In chemistry, pH is a measure of the degree of acidity or alkalinity of a compound. The pH scale ranges from 0 to 14; A pH close to 0 means it is very acidic, a pH close to 14 means it is very alkaline, and a pH of 7 indicates a neutral condition. In plant cultivation, the pH of the soil used to grow crops has a serious effect on plant health and growth. While most plants can grow at a pH of around 6.0 - 7.5, the best growing conditions for some plants are achieved within a narrower pH range, so for those of you who are serious about growing plants, it's important to learn the basics of regulating soil pH. See Step 1 below to start learning how to lower the pH of your soil.
Step
Part 1 of 3: Testing Soil pH
Step 1. Test the soil pH
Before adding anything to change the pH of the soil, always test it first so you can see how much it differs from the target pH. You can purchase a pH test kit at a garden supply or hardware store, or you can take a soil sample to your local agricultural service for a professional test there.
Step 2. Dig five small holes where you planted
Soil pH can be determined easily with a commercial pH meter. These tools are usually sold at hardware stores or garden supply stores and are relatively inexpensive. To get started, prepare a sample of the soil you will be testing. Dig five small holes (about 15 to 20 cm deep). Choose a random location in your garden; that way, you can find out the "average" pH of the soil. Remove the dirt from the hole you made.
Remember that these instructions are general in nature - you must follow the specific instructions on the packaging of your pH meter
Step 3. Take a sample from each hole
Next, use a shovel to pick up small slices of soil from the side of each hole. These slices should be crescent-shaped and about 1.2 cm thick. Try to take an equal number of samples from each hole. Put this soil sample in a clean bucket.
Try to sample enough, so overall you get around 0.94 liters or so. For some test methods, this number is quite a lot
Step 4. Mix the soil in a bucket and spread it on the newspaper until it dries
Allow the soil to dry until the moisture has completely evaporated.
Make sure your soil is completely dry before moving on to the next step, as moisture will cause the pH measurement to be inaccurate
Step 5. Use your tool to determine the actual pH of your soil
Depending on the tools you have, the measurement methods will vary. For many normal pH measuring devices, you will need to place a small amount of soil in the test tube, add a few drops of solution, shake it, and let it settle for a few hours. Eventually, the color of the solution will change - then by comparing the color of the solution with the chart provided in the tool, you can determine the pH of your soil.
Other types of tools are also available, so make sure you follow the instructions that came with your tool. For example, an electronic pH tester can determine pH automatically using a metal rod
Part 2 of 3: Using the pH Lowering Technique
Step 1. Add organic ingredients
Many types of organic matter, such as compost, animal manure, and acidic mulch (such as pine straw) can gradually lower the pH of your soil over time. As organic matter decomposes, bacteria and other microbes will grow and obtain food, creating acidic byproducts during the process. Since organic matter takes time during decay to change the pH of the soil, this option is suitable for long-term goals, but will not produce noticeable changes in the short term. Many plant growers choose to add organic matter to their soil once a year to slowly lower the pH gradually.
Organic matter can also provide benefits to your soil - the most obvious of which is improving its aeration and drainage
Step 2. Add aluminum sulfate
To lower the pH quickly, don't rely solely on the gradual decay of organic matter. Use plenty of soil-acid additives at your local plant store. Among these additives, aluminum sulfate is the one that will give the fastest results. Aluminum sulfate gives soil acidity after dissolving. In plant cultivation, this means that aluminum sulfate works instantly. Therefore, aluminum sulfate is a good choice for rapidly lowering the pH.
Depending on the initial pH of your soil, the amount of aluminum sulfate you should use can vary widely. Under very common conditions, to lower 1 pH scale (e.g. from 7.0 to 6.0; or from 6.0 to 5.0, etc.) of 1 square meter of land, you would need approximately 0.6 kg of aluminum sulfate. However, using too many additives can also be harmful to your plants, so also check online resources such as here for detailed information on proper usage
Step 3. Add sulfur
Another additive that can also lower soil pH is sulfur solids. Compared to aluminum sulfate, sulfur is generally cheaper, stronger (in terms of the amount of material required) and works more slowly. This is because sulfur must first be metabolized by soil bacteria to become sulfuric acid, and this process takes time. Depending on soil moisture, the number of bacteria present, and temperature, sulfur can take up to several months to have a noticeable effect on soil pH.
As stated above, compared to aluminum sulfate, you generally need less sulfur to produce the same change in pH. Generally, you need about 90 grams of solid sulfur to lower the pH scale by 1 on 1 square meter of soil. For more complete usage information, read this resource
Step 4. Add sulfur-coated urea
Like sulfur and aluminum sulfate, soil additives containing sulfur-coated urea can also increase soil acidity over time (lowering its pH). As an additive, urea can have a relatively fast effect, so that it can show results one or two weeks after being added to the soil. Sulfur-coated urea is a widely used additive in fertilizers, so if you plan to fertilize your plants, you can opt for a fertilizer that contains this type of urea to save your effort.
