Build the Best Organic Soil for Your Growroom

By Monica Mansfield
Published: May 14, 2019 | Last updated: June 13, 2022 10:26:08
Key Takeaways

Of the gardening styles, one of the lowest maintenance ways to garden is to use rich, organic soil for your medium. If you build your soil properly, your plants will be fed directly from the soil.

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Avoiding Soil Pests

Building your soil indoors is different than building your soil outdoors, however, you can still use organic gardening principles in your growroom. You just have to make a few key changes. First, you never want to bring outdoor soil indoors unless you’ve pasteurized it. It is better to start with a soil mix from your local garden center. You don’t want to bring a bunch of soil critters into an indoor environment or you could create pest issues.


In an outdoor environment, all of these critters have limitless space to live and eat, but in an indoor environment, they are limited to the soil and plants in your room. When they run out of food in the containers, they will probably start causing problems for your plants. Harmful pests, like spider mites or fungus gnats, like to live in the soil and you definitely don’t want to invite them into your garden.

Using Compost and Soil Amendments

Once you have your base soil it is time to start amending. Outdoors, compost from your compost bin would be the first thing you add. However, using compost indoors is a surefire way to invite pests into your grow room. A great alternative is to add humus to your soil, which you can find at most hydro shops. Humus is organic forest matter that has been decomposed over thousands of years. It is packed full of micro-organisms and contains diverse populations of beneficial bacteria and fungi. Humus is pure black gold and smells like a walk through the forest.


The beneficial micro-organisms, or mycorrhizae, are part of what makes humus essential to mixing the best soil. Mycorrhizae have evolved with plants over millennia and have a symbiotic relationship with them. They help create massive root systems, and as they say, “The bigger the roots, the bigger the fruits!” Mycorrhizae attach to the roots of your plants and eat the plant exudates. They digest the waste and feed it back to the plants, continuously recycling nutrients.

Outdoors, these little creatures are part of a vast underground network, the soil’s internet, and can send their filaments out for several hundred miles or more to obtain food for their plant host. That’s why no one has to feed the forest. Indoors, in containers, they can’t travel if they run out of food, but they will create an ecosystem where you don’t have to feed your plants as much fertilizer throughout their lifetime. Some plants may not need extra food at all, or maybe just a little every couple of months, or once a week. You’ll want to watch your plants for signs that they are hungry and feed accordingly.

There are thousands upon thousands of different mycorrhizal species in humus, but I would recommend adding more beneficials directly to the root zone when you transplant. These usually come in a powder or granular form at your local indoor garden store. By putting them in the root zone as early as possible, you inoculate them right away and they have more time to colonize. This is especially important indoors because they usually only have a few months to colonize and work with your plants, whereas outdoor colonies are already established and they can get right to work. Watering regularly with compost tea is another great way to make sure you have strong mycorrhizal colonies.


Earthworm castings are an essential amendment to add to your soil. They dramatically enhance growth and retain moisture so you don’t have to water as often. They are rich in beneficial microbes, nitrogen, calcium, magnesium, phosphates, potassium, potash, and many other micro-nutrients and minerals. These nutrients are readily available to your plants but won’t cause nutrient burn.

Seaweed is a must-have in the garden. It contains a wide range of nutrients and growth hormones (auxins, gibberellins, and cytokinins), stimulates soil bacteria, and improves soil tilth and moisture retention. It also helps to prevent pests, disease, and drought.


Although there is some debate about whether or not rock dust is effective in the garden, many gardeners share stories of greatly improved yields when they mix rock dust into the soil. Rock dust is full of the minerals that our current-day soil is lacking. By including these minerals back in the soil, you enhance the growth of your plants, and those health benefits pass on to your body when you eat your harvest.

Perform a Soil pH Test

Once you’ve added these basic amendments, I recommend doing a pH and soil test to see if you need to adjust your pH or are low on any vital nutrients. These amendments alone should provide what you need, but if you need to add more of a specific nutrient, you have many options to choose from. You can also top-dress with amendments throughout the life cycle, or fold them in to the top layer of soil.

Guano is a great way to boost nitrogen or phosphorous in your soil, depending on the kind you buy. You’ll find most shops carry two different types of guano—one high in nitrogen and one high in phosphorous. You’ll also find that you have the option to choose between bat or seabird guano. If sustainable practices are important to you, choose seabird guano. When companies harvest guano from bat caves, they will often end up destroying the habitat which leaves the bats homeless.

Blood meal is an excellent slow-release nitrogen source. You’ll notice a significant difference in plant growth when you use this.

Alfalfa meal is a plant source of nitrogen and contains plenty of trace minerals.

For an added boost during the flowering cycle, top-dress or fold in some rock phosphate or bone meal, both of which are good sources of phosphorous and calcium.

Sol-Po-Mag, also known as sulfate of potash-magnesia, is a quick release source of potassium and contains high amounts of magnesium.

Dolomite lime will raise the pH of your soil, as well as add a healthy dose of calcium and magnesium. It is known for promoting healthy plant growth.

If your pH is too high for your acid-loving plants, you can use sulfur to bring it down.

If you were gardening outdoors in the ground, you probably wouldn’t need to add more nutrients during the life cycle. However, because you are growing in containers, you may find that at a certain point, your plants start to get hungry again. Look to your leaves for clues. Yellowing, spots or twisting may indicate a nutrient deficiency. You might need to top-dress with an amendment or add some organic liquid fertilizer to your water. You can also make a tea out of earthworm castings for a nutrient boost. Word to the wise: when growing organically, it is best to avoid synthetic fertilizers because their salt content harms beneficial microbes.

If you build your soil using these superstar amendments, your plants will receive superior nutrition and thrive, while cutting down on your fertilizer bill and work load in the garden. Who doesn’t like to maximize their yield with less work?


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Written by Monica Mansfield | Homesteader, Owner & Writer of The Nature Life Project

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Monica Mansfield is passionate about gardening, sustainable living, and holistic health. After owning an indoor garden store for 5 1/2 years, Monica sold the business and started a 6.5-acre homestead with her husband, Owen. She writes about gardening and health, as well as her homestead adventures on her blog at

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