Why is soil important for our plants’ health? How to choose a potting mix? What is the meaning of soil drainage for plants? What types of potting soils are there?
We presented all of these questions and more to Amanda Lander.
Amanda is an enthusiastic gardener with experience gardening in two very different climates. She has a passion for all things green and growing, especially native plants and their many uses. She tries and tests all she learns and loves to share that practical, proven knowledge with others
Amanda, Why is soil important for plants?
Soil is important for plants because it provides the nutrients and physical support needed for growth. It also regulates the moisture available to plants’ roots and contains microbes and bacteria that enable plants to take up nutrients.
This list can serve as a quick-start guide. The rest of this article will explain why and how different types of soil affect plant growth. Learn about the natural habitat to choose the best soil for each particular plant.
- For trees, acquire or build loam soil.
- For vegetables, use loose, well-draining soil with lots of organic matter such as bagged garden soil.
- For raised beds, it’s best to mix local topsoil and compost if possible.
- For a potted vegetable garden, add perlite to the mix to enhance drainage.
- For flowers and potted plants, use light potting soil.
- For cacti, use formulated cactus/palm/citrus or succulent soil that is porous, pebbly, or sandy.
Common soil alternatives are sphagnum peat moss, coconut coir, mushroom compost, and water beads. Soil alternatives are usually mixed with fertilizer or organic matter.
How can plants grow without any soil at all?
Plants can grow without soil using hydroponics. In a hydroponic garden, add the necessary nutrients to the water in which your plants grow. Provide support to the root systems that would otherwise come from the soil.
Ingredients in potting soil can include natural soil, peat moss, coir, compost, sand, perlite, vermiculite, manure, and bark. A homemade soil mix recipe may call for added organic matter or fertilizers such as worm castings, kelp meal, soybean meal, rock phosphate, rice hulls, or other materials. The average commercial potting mix is labeled “soilless” because it is made of a mixture of soil alternatives with added nutrients.
Perlite is expanded volcanic glass used to improve drainage. It looks like pieces of white styrofoam in your potting mix but is a natural substance that has been superheated to increase its size. Perlite is extremely porous which also helps it to retain nutrients in your soil.
You should add perlite to your soil mix when rooting cuttings or sowing seeds. You should add perlite to heavy clay soil types to increase drainage and aeration. Because perlite improves drainage, it is a welcome addition to soil used for cacti or succulents.
Vermiculite is a soil additive made from compressed dry flakes of silicate material mined from rocks. It looks like golden brown grit and is easy to tell apart from perlite. Like perlite, it has a neutral pH but because it reacts with potassium, magnesium, and calcium in the soil, it can raise the pH of your soil slightly.
Vermiculite is used to enhance water retention for plants that need constant moisture. Use vermiculite to start seeds as it can retain water that seeds need for germination and protect them from fungus.
You need to add fertilizer to plant soil when the soil is deficient in a nutrient or nutrients necessary for plant growth. When you buy potting mix at the store you don’t need to add fertilizer unless you leave it in storage too long and the nutrients become depleted.
After a growing season, nutrients will need to be replenished. Soil in pots is particularly prone to depletion because water that drains from the pot drags nutrients out with it. Added fertilizer or compost can supply the needed nutrients.
There are different types of potting soil to correspond to the different needs of plants. These are some of the major types of potting soil.
- Seed starting mix
- Cactus and succulent mix
- Garden mix
- Indoor potting mix
Before understanding the difference between potting soil, potting mix, garden soil, and topsoil; it helps to understand what soil is. Soil is a type of dirt made up of rocks, decaying plant matter, animal droppings, minerals, moisture, sand, and clay. These soil types may or may not contain natural soil.
- Potting mix contains no natural soil. It is a mixture of soil alternatives and perlite or vermiculite.
- Potting soil may or may not contain natural soil depending on the plants it’s intended for.
- Garden soil is a natural soil-based mix with added organic or inorganic fertilizers. Garden soil is intended to support plants that require a lot of nutrients like flowers, fruits, and vegetables.
- Topsoil is natural soil, either clay or sand, with organic matter added. It is heavy and holds moisture well.
Desert soil is ideal for cacti and other desert plants because it drains water quickly and allows ample airflow. African Violets would fail in desert soil because they are jungle plants and require greater water retention and higher nutrient concentration.
Organic material is important in soil because it holds nutrients and water in the soil. As organic material decays, it gets pushed down through the soil horizons, providing needed nutrients to plants’ roots at different depths.
Soil pH affects the nutrients available to plants, bacteria levels, and soil structure. Extremely acidic or alkaline clay soil can become sticky and hard to cultivate rather than granular and workable.
You can usually learn the ideal pH for your specific plants by reading the tag or seed packet. Some plants prefer more extreme pH levels than the norm. Look your plant up in a garden book or online at sites like The Old Farmer’s Almanac for more information.
As a rule of thumb, most plants grow best in slightly acidic to neutral soil (between 5.5-7 on the pH scale) because the most nutrients are available at those levels. A scale from 0-14 is used to describe pH, 0 being the most acidic and 7 being neutral.
Soil drainage is the ability of soil to release or retain water which affects the structure and ability of plants’ roots to take up nutrients and put them to use. Some plants require long periods of wet roots for healthy growth because they require more water for cellular functions. Other plants would suffer root rot and eventually die if their roots stayed damp too long.
The size and shape of the particles in the different types of soil promote either water drainage or retention. Sandy soil has bigger particles and drains quickly. Soil that is mostly clay may hold too much water for most plants.
Clay, silt, and sand are mineral sediments that affect the physical properties of soil. They affect the ability of water, roots, microbes, and soil-dwelling creatures like worms to move through it.
Clay is extremely fine-grained, which promotes water retention. The texture is sticky and workable but that can prevent plant roots from getting enough oxygen. Too much clay in the soil will make it too stiff for plant growth.
Silt is fine-grained but the particles are not as small as those in clay. Silt promotes water retention and air circulation. Silty soil is among the most fertile because of this ideal balance.
Sand is made of the largest particles and drains water very quickly. Soil that is too sandy is too dry and does not hold enough nutrients to sustain most plant life.
You can reuse potting soil if you amend the soil first. Ensure there are no diseases, pests, or harmful microbes before introducing a new plant. Discard contaminated potting soil.
Potting mix doesn’t go bad but it can develop solvable problems if you store it for an extended period. Old potting soil needs revitalization by mixing in fertilizer or compost of some kind. Most other problems, including mold and fungus gnats, can be solved by drying the soil completely before mixing in new organic material.
Potting soil can become compressed over time as its non-soil components decompose. To fix this problem, mix in more organic material or perlite.
When choosing a soil mix for your plant, find out where your plant’s natural habitat would be. Choose the soil type or potting mix that mimics the soil qualities from that habitat. Pay close attention to water drainage, soil density, and nutrient concentration.
We asked Amanda a few other questions you might find interesting: