The Best Grow Medium (my favorite anyway) potting soil

Potting Soil -- Potting soil is basically topsoil (the top of the soil from somewhere). If you buy a product that says potting soil it is made up mostly of topsoil with various amendments. Potting soil is high in nutrients and organic matter. You can even get your own from outside.  You may want to sterilize it to remove pests especially if nematodes are common in your area. You can sterilize the soil by putting it in the oven at the maximum temperature for an hour, or simply put it in the microwave for 5-10 min until it is steaming. However, it is easier to buy amended topsoil than worry about weed seeds, pests, and diseases which will thrive in your controlled indoor environment, often at the expense of your favorite plants. Moreover, home-dug top soil, even from one area can vary in pH, texture and water holding capacity. If you want something consistent, you can buy various potting soils from HTGSupply.com. Commercial potting soil is usually amended or “lightened up” meaning things like perlite vermiculite peat moss etc. have been added so that the soil does not stick together and plant roots can easily move through the soil. There are specialized commercial potting mixes containing mild fertilizers, additives to help hold water, balance the pH, some even have microorganisms. If you are just starting out, or having problems with growing plants, I recommend using potting soil like fox farm or pro-mix. It will provide nutrients to the plants for several months and most every plant can grow well in these type of potting soils.
The most important thing with soil is soil texture (also see SOIL 5/08). You want something that will hold some water but drain well to allow air to enter the growth medium. Mixes that do not drain well will have low oxygen available to the plants and will have poor plant growth for most plants. An easy test is to grab a handful of your soil and squeeze it. It should stick together, but fall apart if you poke it. If it does not fall apart you need to amend you soil to loosen it up. There are several soil amendments, some can be added to soil, or you can even grow without topsoil (soilless mixes). Soilless mixes tend to have no or low nutrients so they require fertilization at all stages of plant growth. In some cases, if you are using just a soilless medium it can be considered growing hydroponically since all nutrients are added via water solution to the medium the plants are growing in.
Soil can be a renewable resource, but can also be exploited. Once I have used soil indoors, I often add it to my outdoor garden to improve the soil of my outdoor gardens. Soil will lose its nutrients after a growing season or two. You can continue to use the soil but will have to add all nutrients and may risk build up of nutrients to toxic levels. You could flush out your soil between growing cycles like you clean certain hydroponic mediums. In essence, if you use dirt long enough you are growing hydroponically, you are providing all nutrients, maintaining Ph etc.
If you want to be more involved with the uptake of nutrients with your plants, you may want to use soilless mediums since their lack of nutrients means that you must provide (control) all nutrients in all stages of plant growth. Potting soil is easy to use because it provides some nutrients so it “buffers” against too little nutrients, and since it is able to hold onto nutrients, it “buffers” against burning plants with too much fertilizer. Be mindful that soilless mixes allow you to have more control and perhaps maximize impact of nutrients, but they also lack the margin of error that soil provides. Once you can grow great in soil, you may want to move on to hydroponics or try other growth medium mixtures. I have tried many grow mediums, and I still keep going back to soil. It is what plants evolved to grow in, it can be a living thing by itself feeding and nurturing your plants.

Good Growing,
Dr. E. R. Myers

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