Soil and life have evolved together. There is no life without soil and no soil without life. But generally, there is no discernible difference between soil and dirt whatsoever for most of us. At most, it gets on our nerves when it is brought into the house on shoes and it’s what we stick the roots of our favorite house plant into.
However, for scientists and gardeners, the importance of soil is monumental. While dirt is mostly broken-down rocks that contain minerals like calcium, magnesium, and iron, soil is dirt with the added perk of life like bacteria, worms, insects, fungi, and protozoa. Aside from the added advantage of life and nutrients, soil also offers support to plant roots.
Adding a plant to this mix gives you a complex soil food web. It’s not simply a habitat for life either – it is the root of all life on our planet. In order to understand what layer of soil is best for growing plants, we first need to begin with the science of soil.
So, lets dig deeper and find out!
The Science of Soil
All plants use sunlight to photosynthesize carbon from the atmosphere and convert it into plant sugars. An internal transport system, called phloem, transports some of these sugars into the roots for microorganisms to feed on. The microorganisms then use that carbon to turn rock materials, such as sand, silt, and clay, into soil.
The presence of organic matter in the soil gives it a sponge-like structure, allowing it to retain more water. This, in turn, boosts plant growth and helps it thrive. This is also the reason why healthy soil appears to be dark and rich.
Soil consists of the following major components:
- About 40-45% of the soil volume is inorganic mineral content
- About 25% of the soil volume is water
- About 25% of the soil volume is air
- The rest of the 5% of the soil volume is organic content
Achieving the right balance of all four major components is done by controlling the quantity of vegetation, soil compaction, and the presence of water in the soil. Healthy soil is sufficiently enriched with minerals, water, air, and organic matter to promote sustainable growth in plants.
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Importance of Soil for Plant Growth
Soil is one of the most essential elements for all types of plant life in various ways. Other than providing water, nutrients, and support to both plants and trees, soil is a nurturer for green life on earth.
Good and healthy soil is also the key to revegetation. So long as your soil is not nutritious and fertile, any efforts to sprout vegetation would be all in vain. Be it plant in a pot or your home garden, soil provides plants water, nutrients, and oxygen.
From healthy growth and development of seeds to solid support of full-grown trees, it is important to ensure that your soil is rich in quality and properly layered.
Here are a few reasons that make soil so incredibly important for plants:
Root System Support
The different layers of soil are what build the roots system support. For a plant or tree to anchor itself upright and stand strong on its roots, soil is an essential element for it to make it possible.
Like any other living thing, plants need adequate moisture. The soil retains water to provide a continual supply of moisture and nutrients to plants’ root systems. The type of soil determines the amount of water it can retain. For example, clay soil retains water for longer than sandy soil.
Exchange of Oxygen
The structure of the soil allows oxygen to be trapped among the spaces between the particles. The roots of the plants and trees absorb oxygen from the soil and utilize it to breakdown different sugars from the rhizosphere and microorganisms in the soil. These nutrients are then transported to the plant for growth and nourishment.
Nourishment With Nutrients and Minerals
The soil is also responsible for providing plants with the nutrients and minerals needed for growth, development, and plant reproduction. The nutrients and minerals absorbed from the soil by roots are used in producing flowers, seeds, and fruits – or vegetables in some plants. The type of soil determines the quantities of these nutrients available in the soil.
Decomposition of Organic Materials
The microorganisms present in the soil speed up the natural decomposition of organic matter in the soil. These resident organisms breakdown the organic matter in the soil into waste products. These waste products are vital nutrients for a plant’s growth.
The decomposition of the organic matter in the soil provides a continual source of food to plants through mineralization.
Protection from Erosion
One of the biggest external threats to a plant and its growth is erosion. The soil provides protection against the threat of a plant being swept away, especially during rainstorms. By holding onto the roots of the plants, the soil keeps them from being uprooted from the ground during storms and other natural phenomenon.
Similarly, soil also provides nourishment to marine plants, like sea grasses and seaweeds. These plants not only provide food to marine lives but also serve as their natural habitats. Thus, healthy soil protects the coastlines from erosion by sustaining marine plants.
Filtration from Harmful Contaminants
The soil is also responsible for filtering unwanted and toxic contaminants away from the roots of plants and trees. This natural filtration makes nutrients and vital elements easily accessible to the roots, which spurs the growth of plants.
Recycling of Used Matter
The soil has a recycling process that involves the decomposition of organic material and retention of water. Used matter from plants and microorganisms is converted into usable nutrients and minerals to provide new nourishment to plants.
The Different Layers of Soil
Most types of natural soil have three distinct layers of variable thickness and quality. These three layers are known as topsoil, subsoil, and parent material. Each of these three layers has further two or more sub-layers known as horizons. Together, the horizons make up the soil profile.
The horizons of soil range from fertile soil to bottom rock layers. The upper layers are the most fertile, consisting of topsoil and humus. The succeeding bottom layers are the rock layers, consisting of subsoil, regolith, and bedrock.
Each of these layers is unique in its composition, texture, quality, and characteristics. While topsoil is known to be the most essential layer for plants, each of these layers has its own distinct benefits.
The different layers of soil are categorized into the following horizons:
1. The O-Horizon
The first layer of the soil is the O-Horizon, which is the superficial top layer of the soil. This layer is primarily made up of living microorganisms, decaying organic matter or humus, and fresh soil. Due to being rich in organic matter, this layer is usually thin and of dark brown or black color.
2. The A-Horizon
The next layer is the A-Horizon, which is generally known as topsoil. This second layer is highly rich in decaying organic matter and important minerals. The A-Horizon is, especially, responsible for boosting growth in many plants and supporting grasslands.
