Growing your own fruits and vegetables is becoming more and more popular. Gardeners oftentimes struggle with their garden but have no idea why. One of the obstacles includes the need to understand that each plant type has a preferred pH range and type of soil that impacts their ability to thrive. For example, if using any soil type, like Clay for instance, you need to understand is clay soil acidic or alkaline? Therefore, one of the key fundamentals for successful gardening is understanding your soil including the pH of your soil, and how well plant types will thrive in that soil.
In my backyard, I have clay just a few inches below my grass. I originally tried gardening in my backyard using the in-ground conventional row approach and struggled mightily. I had to till the land and pull weeds endlessly while struggling to get a decent harvest. With that in mind, one of the concepts I never understood was the pH of the clay and whether clay was acidic or alkaline.
I’ll explore this topic in this article so you can better understand whether clay or other soil types is acidic, neutral, or alkaline, and the impact it may have on your plants. I will include a section on soil additives and amendments and indicate how they impact the soil’s pH. Finally, included is a list of vegetables along with their preferred pH level ranges.
Is Clay Soil Acidic or Alkaline?
clay will typically have a pH between 8 and 10 naturally but can vary as clay is really a term that describes the physical properties and soil particle size, not the pH value of the soil. The broad pH range you may find with any soil is due to other factors like how fertilizers and water affects the soil’s pH.
What is pH?
The pH describes the relative acidity or alkalinity on a scale from 0 to 14, with 0 being most acidic, 14 being most alkaline or basic, and 7 being neutral. This measure is used to describe the concentration of reactive ions (H+).
The measurement was originally used by the Danish biochemist S.P.L. Sørensen to represent the hydrogen ions concentration, expressed in equivalents per liter, of an aqueous solution: pH = −log[H+] (in expressions of this kind, enclosure of a chemical symbol within square brackets denotes that the concentration of the symbolized species is the quantity being considered). This sounds all complicated. Let’s break it down to what we need to know.
Most food crops prefer a pH of 5.5 to 7.0 but can still have productive plants even outside of this standard pH ranges. A few examples of plants that prefer pH outside of the range defined above include:
- Asparagus: Asparagus like alkaline soil conditions (high pH)
- Blueberries: They like more acidic soil (low pH)
- Potatoes: Potatoes prefer a more acidic soil (low pH)
A more detailed listing of pH preference per vegetable type is provided in a reference toward the end of this post.
The pH of Soil
Your soil’s pH level is a result of interactions among rock, plants, and weather conditions over many years. This varies with climate and regional geography, along with local specifics.
Soil pH is a key variable in soils as it affects the chemical processes that allow the converting and availability of vital nutrients in the soil to the plant. It does this by controlling the chemical structure of the essential nutrients. Plant growth is controlled by this interaction within the soil.
It’s also important, especially for those concerned about your health and staying organic, those synthetic fertilizers, especially those high in ammonium or sulfur, can make soil more acidic. Acid rain can contribute to pH over time. Organic material, including sphagnum peat moss and pine needles, will naturally acidify a bit over time during decomposition. Organic fertilizers can also increase pH over time but at a slower rate.
Ways to Determine the Soil’s pH
There are several ways to determine your in-ground soil pH with varying levels of preciseness.
Soil Testing kits are available from Amazon and your local plant nurseries. Some kits give a broad pH reading by using litmus paper and matching the color of the exposed paper with a chart. Other DIY kits are not complicated and involve placing a soil sample in a bag and adding a supplied reactant to the soil solution, and the resultant color indicates the pH.
There are also pH meters that you can place in the soil and it will give you a reading. The pH meter comes with other features such as moisture, light and nutrient level indications. This method gives you a much better level of accuracy.
The best way to really know your soil pH is by sending in soil samples to soil testing labs done through extension services, such as that provided by the LSUAgCenter’s Soil Testing and Plant Analysis Lab. The nice result of using a service like this is that they give you accurate pH readings and specific recommendations as to how to amend your soil to get to a specific pH range.
First, what is Soil?
At its most basic definition, Soil is a mixture of weathered rock fragments and organic matter that covers the surface of Earth. It is a biologically active home to numerous microorganisms, invertebrates, and plant roots. Native soil is roughly 50% pore space. This space forms a complex network of pores of varying sizes, much like those in a sponge. Soil provides nutrients, water, and physical support for plants as well as oxygen for plant roots.
