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Although many commercial potting mixes exist for purchase, many gardeners choose to make their own. A grower might want to create a custom blend for special growing conditions, if retailers are far, or if they have access to on-farm compost or fertilizer that could reduce overall cost. Because some materials come from potentially unsustainable sources or are not Certified Organic, some people choose to make their own potting soil to embody their personal environmental ethics.
Young plants seek two basic needs in a potting soil mix: nutrition and proper tilth, aka soil texture. Ideally the soil will hold enough water to keep seed and young roots moist but will drain quickly enough that no fungus or rot occurs. The nutrient balance should allow for adequate growth for the four to six weeks in the cell tray before transplanting.
Common Potting Soil Materials
If you’ve already done research on potting soil ingredients, there’s a chance you’ve read about some materials you’re not very familiar with. One of the keys to creating an effective mix that suits your farm ideals is to understand each of these ingredients and how they work.
Coir is a shredded fiber made from coconut husks. It breaks down slowly and holds up to nine times its weight in water, making it an ideal base material. As coir is a by-product of the coco-fiber industry, it is considered a renewable resource but it’s only grown in the tropics.
Sphagnum peat moss also decomposes slowly and is a stable organic material well-suited to the base of a potting mix. It holds even more water by weight than coir, but with a low pH of 4, must be amended to neutralize acidity. Some people are concerned about the over-harvesting of peat.
Compost from food scraps and lawn waste improves soil structure, which increases the exchange of water, air and nutrients to plant roots. In addition, compost can add organic matter and beneficial microbes. If using homemade compost, be sure ingredients are shredded finely and free of disease. If disease is a concern, pasteurize compost by heating it to 200 degrees F in a solar oven or home oven.
Adding fertilizer to a potting mix increases nitrogen, potassium and phosphorous, as well as other elements necessary for young plant growth. Compost adds some nitrogen, but supplementing with blood meal, fish meal or composted poultry manure ensures that plants will have sufficient nitrogen over the span of time they’ll stay in the pot. Choose potassium sulfate or greensand for potassium and bone meal or clay/rock phosphate for phosphorus.
Coarse, sharp or builder’s sand adds no nutritive benefit but contributes to soil weight and air space to a potting mix. Sand is especially useful to balance top-heavy plants expected to grow tall.
Vermiculite is a mined, neutral pH mineral that contributes to holding water and fertilization in the soil. It contains a small amount of calcium and magnesium.
Perlite is a processed volcanic rock product that increases drainage and aeration. Its light density can be used to alter the weight of a mix.
A Basic Recipe
Here is a basic potting soil recipe to help you get started. As you become more familiar with your plants and environmental needs, you can adjust the amounts of each ingredient used. (Go here for more ideas.)
- Mix one part compost, one part peat or coir, and one part vermiculite or perlite by volume. Wear a dust mask to prevent inhalation of small particles during mixing and screening.
- If making a very large batch, use a cement stirrer so that ingredients are evenly distributed.
- Screen the mix through 1/4-inch hardware cloth to ensure even particle size.
- Add 0.6 ounces blood meal, 0.4 ounces clay phosphate and 0.4 ounces greensand per gallon as fertilizer.
Evaluating A Custom Potting Soil
When you create or adjust a potting soil recipe, be sure to test its viability before mixing a larger batch. You can send a sample to a lab for around $15. Find a location near you by contacting your county extension agent.
Practical tests are an alternative to a lab analysis. Ted Stutz, owner of Ohio Earth Food, recently found himself needing to change an organic potting soil blend due to the unavailability of an ingredient. First he made small test batches.
“These prototypes are then used to start tomato, arugula, onion and green pepper plants (to represent what the majority of my customers use the soil for) under grow lights in very controlled conditions,” Stutz says. “Results are monitored from 96 cell seed starts and through a transplant into a small pot.” He and his staff then evaluate the soil performance.
Using Potting Soil
Whether you mix your own soil or purchase new, it’s helpful to lightly wet the soil for a week or two before use. This allows any potential microbial action to begin. Then wet it again when you’re about to seed, so that the soil is moist and able to absorb more water during the growing cycle. If you don’t use up the pre-wetted soil, allow it to dry and then store in an air-tight container.
When you mix your own soil, you’re in charge of the sourcing, cost and fun. Keep careful records and you improve your mix from season to season.