I got into farming years ago because I like being self-sufficient. I want to know where my food comes from and how it has been treated, and I love the work. For all those reasons, I make, not buy, potting mix.
JWallace
For starting seeds, use a fine, light-weight mix made from screened ingredients that are low in nutrients.
The recipes I use reflect the materials I have on hand and the purpose of the mix. For starting seeds, I use a fine, light-weight mix made from screened ingredients that are low in nutrients. If I’m potting up tomatoes from medium-sized to large pots, I need a fertile mix with good water retention and drainage. But if I’m making 2-inch soil blocks, cohesiveness of the mix is critical. Hanging baskets need sources of long-term fertility (e.g., compost) but also need to be lightweight. For all uses, it’s important that the mix can retain water but also allow water to drain. Poor drainage and/or poor air circulation can create conditions conducive to root disease.
Major ingredients
Compost often makes up 20-60 per cent of the mix (by volume). Compost has a high cation exchange capacity (ability to hold nutrients) and water-holding capacity. It can provide nutrients and has a pH of 6-8. Compost also contains microorganisms that can help control soil borne diseases.
The key is to use high-quality, fully cured compost. With commercial compost, suppliers can provide details on the nutrient content. This doesn’t mean, however, that it is better than farm-made compost. I once bought an expensive truckload of compost made from crab meal and wood waste. It would make a good mulch but was useless as a potting mix. Water poured right through it and the nutrients were tied up by the high-carbon wood waste. It also seemed like there was no microbial life in it. Lesson learned: use compost that feels and smells like great soil.
JWallace
Vermicompost or earthworm castings (excretions) can make up 10-40 per cent of the mix instead of compost.
Vermicompost or earthworm castings (excretions) can make up 10-40 per cent of the mix instead of compost. Too much may lead to an excessive salt content that can harm seedlings. Vermicompost often has a higher nutrient content and is more microbially active than compost.
Peat moss is a lightweight ingredient which holds water and releases it slowly, and provides air. However peat contains no nutrients and is acidic. Lime or wood ash is needed to buffer its acidity. Note that many brands of peat moss (and potting mixes) contain chemical wetting agents; certain wetting agents are said to harm soil life (particularly mycorrhizal fungi) and may be carcinogenic. Yucca extract is a natural wetting agent as is hot water. Simply pour hot water on peat moss and let it sit overnight before adding it to the mix.
Coir (coconut fibre) performs a similar function to peat but has a neutral pH and contains potassium and micronutrients. The sustainability issue is the long-distance shipping (and related greenhouse gas emissions.)
Sand provides drainage. You can get sand from quarries or “builder’s sand” from hardware stores. Avoid sand from the seashore — its salt content is too high.
Soil provides bulk and microorganisms. Too much soil can lead to poor drainage and air circulation.
Vermiculite is a mineral expanded by heating with natural gas. It improves aeration and drainage, and adds potassium, magnesium and other micronutrients.
Perlite is volcanic rock that has expanded during heating with natural gas. It improves aeration and water-holding capacity.
JWallace
Wool holds moisture, and provides some nutrients and air flow. Dirty sheep’s wool is readily available and cheap (or free).
Wool holds moisture, and provides some nutrients and air flow. Dirty sheep’s wool is readily available and cheap (or free).
Minor ingredients include:
Lime increases pH. Calcitic lime adds calcium; dolomitic adds both calcium and magnesium.
If you are making enough potting mix it pays to buy ingredients in bulk.
Wood ash increases pH and adds potassium.
Colloidal phosphate is fine rock phosphate.
Greensand (iron-potassium silicate or glauconite) from the ocean floor provides potash and micronutrients.
Bone meal provides phosphorus.
Bloodmeal, alfalfa meal, crab meal or soybean meal can provide nitrogen.
Biochar is said to stimulate microbial activity and increase nutrient availability but research into its use in potting mixes have inconclusive results.
Kelp meal provides micronutrients.
Dried nettles, horsetail, comfrey or yarrow can provide nutrients and possibly other benefits, such as disease suppression.
Humates are made from lignite, a carbon compound. They may improve a plant’s ability to take up nutrients and water.
It may be tempting to overfeed the seedlings, and certainly high amounts of bloodmeal or sprays with fish emulsion will lead to lush foliage, but often at the expense of root development. When seedlings need to get their nutrients from compost, their roots develop the root hairs and microbial relationships they will need to survive in the garden. Plant tissues with high nitrate levels are particularly vulnerable to aphids.
Recipes
Basic
Equal parts of:
- compost
- sand
- garden soil
Cornell
- 70 per cent peat
- 25 per cent coarse perlite
- 5% vermicompost
- 1.5 g/L dolomitic limestone
- 3 g/L greensand
- 3 g/L rock phosphate
- 2 g/L dried blood
ATTRA1+
Equal parts of
- peat
- bone meal
- perlite
- compost (or leaf mould)
- worm castings (optional)
Coleman’s soil block mix++ (2 bushels)
- 3 buckets* brown peat
- ½ cup lime
- 2 buckets coarse sand or perlite
- 3 cups base fertilizer (equal mix of blood meal, colloidal phosphate and greensand)
- 1 bucket soil
- 2 buckets compost
*10-quart buckets
Mix all ingredients thoroughly in order.
Coleman’s mini-block:++
“Seeds germinate better in a ‘low-octant’ mix, without any blood meal added. The peat and compost are finely screened through a ¼-inch mesh,” writes Coleman.
- 4 gallons brown peat
- 1 cup colloidal phosphate
- 1 cup greensand (optional)
- 1 gallon well-decomposed compost
+These recipes are adapted from Eliot Coleman's book The New Organic Grower, 2nd edition (Chelsea Green Publishing, 1995) and are printed with permission from the publisher.
General instructions
JWallace
If making soil blocks, cohesiveness of the mix is critical.
- Gather ingredients in the fall and store them where they won’t freeze.
- Pour hot water over the peat moss and let it sit overnight or longer.
- Mix ingredients ahead of time. Recommendations range from a few days to three months.
- Test the mix by performing a bioassay. Plant seeds (e.g., alfalfa, oats) in the mix. If germination rates are low, check the germination rate by sprouting the seeds in a damp paper towel. If the germination rate is poor in the mix or if the seedlings look sickly, avoid using the mix. Or you can send it to a lab (e.g., one that does soil tests); you might be able to correct it once you learn the pH and nutrient content.
Sterile compost: A half-baked idea
When I first starting making potting mix, I read that compost should be sterilized by pasteurizing it in the oven. I’m not known for my housekeeping skills but baking compost seemed too messy, even for my low standards. So I used compost straight from the curing bin. Not only did I save time and mess, but I also created a better potting mix.
Damping off was no longer a problem once I switched from a sterile commercial potting soil to my own mix with unsterilized compost. (Granted the damping off was partially caused by other factors, but the problem went away before I learned to give seedlings more light.)
Plants benefit from growing in mixes with an abundance and great diversity of microorganisms, such as those in compost. A sterile mix is a bit like your body after you’ve taken strong antibiotics. You no longer have the defenses to fight off secondary infections.
- Janet Wallace