Organisms in the soil are nature’s primary recyclers, turning dead cells and tissue into nutrients, energy, carbon dioxide and water to fuel new life. Earthworms are our farmer’s helper and helps by creating tunnel, and devouring organic matter and leaving castings to enhance our soil’s health.
Soil Porosity
Soil porosity is a measure of the void or empty spaces in soil. This includes all the pore spaces in your soil. It’s measured as a percentage between 0% and 100%.
Properties of soil that determine soil porosity include texture, structure, compaction, and organic matter.
Soil Texture
Soil Texture describes how coarse or fine soil is. The coarsest soil particles are sand. Silt particles are smaller than sand. Under a microscope, both look like rocks, but silt is much smaller.
Soil texture directly affects porosity. Pores between sand particles tend to be large, while those between silt and clay particles tend to be small. Sandy soil contains mostly macropores and has rapid permeability but limited water-holding capacity. On the converse, Micropores predominate in soils containing mostly silt and clay, creating high water-holding capacity.
Soil Structure
Individual particles of sand, silt, and clay bend together with organic matter, forming aggregates called “Peds”. Peds is what provides structure to soil. Aggregation is a natural process caused largely by biological activity, such as earthworm burrowing, root growth, and microbial action. Soil organics work as a binding agent that stabilizes and strengthens “Peds”. The space between “Peds” are the soil’s macropores which help improve permeability, drainage, and oxygen levels in the soil profile.
Soil Compaction
Soil structure is fragile and can be damaged or destroyed by compaction, excessive tillage, or tillage when the soil is too wet. Loss of organic matter also weakens soil structure. Compaction squeezes macropores into micropores and creates horizontal aggregates that resist root penetration and water movement.
Organic Matter
Adding organic matter is the best way to improve the environment for most types of plants in most soils. Organic matter helps build and stabilize structure in fine-textured and compacted soils, thus improving permeability and aeration and reducing the risk of runoff and erosion.
Soil Type
Soil is classified into various types of soil including Clay, Sand, Silt, Loam and Chalky. the difference between each type is the size of the soil particles creating a different texture, water retaining abilities, nutrients, minerals, and of course, pH. For this article, I am only going to focus on Clay soil.
What is Clay?
Clay soil represents an earthy material that contains extremely small soil particles (<4 microns) which creates a very dense, heavy, clumpy soil. These very small particles were created as rocks weathered and experienced soil erosion for thousands of years. Kaolinites and Smectites, Clay minerals, are formed when the rocks experiences this weatherizing and hydrolysis over time.
Clay soil pH will typically have a value of between 8 and 10 naturally but can vary as Clay is really a term that describes the physical properties and soil particle size, not the pH value of the soil.
Characteristics of Clay Soil Include:
- It holds water and nutrients relatively well but it’s so clumpy and dense that most plants struggle to thrive in heavy Clay Soil.
- Clay drains poorly.
- Clay soil develops plasticity as it gets wet.
- Clay soil hardens as it dries up.
- Firing Clay will harden the Clay material up and causes irreversible changes to it’s structure.
- Most Clay has a blandish grayish-to-white-to-tan color along with streaks or spots caused by impurities in the material. For example, some Clay has traces of iron oxide with leaves a reddish or brownish color streak through it
What is Red Clay?
As stated above, Clay is formed by the weathering of decomposition of rock. All rocks contain minerals, and when rocks containing iron oxides weather, they produce red clay. Granite and basalt are common rocks that contain iron oxides. Red clay soil consists of fine particles that are more than 1000 times smaller than a grain of sand.
What Soil Amendments Can Change Soil pH?
Managing soil pH is essential to creating great soil health and fertility enabling ideal growth conditions for our plants through the introduction of various soil additives and amendments. The pH level of your soil controls the solubility of nutrients as well as toxic metals. Getting this right, or wrong, makes a huge difference. Getting to a different pH level requires some research to determine the best way to adjust your soil’s pH.
The first choice, and good for your soil’s overall health, is the introduction of organic matter, especially compost, to your soil. Organic matter generally has a more neutral pH so adding to the soil effectively moves the pH toward neutral. Seasonal soil additives of compost should move your soil away from being overly acidic or alkaline soil, plus making your soil full of nutrients that the plants can absorb as the pH moves closer to the magic range.
