How Soil pH Balance Affects Your Garden

by Matt Gibson

The pH balance of your garden’s soil refers to the calculated level of acidity or alkalinity in the particular area tested. The scale ranges from 1.0, which is highly acidic, to 14.0, which is basic or highly alkaline, while 7.0 is neutral. The sweet spot for most plant life is from 6.0 to 6.5, which is considered slightly acidic. Most gardens, however, will do just fine anywhere between the range of 5.5 to 7.5 pH.

There are some plants that thrive in an even more acidic soil than that, in the 5.0 to 5.5 range, such as blueberries, azaleas, rhododendrons, and heathers. When growing these varieties, you may want to consider containers so that you can manage the pH levels of these acidic soil lovers separately from the garden beds. Remember that pH levels can vary widely, so it’s a good idea to test your soil in several different areas of your garden before treating your soil.

The pH balance of your garden’s soil is important because it determines the availability of nutrients in the soil you use as well as the acidity levels of the soil, which directly affect your plant’s ability to use those nutrients. A soil that is too acidic or too alkaline will lead to a garden that is suffering from nutritional deficiencies. So, if you’re struggling with keeping plants alive, or just interested in getting the most out of your garden come harvest time, you may want to check the pH level of your soil and start taking steps to get the levels as close to the pH sweet spot as you possibly can.

Testing Your Soil

Contact your county extension office, and ask for soil testing instructions and information. They should be able to lead you to their website and show you where to access all the forms you will need in order to get started and get your soon-to-be-measured samples in the right hands for testing. Professional testing is a lot more thorough and accurate than using home testing kits and is highly recommended, especially if you just moved into a new living situation and are starting a new garden.

Getting a Good Soil Sample

Before taking a soil sample, clear away any litter, leaves, or plant residue on the surface area of the soil. Avoid any areas where ashes or manure have been dumped, any firepit-like areas or places where brush has been recently burned. Make a V-shaped hole by cutting directly into the soil with a trowel or shovel six to eight inches deep into the ground. Take a one-inch strip from a one-inch wide slice of soil cut to the length of the hole you made.

Repeat this procedure at several random spots throughout your garden, and mix the samples together in a clean jar or bucket. Then, to send off for testing, measure out a cupful of soil, dry it indoors for two or three days,then store it in a plastic bag with your information on it. Send off the mixed soil sample with all the proper forms and fees (if necessary).

Acidic Soil (pH 6.0 and below)

Highly acidic soil is often found in areas that receive heavy rainfall and have forest cover. Lightweight alkaline nutrients, such as calcium, get washed away, while heavy acidic compounds, such as iron, stay in place. That is why areas with excessive rainfall tend to have more acidic soil than dryer climates.

Slightly acidic soil is great for most garden plants, but if your pH readings show 5.0 or lower, your garden’s soil is too acidic, and steps will need to be taken to treat the soil to reduce the acidity and optimize your soil.

To reduce the acidity in your garden’s soil, you will need to add garden lime, or calcium carbonate, which is readily available at garden centers and nurseries near you. How much lime does it take to get your soil to the optimal pH? If your garden area is approximately 100 square feet, five to 10 pounds of garden lime should raise your soil’s pH a full point (ex: from 4.0 to 5.0). Since measuring the amount of lime you need to raise the pH level of your soil is not an exact science, it’s always a good idea to start small. You can always add more limestone if necessary, but taking it out once you’ve added it is quite impossible.

The lime treatment should be worked into the top few inches of soil, but it can also just be tossed on top to work itself in naturally if you have plenty of time before planting season. Lime treatment can be added to your garden anytime of year. However, it is best to work it into the soil a few months before planting for a particular season so that the treatment has a chance to infiltrate the soil and work its way into deeper zones.

Alkaline Soil (pH 7.0 and above)

Alkaline soil is usually found in areas that receive very little rainfall. Aluminum sulfate, calcium sulphate iron sulphate, garden gypsum, and ground sulphur can decrease your soil’s alkalinity. All of these soil treatment varieties are available at your local gardening supply stores as well as most plant nurseries. Aluminum sulphate is the most commonly used soil treatment for lowering alkaline soil because it changes the pH balance of the soil it’s added to immediately as the aluminum produces acidity as soon as it dissolves. Ground sulphur, for example, can take several months to fully incorporate itself into the soil.

Many experienced gardeners, however, are wary of using sulphur to change the pH of their soil, suggesting instead that one should add compost and other organic matter to their garden soil and stick to plants that work well in high alkaline environments. When growing plants that prefer a more acidic soil, containers can be a great way to create a separate environment where the pH levels are more easily controlled. Once you get your soil to the optimal pH level, you should test the soil yearly so that minor adjustments can be made to keep the pH balance in the ideal range. Maintaining healthy pH levels is an ongoing task, as rain will leach out calcium and other alkaline compounds, constantly raising the pH level, and fertilizers that you add to balance this shift are constantly lowering the pH level. Therefore, lime and sulphur will need to be added regularly to keep the soil from becoming too acidic or too alkaline over time.

Clemson University created this helpful guide for determining how much sulphur needed to lower the pH level of your soil. The guide also covers how much lime you will need to raise pH levels if your soil is too acidic.

Soil Sample Test Results

Most professional soil samples will come back with specific fertilizer recommendations that target three key elements: nitrogen, phosphorus, and potassium.

Nitrogen
Nitrogen helps promote leafy plant growth, and nitrogen levels can be adjusted by adding manure. It’s best to add fresh manure in the fall so that it has time to break down over the winter just in time for spring planting. Nitrogen can be added in many forms, such as dried blood (or blood meal), alfalfa, cottonseed, or soybean meal. The nitrogen in these products is released very quickly, however, so it’s best to wait until spring to add it to the soil.

Phosphorus
Rock powders are excellent enhancements for your soil, and they last for three to four years once added to the soil. Rock phosphate, bone char, and bone meal are the most common phosphorus soil treatments. Rock phosphate, aside from phosphorus, also provides magnesium and many valuable trace minerals. Phosphorus is essential to a healthy garden, as it aids in strengthening stems, germination, fighting off diseases, and absorption of minerals.

Potassium
Potassium-rich soil gives plants the power to flower and produce fruit. It also helps to regulate water flow in your plant’s cells and aids in fighting off diseases. When you are gardening in soil that is lacking in potassium, you will probably see your plant’s experience stunted growth and weak stem structure. Wood ashes, granite dust, and greensand (glauconite) are all commonly used to add potassium to soil.

Though pH balance is only one of many environmental factors that directly affect plant growth, the pH value of your garden’s soil is a major player in nutrient availability. Maintaining the perfect balance of alkalinity and acidity is a challenging but rewarding task that every gardener should attempt to master.

Want to learn more about how pH balances in soil affect your garden?

The Old Farmer’s Almanac covers Soil pH Levels for Plants
The Old Farmer’s Almanac covers Soil Testing for a Better Garden
Burpee covers Understanding Soil pH
Craftsy covers Soil pH Levels
Fine Gardening covers The Four Things You Need to Know about Soil pH
Good Housekeeping covers What To Do If Your Soil Is Too Alkaline
Mother Earth News covers Your Garden’s Soil pH Matters