It can be overwhelming, especially to a new grower, to try and wade through all of the information out there about growing mediums. Every manufacturer wants you to buy their brand of media. Regardless of the type of media chosen however, all growing mediums need to serve some similar functions or they won’t grow much of anything. All mediums need to support plant growth. They do this by providing physical structure to hold the roots in place, they either provide or allow for the transfer of air, water and nutrients to get to the plant and they do not impede root growth. Let’s look at a few of the more common media out there and their respective benefits and characteristics.
These are far and away the most common of media choices out there. There are hundreds, if not thousands, of different formulations available. In general though they all have some combination of similar ingredients: peat moss or coir, vermiculite, perlite, and sometimes contain fertilizers, lime, sand or even mycorrhizae.
Soilless mixes are usually good, all-around mixes suitable for growing most vegetable and ornamental plants. They generally have a constant pH, drain well, hold nutrients well and come without any embedded insects or diseases. Soilless mixes are the easy choice for most novice to intermediate growers. Even many long-term professional growers will use these mixes and may add their own preferred amendments or create custom blends, using these mixes as their base material.
Some experienced growers will grow directly in compost. The benefits to compost are that they typically have high amounts of both macronutrientsand micronutrients, usually contain no synthetic materials, retain water and nutrients sufficiently without holding too much of either, and if its source was properly managed, it should not contain any insect, diseases or weed seeds. The reason that this is not necessarily the medium to start out with for your crops, is that the quality of compost is widely variable.
In order for organic materials to turn into consistent and high-quality, nutrient-rich compost, it needs to be managed from start to finish. It needs to have been made with biodegradable items that were in a ratio of 20-30:1 of carbon to nitrogen feed stocks and in sufficient volumes that the piles reach a temperature of 140 – 160 F (60-71 C) to kill weed seeds, diseases and insects. If you are unsure of its source, use compost as an amendment to your media only, and not as the sole component.
Like compost, some growers may rely on some type of sand as their primary medium, but not without proper management and oversight. Sand serves many purposes as a grow medium and satisfies the criteria for supporting roots, and allowing air, nutrient and water to pass, as well as allowing for root growth. Additionally sand is great for outdoor container production because it adds significant weight to the container meaning they are less likely to be blown or knocked over.
Sand as a media needs to be used carefully as it can be “too good” at allowing water and nutrients to pass through. Sandy mediums need to be watered much more often than other media. Fertilizers that are water soluble are often leached out and much of it goes to waste in sand. If sand is chosen for its benefits, it should be managed with frequent watering and organic fertilizers which are not water soluble.
Rockwool, a fiber made from actually melted rocks, is sometimes used as a growing media for hydroponic and/or submerged crops. It offers good structure, and holds water well. It comes in various forms such as sheets or blocks making it a customizable media.
There are some considerations or precautions to consider when using rockwool: Its dust is hazardous to breathe, it is not biodegradable, and it can hold too much water leading to root rot. Rockwool, like some of the other media mentioned above can be used for the successful production of plants, but care needs to be taken to get the most out of it.
Gravel or pumice, clay pellets or any other small rock can be used as a growth media. Like sand, it is great for drainage, and adds stability to container grown plants. Like sand also, it does not hold nutrients well and is prone to drying out quickly. If any type of stone is selected as a medium, avoid smooth stones. Porous rocks are much better at holding some amount of nutrients and moisture for your plants. Fertilizers for plants grown in stones should not be water soluble.