When it comes to soilless growing mixes, the possibilities are seemingly endless. There are dozens of different materials that could be used, and the ingredients can be blended in an almost-infinite number of ways. Soilless mixes can be divided into two main categories: seed-starting or potting mixes. Each type serves a different purpose and has different ingredients and ratios of each ingredient.
Seed-starting mixes are designed for starting young plants, and potting mixes are for more mature plants. Seed mixes are often comprised of finer materials than potting mixes, as they are designed to hold more moisture.
The reason both are categorized as soilless mixes is because they contain no actual soil. This is for several reasons. When plants are grown in containers, their roots are limited to the volume of those containers and they cannot spread far and wide like plants grown in the ground. It is important that the media they are grown in is porous enough to allow for easier root growth and maximum air, water, and nutrient circulation.
Garden soils are dense and can restrict root growth in tight quarters. They also do not drain as well as soilless mixes and plants suffocate in overly moist environments. An additional reason soil is not often used in potting mixes is to reduce the chance of introducing pathogens or pests into the garden.
What are the Main Ingredients of a Potting Mix?
Unless you are buying a mix for a very specific type of plant, such as cacti, succulents, or orchids, most commercially available, general-purpose soilless mixes will include peat moss, perlite, vermiculite, lime and some sort of fertilizer.
Peat moss, usually Sphagnum peat moss, is made up of dehydrated plant remains from poorly drained bogs. Most of the peat moss sold in North America is mined from Canadian peat bogs, though it is also mined in Ireland and Germany. Peat moss can absorb 10-20 times its volume in water and adds aeration to potting mixes. It cannot be used by itself, however, as it has a very acidic pH, which is too low for most plants to properly grow.
Perlite is a lightweight volcanic glass that increases aeration and drainage. It is used more heavily in potting mixes than in seed-starting mixes. Perlite pieces are easily identifiable in growing media, as they are the white pieces.
Vermiculite is a very fine mica, which, like peat moss, has a high capacity for absorbing and holding water, but it has several other beneficial properties as well. Its presence in growing media adds magnesium and potassium, but more importantly, it allows for the ready exchange of nutrients in the soil so plants can use them more efficiently.
When lime is added to a soilless mix, it is either to raise the pH or because it adds calcium and magnesium. Mixes with high amounts of peat moss need to have the pH balanced closer to 6 or 6.5 and lime is often the additional ingredient to get it there.
Use a growing media with some lime in it if you’re growing crops that are susceptible to blossom-end rot, such as tomatoes and peppers. Blossom end rot, which appears as blackened spots on the bottoms of the fruits, occurs as a result of a calcium deficiency and not from a fungi, as is often thought.
Most mixes also include materials that add nutrients to the mix. Bone meal, rock phosphate, dried blood, kelp, and compost are often added to organic growing media; synthetic or petroleum-based fertilizers are often added to non-organic mixes.
Additional Ingredients in a Potting Mix
Sand is added to the media if extra weight is needed. As one of the other purposes of growing media is to secure the root system and the plant itself, a mix that is too light is not able to provide enough physical stability to growing plants. Sand particles vary greatly in size and may also be added to improve or reduce drainage and aeration, depending on the needs of the gardener.
Bark is often used in potting mixes, especially those meant for larger or more mature plants. It can be used as a substitute for peat moss, as many barks will absorb moisture, aerate the mix and also lower the pH of the media, though not quite as much as peat moss will. It is important to use aged bark, as freshly ground wood matter will steal nitrogen from plants. Sawdust can be used in conjunction with or instead of bark, but the same caveat applies. Sawdust should also never be derived from treated wood products.
Rice hulls are gaining popularity in mixes, as they do not cause the same nitrogen depletion that wood products can. Rice hulls are a by-product of a renewable crop, and they can help improve the drainage of a soilless mix.
Coco coir is also made from a renewable crop, and improves the drainage of the mix, holds almost 10 times its own weight in moisture and provides a ready breeding ground for beneficial micro-organisms.
Compost as Soilless Media
Compost is sometimes used as an amendment in soilless media, but it is also a soilless media by itself. Plants can germinate, grow and produce food crops entirely in compost. Composts are primarily derived from properly decomposed plant and animal materials and wastes.
When a compost has the proper balance of nitrogen and carbon, it contains all or the majority of the macro- and micronutrients that healthy plants need to grow and thrive.
On a side note, animal manures are a great, nutrient-dense addition to any compost pile, but any compost containing animal manures should not be used unless the pile has been subjected to prolonged temperatures of between 130 and 170°F so any potentially dangerous or harmful pathogens in the manure are killed.
For food crops, the manure needs to be aged for four months before applying to any soil containing food crops for human consumption.
Tips for Making Your Own Potting Mix
Once you have a handle on the function of each of the components of growing media and know what you wish to grow, you can try blending your own mix. This is not a cost-saving measure when you factor in the dollar value of your time, but it will give you a mix containing all the ingredients, in the proper proportions, to meet your unique needs.
Make sure that wherever you do, your mix is clean. Any benefit you hoped to obtain from making your own soilless mix will quickly evaporate if all of your plants are suffering because of contaminated mix.
There will also likely be a learning curve and some trial and error to determine which combinations of ingredients work best for your plants, their water availability and, to a lesser extent, their climate, but for the gardener who wishes to take it up a level, making your own growing media is a worthwhile challenge.