Soilless Mixes

By Eric Sideman, Ph.D.

Soilless mixes were developed for use in containers for seedlings, because field soil does not work well. Soil alone is heavy and poorly aerated. It tends to become waterlogged and sticky when wet. Then it shrinks when it dries, pulls away from the container edges and turns into a little brick, which is difficult for plant roots to penetrate. Furthermore, field soil may be a source of diseases that retard growth and may kill seedlings.

An ideal mix will:

• be dense enough to hold up the seedling;
• retain moisture;
• be porous enough so that excess water drains and the mix remains aerated;
• be free of weed seeds and plant pathogens;
• be low in salinity (1 to 2 mmhos);
• have a pH of 6.5;
• have adequate amounts of nutrients available.

Many ideal mixes are available in the marketplace. They are made primarily from peat, perlite and vermiculite. But most commercial mixes on the market do not meet organic standards and would not be permitted in certified organic production, because they also contain synthetic sources of nutrients (fertilizers). A few commercial mixes do meet organic standards. They too use peat, but instead of synthetic chemicals, the organic mixes rely on compost, natural rock powders and organic sources of nutrients. Johnny’s Selected Seeds and Fedco both carry approved seed starting mixes.

Many growers make their own mixes. Common ingredients for the major portion of mixes include peat, sand, vermiculite, perlite, compost and lime. Below are the basic characteristics of each of these. When designing a mix, growers look at what they need from each ingredient with regard to moisture holding capacity, aeration, nutrients, etc., then determine proportions that work. Here are the major characteristics of these ingredients:

Peat:

• high moisture holding capacity;
• low pH (compensate with limestone by adding 2 to 3% by weight at least five days before planting);
• very little to no nutrients;
• questionably a renewable resource. The Canadian peat industry claims to be harvesting peat from bogs at a rate no higher than it grows. Still, some growers will substitute coir (coconut fibers) or leaf mold for peat. Coconut fibers have their own environmental issue, since they have to be shipped long distances. Furthermore, if you use coir, make sure it is pesticide free.

Sand:

• increases density for greater support;
• improves aeration;
• contains no nutrients.

Vermiculite:

• has good water holding capacity;
• improves aeration;
• has a neutral pH and good buffering capacity;
• has a high cation exchange capacity (CEC);
• is sterile;
• contains some magnesium and potassium.

Perlite:

• greatly improves aeration;
• has a neutral pH but no buffering capacity;
• has no CEC;
• contains no nutrients;
• is sterile.

Compost:

• is a good source of plant nutrients;
• has good moisture holding capacity;
• has a high CEC;
• becomes waterlogged easily.

Compost for a potting soil should be the best compost. It must be mature, with a proper C:N ratio, be low in salts that would otherwise interfere with seed germination and be porous. Compost samples can be analyzed by the University of Maine Soils Lab. An ideal compost should have these characteristics:

• pH — 6 to 6.5;
• salt — 1 to 3 mmhos;
• C:N — 15: to 25:1;
• bulk density — 10 to 30 lbs./cu. ft.;
• NO3 (nitrate nitrogen) > 500 ppm;
• NH4+ (ammonium nitrogen) <100 ppm.

Recipes

Here are some recipes for soilless mixes. I suggest that you try your own based on these and the information above and test it before you do any large plantings. The advantage of commercial mixes is that they are consistent, and the disadvantage of homemade mixes is that they often are not dependably consistent.

Mix 1

Mix 2

Mix 3

Mix 4

NOTE: All of the mixes above have concentrated sources of nitrogen, e.g., blood meal or crab meal. You can substitute alfalfa meal (not pellets, which mold), fish meal or soy bean meal. You can also make any of these mixes without these sources of N and provide the needed nitrogen in periodic watering with compost tea or fish emulsion.

What I do, rather than adding fertility, is pot up to larger pots every three weeks or so, which gives the seedlings new mix.

