If you’ve spent any time shopping for soil, you’ve likely noticed that there are quite a few different kinds available. You have indoor soil, outdoor soil, all-purpose soil, flower soil, vegetable and herbs soil, highly-acidic soil, low-acidic soil…and many, many more. So many ways to supplement soil.
Perhaps you’ve wondered why. After all, potting soil is potting soil, right? Well, not so fast. There are so many different kinds because different plants have different needs, and growing plants inside is different than growing them outside.
This guide will help you learn the importance of soil and how to supplement it when it’s not quite up to snuff.
Why Soil Is Important for Indoor Plants
So why do soil or other types of growing medium matter? Well, it matters for a few reasons how you supplement soil.
Anchorage: As roots expand, they need something to latch onto. Soil or a soilless medium provides that.
Oxygen: Most indoor soil or medium provides aeration, which is space between the particles that allows oxygen and air to get into. Oxygen helps plants break down sugars and release the energy needed to live and grow.
Water: Growing media contain the ability to hold onto moisture from water. When directions say something like, “provide well-draining soil,” this refers to the soil’s ability to hold onto water instead of letting it drain straight through the pot. Some plants need more moisture retention than others.
Temperature control: Soil or plant medium protects plant roots from fluctuations in temperature. This holds especially true during overly hot or cold periods.
Nutrients: Soil supplies nutrients back into the plant. Often, when you add fertilizer or some type of compost to the soil, it soaks up the nutrients and holds onto them until the plant needs them.
Indoor Soil vs. Outdoor Soil
If planting indoors, it’s essential to use indoor potting soil instead of outdoor potting soil. Outdoor is heavier, which doesn’t allow enough aeration, and causes it to stay wetter longer, leading to root rot. Indoor potting soil typically contains a different mix of soil media, such as vermiculite, perlite, etc. Indoor and outdoor mixes also provide different nutrients based on the expected environment the plants will live in.
An all-purpose potting mix is somewhere in the middle, though you may need to add specific feeds or solutions to get it to the place your plants need.
Ingredients Used For Indoor Soil Mixes
Of course, you don’t have to buy indoor potting soil mixtures. You can make your own to save money, tend to the needs of a specific plant, or just for the DIY nature of it.
Typical indoor soil mixes include:
Organic or Synthetic Fertilizer
Organic fertilizers result from decomposed organics and are easier for plants to digest. Unless otherwise stated, they are slow-release and provide consistent nourishment. Synthetic fertilizers are created from non-organic materials or chemicals. They can be designed to release nutrients quickly or slowly.
Peat Moss
Peat moss helps soil hold moisture and releases it over time for your plants. This helps prevent nutrients from being washed out during watering.
Limestone
Limestone helps soil by reducing acidity and increasing alkaline to help maintain a neutral pH level range of 5.5 to 6.5.
Coarse Sand
Heavy soil might not provide enough aeration for your plants, so coarse sand is used to make the soil more porous. This helps drainage and gives roots more room to expand and penetrate the soil.
Vermiculite
Vermiculite is a mineral used to encourage seed germination and improve soil structure. Like sand, it provides aeration. It also holds water and does not rot.
Perlite
If you’ve ever wondered about those small white balls in potting soil, it’s not tiny pieces of foam; it’s perlite. Perlite is a granular material made from expanded volcanic glass heated to 1000 degrees C to make it pop, like popcorn. It lasts a very long time, and it helps soil be more porous and drain better.
Coconut or Coco Coir
Coir is the fibers between the shell and the outer core of a coconut. They add to water absorbency, retention, and drainage of the soil. You can also use coco coir on its own to sprout seeds, propagate plants, and provide support in hydroponic systems.
Pumice
Pumice is a volcanic rock with a rough texture that contains few trace minerals but is used to improve soil structure. It loosens heavy soil and keeps it from compacting, which improves drainage and aeration. For succulents, pumice might constitute up to half of your potting media.
Compost
Compost is decomposed organic material. It’s made with just about any type of organic material that breaks down, such as food scraps, leaves, and twigs. Compost helps balance soil density and provides essential nutrients for the soil.
Tree Barks
Pine and fir tree bark is often used in soil mixes. Bark is used to retain moisture, limit compaction, and protect the plant from heat and cold.
Clay
Clay isn’t used often in indoor mixes. It’s heavier, holds more water, and doesn’t drain well. But, it may be helpful for certain indoor plants to add clay to your mix.
How to Choose or Make the Right Mix
Ultimately, choosing the right mix is about finding out what your plants need. Typically, indoor plants require good drainage to help them avoid root rot. So, a general rule of thumb for developing or choosing indoor potting mixes is to lean on the side of combinations that are “light and fluffy.”
Here are a few common indoor plant types and their soil media.