The content of sulfur-coated urea varies with each type of fertilizer, so consult your fertilizer application guide to determine the right amount for your plant's needs
Step 5. Add other acidic additives
In addition to the additives mentioned above, many other compounds can lower soil pH. Many of these compounds are often included in certain fertilizer mixes, while others are sold separately. The time and amount needed can vary widely, so pay attention to the instructions for use on the package, or ask an experienced fertilizer dealer for advice. Additional ingredients that can lower the pH of your soil are:
- Diamonium phosphate
- Ferrous sulfate
- Peat
- Ammonium nitrate
Step 6. Plant plants that are resistant to alkaline conditions
If your soil is too alkaline to grow plants that need acidic conditions, growing plants that can grow in alkaline conditions can lower the pH of your soil as it grows. As plants grow, develop, and decay, organic matter returning to the soil will promote bacterial growth and gradually lower the pH of your soil (similar to adding organic matter in the form of animal manure or mulch to the soil). This method is the slowest option for lowering the pH of your soil, as the plants must first grow in order to start providing decaying organic matter in the soil. Some plants that can grow in alkaline conditions include:
- Certain evergreen shrubs (e.g., boxwoods, California lilacs)
- Certain deciduous shrubs (e.g., lilacs, mock oranges, Forsythia species)
- Certain perennials (e.g., pick, hellebore)
Part 3 of 3: Knowing When You Need to Lower Soil pH
Step 1. Lower the soil pH for shrubs such as rhododendrons and azaleas
Some types of flowering shrubs, such as rhododendrons and azaleas, require moderately acidic soil to grow. This plant often originates in areas of high rainfall, such as the Pacific Northwest in the United States (high rainfall tends to lower soil pH). For a shrub like this, a pH range of 4.5 - 5.5 is optimal growing conditions. However, a pH of 6.0 is usually acceptable.
Step 2. Lower the soil pH for flowering plants such as begonias and hydrangeas
Many brightly colored flowers such as petunias and begonias grow best in acidic soil. For some of these flowers, changing the acidity of the soil from "slightly" acidic to "very" acidic can bring about a noticeable change in flower color. For example, growing hydrangeas in soil with a pH between 6.0 - 6.2 will give pink flowers, whereas lowering the soil pH to around 5.2 - 5.5 will give purple/blue flowers.
The blue color of hydrangeas grown in low pH soil comes from aluminum. When the soil pH is low, it is easier for hydrangeas to absorb aluminum from the soil, which will show up in their petals
Step 3. Lower the pH for evergreens
Many perennial evergreens grow in slightly acidic soil. For example, spruce and pine can grow well at a soil pH of around 5.5 - 6.0. In addition, straw from these plants can be incorporated into neutral or alkaline soils as organic matter which will lower the pH along with straw rot.
Step 4. Lower the soil pH for a particular berry tree
Perhaps the plant best known for its acidity is the blueberry, which grows well in very acidic soils (ideally 4.0 - 5.0). However, some other berry species also like acidic conditions. For example, cranberries grow best at a pH of around 4.2 - 5.0, while gooseberries, currants, and elderberries grow best at a pH of around 5.5 - 6.5.
Step 5. Lower the pH to just below neutral conditions for ferns
Most types of ferns grow well in soils with a pH below 7.0 - even plants that grow better in alkaline conditions can also grow in acidic soils. For example, Maidenhair Fern grows better at a soil pH of around 7.0 - 8.0 but can grow even in soils with a pH of 6.0. Some ferns can even grow in soil with a pH of 4.0.
Step 6. Look up gardening instructions for a list of plants that like acidic conditions
The number of plants that can grow or grow well in low pH soils is too great to list in this article. For more information, you may need to read botanical books. You can find them at garden supply stores, or specialty bookstores, although many resources are also available online. For example, The Old Farmer's Almanac's official website has a list of suitable pHs for many plants (you can access them here).
Tips
- Several chemicals that can change soil pH are available in spray form.
- Plants grown in inappropriate soil conditions will not grow well, because certain nutrients will be bound to the soil and cannot be used by plants.
- Avoid using excessive amounts of chemicals, as they can have long-term adverse effects on the soil, as well as the environment.
- The influence of elemental sulfur will last for several seasons.
- Elemental sulfur is generally best used during the spring, and is very difficult to apply to already planted crops.
- Soil pH can affect everything from how well the soil drains to how quickly it erodes.
- Use natural compost whenever possible. This material will benefit plants by increasing the available nutrients. Composting is also a good way to recycle lawn clippings and kitchen waste.
- Elemental sulfur and compost will facilitate biological reactions, while aluminum sulfate and ferrous sulfate trigger chemical reactions.
Warning
- Too much aluminum sulfate can poison plants.
- If you spill urea, aluminum sulfate, or sulfur on plant leaves, rinse thoroughly with plenty of water. If left unchecked, the plant's leaves can "burn" making your plant appear damaged.