Footnotes

1. When using blood meal be aware that when it first gets wet and starts to decompose, it gives off ammonia that can kill plant roots. I suggest that you wet the potting soil about a week before you plant into it and make sure it stays aerated during that week.

2. When using topsoil you may want to “sterilize” it, because of potential plant pathogens. This can be done on a small scale in your home oven. Bake it at 350 degrees F. for 45 minutes or until the soil is about 180 degrees F. for 30 minutes. This should kill the pathogens but leave enough beneficial soil microbes alive.

3. Black peat is a more humified peat that is sometime referred to as peat humus. It is not often found in commercial markets, but if you look for the darkest peat with short stems, that will do fine. True black peat cannot be used alone, because it becomes slimy and muddy when wet. One grower told me that when he could not find black peat, he just dropped it from the mix and increased the amount of compost a bit.

4. Certified growers would have to use an approved compost.

About the author: Eric is MOFGA’s “extension agent.” You can contact him with questions about your farm or garden crops at [email protected] or at 946-4402.

Fish Emulsion in Media Suppresses Damping-off

Canadian scientists suggest that incubating peat mix and soil with fish emulsion may enhance plant growth and suppress seedling damping-off diseases caused by Rhizoctonia solani and Pythium aphanidermatum. Fish emulsion (1% to 4% by weight) or equivalent inorganic fertilizer (N-P-K) was incorporated into pathogen-infested peat mix and incubated in plastic bags for 1, 7, 14 or 28 days before radish and cucumber seeds were sown. Plants were rated after 14 days for incidence and severity of damping-off.

Peat mix incubated for one day with fish emulsion offered negligible protection, but after seven days incubation, 70% to 80% of the seedlings remained disease-free in peat mix amended with 4% fish emulsion; and after 28 days, all concentrations of fish emulsion provided equivalent protection.

The inorganic N-P-K treatment, adjusted to reflect N-P-K levels in the fish emulsion, provided no disease control, indicating that disease protection was not due to increased nutrition.

Incorporating 0.5% (by weight) fish emulsion into soil five days before planting radishes effectively controlled damping-off.

Fish emulsion (2% and 4% by weight) also effectively and consistently suppressed damping-off of cucumber seedlings in muck soil naturally infested with damping-off pathogens.

Pasteurizing peat mix then re-infestating it with R. solani resulted in a much higher level of disease than that in unpasteurized, infested peat mix. Adding fish emulsion restored disease suppression within seven days.

Fish emulsion may not be toxic to the pathogens but may create a biological climate in peat substrate or soil that suppresses the disease. Fresh and dry mass measurements of plants produced in 4% fish emulsion were two to three times greater than in nonfertilized peat but were comparable to those receiving equivalent N-P-K.

Source: “Suppression of Rhizoctonia and Pythium damping-off of radish and cucumber seedlings by addition of fish emulsion to peat mix or soil,” by Pervaiz A. Abbasi, Kenneth L. Conn, and George Lazarovits, Canadian Journal of Plant Pathology, 26(2), April-June 2004, 177-187; abstract at https://pubservices.nrc-cnrc.ca/rp-ps/absres.jsp?jcode=tcjpp&ftl=k04-012&lang=eng.

Organic Crop Rotation Study Shows Favorable Results

An organic crop rotation is at least as sustainable as no-till farming or chisel tillage in terms of nitrogen loss and corn yields, according to an Agricultural Research Service (ARS) study. The five-year study showed that a three-year rotation of organic corn, soybeans, wheat and a legume cover crop had nitrogen losses and corn yields similar to those on land where either chisel-tillage or no-till farming had been used. The organic rotation relied on poultry litter, soybeans and a hairy vetch legume cover crop as nitrogen sources. The highest risk of leaching nitrogen to groundwater was on fields with no-till or chisel tillage where both commercial fertilizer and poultry litter had been used.

Source: Source: ATTRA Weekly Harvest Newsletter, Dec. 1, 2004; https://attra.ncat.org/newsletter/archives.html; https://www.ars.usda.gov/is/pr/2004/041126.htm