Why is Understanding Soil so Important?
If you are planning on starting a garden, whether it’s in containers or raised beds, or directly into the ground, your soil is going to be the foundation for your plants. They are going to get the majority of the nutrients they need to survive and grow by absorbing them from the soil they are planted in (want to learn more about nutrients your plants need? Read our article here: Growing Plants: Essential Nutrients for Healthy Plants). Understanding what soil is, what kinds of soil there are, and how the composition impacts your plants is essential to getting the most out of your garden.
I know you have heard terms like humus, clay, and pH thrown around, but do you really know what they are? Do you understand what impacts they have on your plant’s ability to grow? Understanding the soil’s makeup and acidity help determine what nutrients are available for your plants to grow. Once you understand that, you can either decide what plants would survive the best in the soil you have available or you can amend the soil to cater to the plants you want to grow. It may seem like a lot, but gardening is a lifelong learning process, so don’t stress! Let’s work on the basics:
What is Soil?
The first thing you need to know about soil is what it is. The answer may seem pretty obvious since we see it everywhere we go, but understanding what it actually is will make you a more informed gardener. You may remember from school that it’s the upper layer of the earth’s surface, but that doesn’t really help you understand how to grow bigger and healthier plants. So we have to get down to a more detailed level.
Soil is a porous material that is a combination of living and non-living materials. It is composed of rock and mineral particles, water, air, and decaying organic matter (humus).
- Rock and Mineral Particles: Rock and mineral particles make up about 45% of soil. These particles are made by the breakdown of the earth’s crust into smaller pieces, making sand, silt, and clay. Each one of these components has it’s own ability to store water and it’s own role in helping with the breakdown of organic material.
- Water: Soil is approximately 20-30% water.
- Air: Soil is approximately 20-30% air.
- Organic Material: Organic material makes up about 5% of the soil1 and is made up of organisms (10%), roots (10%) and humus (80%).2 Humus is the dark material that forms in the soil as plants and animal matter decays. As this matter decays, it is breaking down to it’s basic chemical elements that are being added to the soil, many of which are the essential micro and macro nutrients that plants need to grow and survive.3 Humus is created naturally, but if your soil is not creating enough for your needs, you can achieve it through composting and amending it into the soil.
While each component is an important factor in your plant’s health, the porousness of the soil is also vital. The soil’s porosity is determined by the rock and mineral particle sizes, the root system in the soil, and what organisms are living in the soil. This porosity is vital to your garden because it will determine if and how well your soil holds water. As a gardener, you know how important the soil’s ability to hold water is and what a delicate battle it can be. For most plants, you want the soil to hold water so that your plant’s roots can access it easily, but you don’t want it hold so well that your roots will rot from standing water. You also don’t want the water to drain quickly; otherwise, your plants will struggle to get the water they need to survive. This element of the soil will also determine how frequently you need to water your plants.
Types of Soil:
Soil type is determined by the proportion of sand, silt, and clay that is in your soil. Sand is the largest particle, the silt, then clay. The proportion will greatly impact the soil’s characteristics in terms of composition, available nutrients, and water retention.
- Clay Soil: This type of soil is over 25% clay. It is nutrient rich in most cases because of the binding between the nutrients and clay minerals, but it is a heavy, dense soil because of its high-water content. This kind of soil drains very slowly because of the small spaces between clay particles are easily compacted, reducing its porosity. This type of soil also takes longer to heat up in the spring, potentially delaying planting. When the soil does heat up though it will often crack. You can identify this type of soil by getting it wet and seeing how easy it is to shape. If you can roll it into a log and it gets shiny when rubbed, you have a heavy clay soil.
- Sandy Soils: This type of soil is mostly made up of sand and contains very little clay. This kind of soil drains water very quickly which means it dries out quickly, making it tricky to ensure your plants are getting the water it needs. This kind of soil also lacks nutrients in most cases since its composition allows nutrients to wash away easily. This soil heats up quickly in the spring but is also high in acidity, making it challenging to find the proper plants that need high acidity environments with low water needs. You can identify this soil by its texture. If the soil feels gritty and you can not shape it, you have a heavy sand soil. If you are getting a little bit of shape formation but not much, it could be an indication that you have a sandy loam soil.
- Silt Soils: These kinds of soils are the best of both worlds, being made up of mostly intermediate sized rock and mineral particles. They drain better than clay soils, hold water better than sandy soils, and are fertile. Pure silt soils are rare in the gardening world but you would be able to identify it by having a soapy texture that does not shape easily.
- Loam: This kind of soil is a mixture of clay, sand, and silt and is classified as clay-loam or sandy-loam based on its majority composition. This kind of soil does not have the extreme features of its counterparts, and many gardeners find them easy to work with. The soil is fertile, drains well, retains water, is pleasant to work with, and can be compacted easily. 5
Once you identify the kind of soil you have, you can work on adding amendments to the soil to give it more plant-friendly characteristics or modify the style of garden you are going to have. For example, if your soil is heavy clay soil, you may add amendments that will allow better drainage and less compaction, or you may decide to build a raised garden bed and start with store bought soil. If you have sandy soil you would need to add amendments that retain water and most likely nutrients.
Soil pH is a measure of the soil’s acidity or alkalinity, with 7.0 as neutral. This is vital information because it directly impacts the availability of nutrients in the soil and the microbial population in the plants, which will convert essential nutrients like sulfur and nitrogen into a form plants can absorb. In soils with a low pH, macro-nutrients (like nitrogen, phosphorus, and potassium) are less available to your plants. In soils with a high pH, micro-nutrients (like calcium, magnesium, and sulfur) become less available.4 This is why most plants like to grown in the middle pH range of 5.0 – 8.0 because it allows them optimal nutrient absorption.
It is important to test your soil’s pH before planting to know if you need to amend it to bring it to the proper range for your plants. This can be done with tests that have you add a sample and mail it in to be analyzed or you can buy a meter like this one that measures it within minutes when inserted into the soil. So, you may be thinking, “Ok, my soil pH is a 5.0, now what?” Good question since you are now only operating on half the information you need.
So what’s next? Now you need to understand the pH requirements of your plants. You can do a simple internet search to find out or I like using this Old Farmer’s Almanac chart for a quick guide. For example, if you are planning to grow blueberries, you are good to go since they like the soil’s pH to be between 4.0 – 6.0 but if you want to grow kale then you have some work to do since they like their soil’s pH to be 6.0 – 7.5. Once you have determined your soil’s pH and what you need it to be, you can add amendments to the soil to get it where you need it to be for optimal plant health.
Now that you understand the components of soil composition, the different types of soil, and how soil pH factors in, it’s time to figure out what kind of soil you have and how it will impact what you want to grow. Once you’ve figured that out, you can start amending your soil to cater to the plants you want to grow, or determine if you need to start a garden in a raised garden bed or in planters. Stay tuned for more gardening basic articles to get the most from your garden!
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- What is Soil? (2018, January 11). Retrieved from https://serc.carleton.edu/integrate/teaching_materials/food_supply/student_materials/1029
- Shapley, P. (2010). Soil Composition. Retrieved from http://butane.chem.uiuc.edu/pshapley/Environmental/L28/3.html
- National Geographic Society. (2012, October 09). Humus. Retrieved from https://www.nationalgeographic.org/encyclopedia/humus/
- Prince, R. (n.d.). Plant Nutrients. Retrieved from http://www.ncagr.gov/CYBER/kidswrld/plant/nutrient.htm
- Soil types. (n.d.). Retrieved from https://www.rhs.org.uk/advice/profile?pid=179