Mixing your own potting soil

Mixing your own potting soil

Finding the perfect potting soil for seeding vegetable starts poses a particular problem to the organic and frugal small farmer. Organic potting mixes must use organic amendments to provide enough nutrients released over time to sustain the plant while it's in pot, but not so much initially as to inhibit seed germination or attract pests and diseases. Organic amendments can get expensive, and we are always concerned with what other farming and resource harvesting practices our own farm is supporting.

It's an area of small farming that could be a lot more creatively explored than it is, with successes potentially leading to major savings for the farmer. Potting soil produced for a low cost, on-farm and from a mixture of various farm by-products; that would be a dream! The obvious and major difficulty though lies in the fact that an unsuccessful potting soil experiment can be devastating to the start of your season. Narrow planting windows in short season climates can mean no second chances for replanting in the event of early crop failure due to inadequate potting soil.

I have been exploratory with my seedling soil mixes in the past but my current method of experimentation is pretty conservative. It involves planting most crops with my standard potting mix listed below, and using experimental potting mixes in a few flats, planted to the same crops as the control. I don't have a separate greenhouse space to quarantine the experimental flats in case they grow stressed plants which attract pests and diseases, that would be ideal, but I do try to make the experimental mixes as low risk as possible. This means separating them as much as I can from the standard-mix plants I'm counting on, vigilantly watching for stressed plants, pests, or diseases, and not allowing standing or splashing water which can spread bacteria and fungi. Its worth it to experiment because a successful potting mix containing some ingredients produced on the farm, like my own compost, could mean a significant cost savings and would be a more sustainable use of resources.



The standard organic potting mix recipe I use contains peat. Peat is sort of a damned if you do, damned if you don't material. Damned if you do because its widely considered to be a non-renewable resource at the rate its being used (mostly for horticultural applications) and because intact peat bogs are some of the planet's major carbon sinks. Damned if you don't because no material is as fool-proof to be successful in starting seeds with, and as economical. This article in the Washington Post says more: https://www.washingtonpost.com/lifestyle/home/should-sustainable-gardeners-use-peat-moss/2017/05/09/1fc746f0-3118-11e7-9534-00e4656c22aa_story.html?utm_term=.5fd5106dd526


It's a controversial issue, but I have made peace with the limited use of spaghnum peat moss from Canadian bogs for starting seeds. I wont use it for larger applications where something else will do. Since I'm already committed to trying to use materials generated on-farm, I don't need to wring my hands over exactly how sustainable or otherwise a product is to want to experiment with more local materials.


My current standard potting mix recipe-

I used this while I worked at the Utah State University Student Organic Farm under soil scientist Dr. Jennifer Reeve. It probably pretty similar to what you could find in some bagged organic potting soil, but it's more economical to mix it yourself. And its a lot of fun. Mix ingredients on a tarp, cement pad, or in a wheelbarrow, spraying water on the mixture to keep the dust down. Wear a face mask, especially when dealing with the vermiculite and perlite!

“2 bales sphagnum peat moss (3.8 or 4.0 cubic foot bales)

 1 bag coarse vermiculite (4.0 cubic foot bags)

 1 bag coarse perlite (4.0 cubic foot bags)

 6 quarts of a fertilizing mixture comprised of:

15 parts steamed bone meal

10 parts kelp meal

10 parts blood meal

5 to 10 parts dolomitic limestone (80 to 90 mesh)


Note: This mix works well in small and medium plug trays and 1020 flats for growing lettuce, onions, leeks, peppers, tomatoes, melons, squash, cucumbers, and many flowers. When repotting small plugs into larger cells, add about 1/3 by volume of old leaf mold or compost and more fertilizing mixture. Continue to fertilize twice per week with soluble fish and seaweed fertilizer.”


This recipe has comes from ATTRA, https://attra.ncat.org/attra-pub/download.php?id=47 . It's the Tipi Produce Recipe. There are tons of recipes here. Some are worth exploring for seeding, and others seem like they could work only in some other limited application, ie. potting up established plants, rooting houseplant cuttings, transplanting desert-adapted plants, etc. Just remember the basic functions of a potting soil for before making your own mix: water retention, drainage, and gradual and consistent nutrient supply.


All the ingredients for this potting soil are available at Steve Regan here in Idaho/Utah. Most feed and farm stores, or nurseries should carry these products.


My experimental mixes-

My experimental mixes mostly involve attempts to replace peat for the reasons listed above, either partially or all together. Finished and sifted compost, coconut coir, vermicastings, and field soil can all be potentially successful in replacing some amount of the peat called for, but I'm still trying to find proportions of these that are as successful as this peat-based mix. More to come on this!