Succulents
Due to their shallow root systems, succulents prefer porous, sandy soil that drains quickly. Sand, perlite, and other minerals are best.
Flowers
Flower mixes typically contain a balance of peat moss, bark, vermiculite, and/or perlite. Soil should be lightweight and loamy.
Orchids
Orchids do not grow in regular potting soil. In fact, you’ll kill them very quickly if you try to grow them that way. Instead, orchids require some mix of bark, perlite, moss, or coco coir. The medium needs to be well-aerated and fast-draining.
Herbs
Herbs aren’t as picky as most indoor plants and typically thrive on any good soil mix. But, you want to ensure that it drains well and offers plenty of nutrients. Some herbs are more selective, though. So be sure to consider best practices for whichever herb you’re growing.
Vegetables
Vegetable soil mixes are almost entirely dependent on the vegetables you are growing. Most vegetables prefer a well-draining, loose soil that can hold moisture without getting soggy. Feeding with fertilizer and compost can also help your vegetable plants produce more fruit.
Ferns
Almost all ferns prefer moist, well-draining, and slightly acidic soil. However, consult best practices for the actual fern you attempt to grow or maintain.
Trees and Shrubs
Loam soil, rich in nutrients and high in water drainage and retention, is best for most trees and shrubs.
What It Means to Supplement Soil
Over time or throughout different growth stages, you may need to supplement your soil. Maybe you started with one mix to help the seed germinate, but now you need to add something else after it’s developed. Or perhaps you want to feed your plant so that it grows larger or produces more. You might also have to supplement your soil if your plant begins to look sickly or isn’t getting the nutrients it needs.
Watering, feeding, and harvesting causes nutrients to naturally leave the soil, so checking it every couple of months is a great idea. Based on soil testing, you may need to supplement it from time to time to ensure good plant health. But, you shouldn’t just add things blindly. You’ll need to know what to add and when.
Soil Testing and Why It Is Important
A soil test is a great way to check the health and fertility of your soil. Instead of guessing what your plant needs, tests can show pH levels and help you pinpoint nutrient deficiencies. Knowing what soil needs will allow you to fix any issues.
How to Test Soil
There are three main tests you can perform that will tell you the pH level of your soil, the nutrient (or nutrition) level of your soil, and what your soil mix is made of. Since you should have a great idea of where your soil came from (either a pre-bought indoor potting mix or one that was self-made), we’ll just focus on the first two types of tests.
Typically the best times to test soil is before the growing season, after it, or when your plants seem to not be developing well.
pH Testing
Soil acidity can cause severe problems for your plants if they prefer neutral or more alkaline-based soil. Acidification can decrease the availability of nutrients, reduce plant production and water usage, and stunt the plant’s growth. If left unchecked, the plant can eventually die.
Fortunately, performing pH tests are simple. You’ll need to get a pH tester or testing kit. These can be found online or at your local nursery in the form of test strips or chemical colored dye tests. You can also test your soil with a two-pronged electronic meter. For each of these, you’ll use distilled water and a sample of your soil. Some electronic meters say you can stick the prongs directly into the soil. Still, you may want to try it in multiple ways to confirm accuracy.
A few key things to remember when pH testing:
- Don’t just test the soil on the surface level. Dig down 3 to 4 inches if you can.
- If it’s a large bed, test multiple areas.
- Make sure the water you use is neutral. If not, use distilled water.
- Don’t touch the soil with your hands, or you could contaminate the sample.
- After mixing water and soil, allow the sample time to settle.
- If color matching, verify the color in natural daylight instead of inside.
Nutrient Testing
For nutrient testing, you’ll need to get a kit that measures the amount of nitrogen (N), phosphorus (P), and potassium (K) in your soil. These kits typically involve taking a sample of soil, mixing it with water, then placing it inside a container, along with a capsule, and checking the color that it turns into.
How to Supplement Your Soil
There are multiple ways to adjust the pH levels and add nutrients to your soil. Each requires mixing specific materials or solutions into the soil and is much easier to accomplish with less plant in the pot or container. This is why the beginning or end of the season is best.
Store-Bought Solutions
You can purchase solutions or fertilizers at nurseries and online that will help you balance nutrient and pH levels. Some stores even carry kits that come with testers and solutions in one package.
Self-Made Compost
One of the best ways to control acidic levels and add nutrients to your indoor plants is to develop your own compost. It’s a natural product, so you don’t have to worry about chemicals or burning your plant’s roots. Because it breaks down over time, compost is also a slow-release feed for plants. Also, it’s incredibly cheap since it’s made up of organic matter like food and plant waste.
Developing strong soil maintenance habits is essential to successful home gardening. Providing your plants with the right feed and nutrients at the right time can produce greater yields and more full blooms. It could also play a significant role in how long your indoor plants are around to keep you company.