biernbaum, 2001)
- 2 parts screened compost
- 4 parts sphagnum peat
- 1 part perlite
- 1 part vermiculite
- Lime as needed to adjust pH to 6
Seed mix – standard soilless (Biernbaum, 2001)
- 50–75% sphagnum peat
- 25–50% vermiculite
- 5 lbs of ground or superfine dolomitic lime per cubic yard of mix
- Blood meal, rock phosphate, and greensand at 5 to 10 lbs per cubic yard
Organic seedling mix (Biernbaum, 2001)
- 10 gallon of 2 year old leaf mold, sifted
- 10 gallons of sifted compost
- 5–10 gallons of sphagnum peat
- 5 gallons of perlite
- 5 gallons of coarse river sand
- 3 cups blood meal
- 6 cups bone meal
Soilless potting mix (used by Windsor Organic Research on Transition project, E. Zaborski)
- 1 part compost
- 1 part vermiculite
- 1 part peat moss
Screened with ¼ inch screen to mix together. Per 1 gallon mix add:
- 0.6 oz blood meal (17.01 grams)
- 0.4 oz clay phosphate (11.34 grams)
- 0.4 oz greensand (11.34 grams)
Soil-based seedling mix (Hamilton, 1993)
- 2 parts loam (stacked turf to kill any weed seed and disease)
- 2 parts sphagnum peat
- 2 parts coarse grit (sand)
- 30 g or 1 oz lime for each 2 gallon bucket (9 liters)
- 60 g or 1 oz blood meal for each 2 gallon bucket (9 liters)
Organic potting mix (credited to Eliot Coleman in Kuepper, 2004).
- 1 part sphagnum peat
- 1 part peat humus (short fiber)
- 1 part compost
- 1 part sharp sand (builder’s)
to every 80 quarts of this add:
- 1 cup greensand
- 1 cup colloidal phosphate
- 1½–2 cups crabmeal or blood meal
- ½ cup lime
Soil block mix (Kuepper, 2004; adapted from Coleman, 1995)
- 3 buckets (standard 10-qt. bucket) brown peat
- ½ cup lime (mix well)
- 2 buckets coarse sand or perlite
- 3 cups base fertilizer (blood meal, colloidal phosphate, and greensand mixed together in equal parts)
- 1 bucket soil
- 2 bucket
Read more here:: Organic Potting Mix Basics