Under most cases, to raise the soil pH, liming agents, such as Agricultural Lime, are added to the soil. In some cases, lower soil pH is desired, which can be achieved using soil amendments such as elemental sulfur (S), or aluminum sulfate. In either case, the pH tolerance of the target plant species, the properties of the soil, and the properties of the soil amendment must be considered to achieve the desired change in soil pH.
Common Soil Additives
Aluminum Sulfate/Elemental Sulfur
Soil pH can be reduced effectively by amending your soil with aluminum sulfate or elemental sulfur. Each amendment provides varying speeds to lower the soil’s pH. For example, Aluminum Sulfate is faster acting than elemental sulfur as it’s much more soluble. The advantage of elemental sulfur is that it’s less expensive, particularly if a large area needs to be treated.
Sulfuric acid is another amendment but only utilized by commercial operations due to it’s potential danger in using it.
Ammonium Nitrate
Ammonium Nitrate, and other Ammonium-based fertilizers like Ammonium Sulfate, are convenient to use as they not only fertilize your soil but can also acidify the soil. However, you would only want to apply so that it makes gradual changes to the soil pH. Still, there is much less risk of lowering pH too much when using Ammonium Nitrate than using elemental sulfur. Ammonium Nitrate can also be used with acid-loving plants to maintain an already low soil pH.
Dolomitic Limestone
Dolomitic Lime contains both Magnesium Carbonate and Calcium Carbonate. The ratio is typically 50% Calcium Carbonate and 40% Magnesium Carbonate, with 10% in additional materials. Dolomite Lime is typically amended to soil to reduce soil acidity (increase soil pH), which improves soil health.
Lime (Agricultural)
Agricultural lime is a soil amendment that’s made from pulverized limestone or chalk. The primary active component is calcium carbonate.
Lime is used in soils as a base (alkaline) to help correct over-acidic soil. Agricultural Lime increases the pH of acidic soil. This reduces acidity and increases alkalinity. An additional benefit of amending Agricultural Lime is that it’s a great source of calcium for plants and improves water penetration for acid soils. Further, it improves the uptake of plant nutrients like nitrogen, phosphorus, and potassium.
Too much lime can make the soil over alkaline demands and lead to poor soil health and plant growth.
Common Organic Soil Amendments
Organic soil amendments can also have an impact on the soil’s pH but typically in a less significant manner as specific chemical-based soil amendments.
Coco Coir
Coco Coir is a formal name for the fibrous material that is broken down from the out husk of coconuts. It does a good job with retaining water so as an amendment helps the soil retain water and nutrients. It is a great amendment for sand which lacks the above. The pH of Coco Coir ranges from 5.8 to 6.5.
Coffee Grounds
The use of coffee grounds as a soil amendment is a great way to supplement your soil with some carbon, nitrogen enrichment and organic matter that feed soil organisms. This is especially useful since it’s considered a waste product of making your coffee. What a great way to be sustainable in a way that will benefit your soil.
As a note, used coffee grounds are beneficial to your soil. However, they have not been shown to consistently lower soil pH. Use coffee grounds in your soil with a bit of discretion as it should not replace your compost or other organic elements in your soil.
Compost
Compost’s pH scale varies as it breaks down and matures. In the early stages, the pH is around 5.5 due to low-temperature microbes breaking down the organic matter and turning it into amino acids. As the compost matures, bacteria known as mesophiles takes over and brings the pH up to a more neutral level, between 6 and 8. With that in mind, adding compost to the soil effectively moves the pH toward neutral.
Cottonseed Meal
Cottonseed Meal is an organic fertilizer that lowers the pH of soil, poses little danger of burning plants, and provides a great source of nitrogen, phosphorous, and potassium, and additional micro-nutrients. It provides high-nitrogen content (6-2-2) that makes a great organic amendment to your vegetable garden soil.
Pine Needles
Pine needles are acidic but do not have the capacity to lower the soil pH any perceptual amount. As the needles break down, and are incorporated into the soil, decomposing microbes neutralize them. They are a great mulch that does not greatly impact the soil’s pH, even when incorporated into the soil.