As this layer is rich in nutrients, seed germination and plant growth occur in the A-Horizon. Usually, the color of the A-Horizon layer ranges from greyish to brownish.
The A-Horizon also consists of various soil types, including clay, loam, sand, and silt. Despite its various benefits, this layer is highly prone to erosion from both wind and water. The topsoil is most significant for plant growth.
3. The E-Horizon
The third and the thinnest layer of soil is the E-Horizon. As a result of leaching of minerals from this layer, it is also referred to as the eluviation layer. The leaching also removes aluminium, clay, organic compounds, and other soluble content from this layer.
Consequently, the E-Horizon is lighter in color as compared to the A-Horizon. As the leaching continues down the different layers of soil, all it is left with is silt and sand.
4. The B-Horizon
The next layer is the B-Horizon, which is known as the subsoil. It is lighter in color than its preceding layers and primarily consists of large rocks, bedrock, a few minerals, and a little organic matter. As a result of leaching, the subsoil accumulates big quantity of aluminium, iron, clay, and organic compounds from the layers above.
The process of illuviation also causes leaching of mineralized water from the upper layers. The B-Horizon or subsoil is chemically hostile to plant growth. Naturally, it is the layer where the roots of large trees end.
5. The C-Horizon
As we move down further, the next layer that comes up is the regolith or the C-Horizon. There is very little organic matter in this layer and so plant roots do not reach this layer due to the lack of nourishment.
This layer also consists of a good amount of compacted sediment and cemented geological material, which mostly comprises of fragmented pieces of bedrock.
6. The R-Horizon
The final layer of soil is some 50 meters from the topsoil and is known as the R-Horizon. It lies at the base of the soil profile and is also known as the bedrock. All this layer contains is solid, unweathered rock that is compacted and cemented down to the base.
However, this layer does contain minerals like limestone, granite, and basalt, though none of them are needed for plant growth.
As the type of soil varies from place to place, some types might have additional layers or lack any of the above layers. The thickness of these layers might also differ, depending on all the factors that affect soil formation.
Usually, immature soil consists of O, A, and C horizons, while old and mature soils might have all the above layers and some more.
What Layer of Soil Is Best for Growing Plants?
Now that you are familiar with the different layers of soil, it is easier to understand that topsoil is of the greatest importance to plant growth. Due to being rich in minerals, microorganisms, and essential organic nutrients, topsoil is the primary source for plants to grow and thrive.
The two main resources in topsoil for plant growth are carbon and nitrogen. The carbon in topsoil is responsible for providing energy while nitrogen is a tissue builder. All plants require these two components in a range of ratios, depending on their suitable growth.
An optimum ratio for topsoil is generally less than that of 20:1. A suitable ratio of these components ensures an adequate reserve of energy and tissue building material to boost plant growth.
The soil classification system includes the O-Horizon in the topsoil, along with the A-Horizon. Thus, both the top layers of soil can be regarded as topsoil. There is also a variety of soil mixtures available in the market sold under the name of topsoil. These commercial soils are used for nourishing and improving gardens, lawns, and potted plants.
The Common Myths About Topsoil
If you are someone who takes great joy in gardening, all those bags of soil amendments might leave you baffled about how to improve the quality of the topsoil in your garden.
In fact, the wrong composition of soil can not only make your green friends wither but might also make you give up on your passion altogether.
So, before you concede defeat, there are a few things that you should know. Here are three common myths about topsoil that might be hampering the sustainable growth of your plants.
1. All Topsoil is More or Less the Same
The first and the most important thing to understand for anyone with a green thumb is that topsoil varies substantially – even in your backyard. From your garden bed to your neighbour’s, topsoil can differ a great deal.
All types of soil is made up of sand, silt, and clay in varying amounts, along with a good amount of decomposed organic matter. The most suitable mix of these components not only provides nutrients to the roots to absorb, but also allows for good drainage and makes oxygen exchange easier for the roots.
All of these elements combine to promote the healthy growth of plants. Some topsoil might have a lot of weed seeds as well. Plants with that kind of soil might need a little more weeding.
2. For Good Topsoil, Till it Once a Year
Another important thing to remember is that as long as you have built raised beds with good soil and do not compact it by walking on it, there is no need to till the soil every year. If you have not built a raised bed, it is probably better to create a permanent path so as not to compact your soil by walking on it.
One more winning tip to add nutrients to the old soil and replace the organic matter in it after it has been broken down is to add about 1 to 2 inches of compost. Remember to add the compost once you have cleaned up your garden in the fall and your topsoil will be as good as new for the spring!
3. There is No Need for Fertilizer if the Topsoil is Rich
All types of plants draw their nutrients for growth from the topsoil. While your topsoil might be good to go for a long time, the nutrients have to be replenished every year. This is essentially important to spur growth in growing plants, annual flowers, and vegetables since they tend to use up all the nutrients of the topsoil.
Fortunately, replenishing topsoil and restoring nutrition in it is incredibly easy and inexpensive. All you have to do is add some high-quality compost and granular or liquid fertilizer to the topsoil.
Topsoil is a primary component that provides nutrients to plants. Therefore, it the most important layer of soil for enabling growth in plants, trees, and grasslands. If you have to increase the volume of your soil, build up a garden from scratch, or simply want to give a leg-up on plant growth, buying topsoil is the best way to do it.
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The better the quality of your topsoil, the less fertilizer input you would need for your plant, and the more long-term sustainability you can achieve. Now that you know what layer of soil is best for growing plants, add some quality topsoil, and heal your green friends!
I am Elsa, love gardening. I spent lots of time with plants, flowers, it gives me lots of happiness.
I am sharing all the practical tips on how to grow various plants, flower plants, vegetables in the garden. Read more about me.