Seeding Mixes

Spring 2000
Nursery crops in compost
Nursery crops growing in grow-tubes filled with a compost-based medium. You can check the performance of plants growing in such a medium by growing a few in a commercial, non-compost based medium for comparison. English photo.

By Jean English

Working with compost-based seeding mixes is not a static thing. That was the main message Dr. Will Brinton of Woods End Lab imparted in a MOFGA-sponsored talk at the Maine Agricultural Trades Show in January. Among the factors that may change the composition and production of such mixes are the National Organic Standards; questions about the safety of using bone and blood meal in mixes (Woods End has had a lot of calls about these in relation to Mad Cow Disease lately, and Europe is not allowing any “blood-type ingredients” in potting mixes); and concerns about the use of peat.

“There’s a strong anti-peat sentiment in Europe,” Brinton reported. “It’s not so strong in the U.S., and the Canadians claim to have a virtually endless peat supply.” For those who want to avoid the latter, Brinton said that a good leaf mold can be substituted 1:1 for peat moss. One organic grower in Massachusetts has found that a mixture of 55% leaf mold, 44% dairy/horse manure compost, and 1% limestone worked well for him. However, “not many people are good at making leaf mold compost any more,” said Brinton. The type of leaves molded (oak, maple … ) doesn’t seem to matter, said Brinton, although he has not done controlled experiments to study this factor. Eliot Coleman, he continued, has a simple, favorite recipe: compost and soil; no peat.

Pathogens are also a concern. “IFOAM now recommends testing organic amendments, which I assume includes organic potting mixes, for pathogens,” such as E. coli and Salmonella, said Brinton, and the new OMRI rule says that if a compost tests positive for Salmonella, it is no longer compost but a raw material – so the grower has to go through a longer waiting period before harvesting produce raised with the material. Chicken manure, Brinton pointed out, can be very high in Salmonella, but the pathogen can be destroyed by good composting.

Beyond concerns about Mad Cow, National Standards, pathogens, and the renewability of peat, Brinton explained that compost-based mixes are inherently variable, as opposed to chemical-based seed mixes. He described a visit to the Pro Mix factory, where the “quality control is amazing; I’ve never seen an organic mix that can stand up to that standard. They (Pro Mix) criticize organic mixes for that reason, but you have to look at it differently.”

One difference is that compost-based mixes can be used to control diseases, especially Rhizoctonia- and Pythium-caused damping off. Also, “there are dramatic changes in root development with compost mixes versus chemical seeding mixes,” said Brinton. He showed a slide in which the roots growing in a compost-based mix were much more branched and had a much greater proliferation of small root hairs. He has seen this with all kinds of plants.

One way to test a potting mix, in fact, is to dump it out of the pot and look at the roots that grew in it. “They should be well developed,” Brinton explained. “If the roots are around the outside (of the potting mix), you’re having trouble with oxygenation; they’re not growing into the mix, they’re avoiding it. This results from immature compost which is drawing oxygen to keep breaking down. The effect on plants is very similar to that of being waterlogged. However, when the roots grow out around the side edge of the pot to avoid this, then they become prone to drying out.”

Among the variables that make compost-based mixes more difficult to standardize is the variability of compost ingredients from year to year and even from one batch of manure to the next. “Because you’re doing it organically, you have to take what you can get.” Other variables include the moisture content of the compost pile; the amount of turning; the maturity of the compost; and storage conditions – both at the site of production and by the grower. “The grower has to take some responsibility for proper storage,” Brinton believes.

When making compost, Brinton reminded listeners to make sure the middle of the pile is kept moist but not overly wet, and that the pile is turned sufficiently, otherwise it may remain in distinct layers, most of them uncomposted. “If it’s a small operation, the best thing is a lot of turning,” he said. To prevent excess moisture, piles can be covered with straw or polyester fabrics that shed water. “We have even found you can use ordinary plastic tarps, so long as the entire pile is not covered and air gets in around the edges,” Brinton added.

Stop adding to the pile long enough before using it so that it contains no uncomposted material, too, he said. Another way to test compost is to grow plants in it and use Pro Mix or another chemical mix as a control. Your young plants should grow at least as well as those in the chemical mix. Also, you can grow garden cress and/or wheat in the compost mix and in Pro Mix; the plants will grow very slowly if excess organic acids are present, or if the compost is not mature. Cress is a good crop to test because it is sensitive to high salts, and wheat can test for available nitrogen.

Brinton is trying to help makers of potting mixers achieve a mix that contains 35 to 60% compost, but this can be difficult, because the compost can be too rich in nutrients. “One type we had to dilute 30:1 to get the salts low enough,” he said.

When asked about types of manures and bedding, Brinton said that straw-based manures work best, but composts can work with wood. Sawdust and some bark, however, “can pull all of the soluble nitrogen out of the soil mix within three days.” Peat moss, on the other hand, does not soak up nitrogen very quickly. Hardwood shavings are better than softwood, he added, because terpenes and other chemicals in the softwoods inhibit seedling development.

Brinton recommends against using municipal yard waste from recycling centers in compost mixes. When he has looked at this material, he’s found plastic, glass, the sole of a shoe, and other unwanted ingredients. “It’s an objectionable contaminant, and people may get cut” from such contaminants, “especially if they’re rubbing it through a screen.”

Here are some simple formula and test goals for organic seedling mixes modified after Woods End’s own booklet on organic potting mixes, (available from Woods End for $10.00 +P&H).

Compost-Based Seedling Starter

Mature compost: 30-50% by volume

(mature compost best if containing 15-20% initially of cow manure or 5-10% of chicken manure)

Peat moss, sphagnum, screened, 50-70% (plus 2% of weight dolomitic lime)

– or –

Leaf mold compost, well-aged 50-70%

(the above two can be used together, blended 1:1 for good effect).

Rock minerals < 5% of weight of mix

Dried seaweed powder 0.5% of mix weight


Test criteria for organic potting mixes:

Density should be about … 0.25 g/cc = 15 lbs./cu. ft. (soil is 60-70 lbs./cu.ft.)

pH should be … 6.0-6.2; too low will stop early growth; too high gives poor nutrient results

Available NO3 should be … 100-150 mg/liter

Potassium should be … 500-800 mg/l

Salt (Maximum) should be … 1-2.5 mmhos/cm


Woods End Laboratory offers an organic potting mix analysis. Several state labs, such as Cornell, UVM and UMass, also offer potting mix analyses. Brinton stresses that the results of lab analyses depend on an interpretation using one of several possible models for nutrients in potting mixes. For more information, see or email [email protected]; Woods End Agricultural Institute, PO Box 297, Mt. Vernon ME 04352; Tel. 207-293-2453; Fax 207-293-2488.

– Jean English, with input after the Trades Show from Will Brinton

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