Sphagnum Peat Moss
Most Sphagnum Peat Moss found in garden centers has a neutral or slightly acidic pH. Therefore, has no significant impact on soil pH. However, Canadian Sphagnum Peat Moss has a pH in the range of 3.0 to 4.5 so it will effectively lower your soil pH.
Wood Ash
Calcium makes up about 20% or more of wood ash. That’s important since that’s the primary active ingredient in garden lime. So, if your soil is acidic, amending it with wood ash can raise your soil pH level. You can substitute four cups of wood ash for one pound of agricultural lime. Note that if you soil is already neutral or alkaline, wood ash could raise the pH too high a level.
Wood Chips
Utilizing wood chips as mulch will not change your soil’s pH, except a slight increase in acidity localized in the very top of the soil. The slight increase of acidity and a drop in readily available nitrogen due to decomposition of the wood chips would occur if the wood chips are worked into the soil and not used just as a mulch.
Worm Castings
Worm Castings are beneficial to the soil for several reasons including the addition of plant accessible nutrients, beneficial microorganisms, and organic matter. As far as changing the pH of soil, worm casting pH is near neutral with a pH of 7. It will not change your soil’s pH to any significant amount.
How much of the Soil Additives or Amendments Should You Use
The actual amount of each amendment is based on the total cubic feet of the soil in your garden bed where you are trying to adjust the pH. That’s the advantage of sending in your garden soil to an extension service. They typically give you recommendations on how to amend your raised bed soil based on your soil’s analyzed pH level.
You can take one of two options. First, for best results, send in a sample of your soil to determine your actual soil pH and recommendations to adjust it. Second, is to use another testing method and make the adjustments yourself. It’s best to go about it slow and not hammer your soil with a ton of anything. Taking care of your soil health is paramount to having a bountiful harvest so be patient while working on the pH of your soil.
pH Preference by Vegetable Type
| Vegetable Name | Approximate pH Range |
| Artichoke | 6.5 – 7.5 |
| Asparagus | 6.5 – 7.5 |
| Beans (pod) | 6.0 – 7.5 |
| Beets | 6.0 – 7.5 |
| Broccoli | 6.0 – 7.0 |
| Brussels Sprouts | 6.0 – 7.5 |
| Cabbage | 5.5 – 7.0 |
| Carrots | 5.5 – 7.0 |
| Cauliflower | 6.0 – 7.0 |
| Celery | 5.8 – 6.5 |
| Chinese Cabbage | 6.5 – 7.0 |
| Corn | 5.5 – 7.0 |
| Cucumber | 5.5 – 7.0 |
| Eggplant | 6.0 – 7.0 |
| Endive | 5.0 – 6.0 |
| Fennel | 5.5 – 6.5 |
| Garlic | 6.0 – 7.5 |
| Green Beans | 6.0 – 7.5 |
| Jerusalem Artichoke | 5.8 – 7.0 |
| Kohlrabi | 6.5 – 7.5 |
| Leeks | 6.0 – 7.0 |
| Lettuce | 6.0 – 7.0 |
| Okra | 5.5 – 6.5 |
| Onion | 6.0 – 7.0 |
| Parsnip | 5.5 – 7.5 |
| Peas | 6.0 – 7.0 |
| Peppers | 5.5 – 7.0 |
| Potato | 5.0 – 6.5 |
| Pumpkin | 5.5 – 7.5 |
| Radish | 6.0 – 7.0 |
| Rutabaga | 6.0 – 7.5 |
| Shallot | 5.5 – 7.0 |
| Spinach | 6.0 – 7.0 |
| Squash | 6.0 – 7.0 |
| Sweet Potato | 5.5 – 6.0 |
| Swiss Chard | 6.5 – 7.0 |
| Tomatillo | 6.5 – 7.0 |
| Tomato | 5.5 – 7.5 |
| Turnip | 6.0 – 7.0 |
| Zucchini | 6.0 – 7.0 |
Conclusion
Understanding your soil’s pH is critical for having a bountiful harvest from your garden. Clay is one of the main soil types you can experience in your in-ground garden so understanding is clay soil acidic or alkaline (it’s pH) and how to amend it to move the pH to the desired range will give you the opportunity to have success.
If you like this content, check out What Zone is Louisiana for Plants? or How Often to Water Cilantro Plant? posts. Also, feel free to check out the Louisiana Simple Living YouTube channel which provides a wealth of raised bed gardening video for you viewing.
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