Soil mixes containing compost and composted manures:

It sounds so good, making the bulk of my potting soil with something I'm already generating on the farm. But there is a lot to consider in using a compost-based potting soil. The first issue I have had with using compost in potting soils is the coarseness and uneven texture of compost. If your compost has any large chunks it can make seeding difficult, reduce soil to seed contact, and cause uneven moisture across the flat. You need to first sift your compost for fine seeding.


The second issue is ultimately the reason I only use compost experimentally in seedling mixes now, until I really have both my composting process and compost potting soil recipe down. Potting soils containing compost have a propensity to become home to damping off fungi: http://utahpests.usu.edu/uppdl/files-ou/factsheet/PLP-021.pdf. Damping off is so depressing! Little beautiful seedlings suddenly rootless and falling over dead. It's a nightmare and it spreads quickly. The reasons it happens in potting soils containing compost could be many, but some main ones

1) Because damping off fungi are soilborne, they came into the greenhouse on the unsterilized compost. 2) Great compost contains lot of nutrients and as such attracts microbiology of all sorts. Contrast this with peat which is nutritionally and biologically inert. 3) Damping off proliferates in overly moist soils and compost containing potting soils can become overly damp and resist drying out.

Some of the reasons we love compost, its capacity to hold water and supply nutrients, are exactly what can make it difficult to use in potting soil.


One exception to this would be vermicompost. I have had great success with using this to replace up to half of the peat in a potting soil recipe. Vermicompost is finer and of a more consistent texture than ordinary compost. I also suspect that it is more consistently “finished,” meaning completely composted, than most ordinary compost. Its too expensive in bags for me to consider that as a viable option. But, if I had an industrial sized worm bin I would use 100% of those castings in potting soil. Maybe someday, we already have enough creatures moving into the house with us during the winter...



Soil that has been allowed to dry out:

Potting soil that has been left in a hot greenhouse and allowed to dry out soon becomes hydrophobic. It will not absorb water. I have used dry potting soil to plant seeds in, then watered the flats to see the water splash violently across the soil surface and the newly planted seeds who knows where. Some standing water will be left on top, not soaking in. Its a great illustration of surface tension!


Only mix as much soil as you need at a time. Store soil outside of the greenhouse where it won't get so hot. Also, store it in a container with holes in the bottom, like a 50 gallon tree pot, so you can water it when you water the plants in the greenhouse. For soil that has become hydrophobic, drench it with water and let it sit wet for a couple of days before planting in it.




If it can be direct-sown, do it:

We just built a 525 sq. ft. greenhouse. I love working in the greenhouse when its frigid outside, extending both the growing season and the amount of the year I get to be outside. All that being said, if a crop can be direct-sown, I do it. In my experience, a lot of crops that experts say “can” be transplanted, like squashes, melons, and cucumbers, simply don't transplant that well and just use resources to maintain in the greenhouse. I'm always searching for the lowest-input methods of growing crops. Cutting out the greenhouse time for unnecessary plants is what I consider a low hanging fruit in that department. I'm also really interested in short season, early maturing varieties and direct sowing allows me to really see what a variety is capable of.


The crops I do greenhouse start I do so either because the season is too short to see them mature, or because pest pressure is just too heavy early in the life cycle. In the latter case, greenhousing is an easy alternative to other control measures. I also start a few cool-season crops in the greenhouse, like cabbage and broccoli. Spring here is brief and it usually gets too hot too fast to direct sow them with much success. This is something I'd also like to do more varietal exploration with though.


The main crops I start in the greenhouse:




Long-season flowers



Napa cabbage (flea beetle and earwig damage prevention)

Zinnia (earwig damage prevention)

Some early lettuces


Plan seeding amounts carefully:

I get it, we all want to be outside as much as possible so planning and record keeping can be a drag. But no amount of pencil and papering before the season can compare to the amount of work and resources it takes to care for a plant for weeks and weeks, just to find you grew too much and didn't need it. That's some labor intensive compost. It's hard to plan for what the market wants, or what your family can use, but a lot of great calculators out there can at least help you get close. The Johnny's Selected Seeds Catalog devotes the first few pages to some charts for calculating greenhouse seeding rates and expected yields: http://www.johnnyseeds.com/growers-library/online-tools-calculators.html


Also, if you were over or under your amounts this year, or right on, record it so you know how much to plant next year! You will never remember!


Have something to add? Write me at julie.givingground@gmail.com

Try these for early spring planting indoors: