Difference between revisions of "Lower Soil pH"

In chemistry, pH is a measure of how acidic or basic a substance is. The pH scale runs from 0 to 14 &mdash; a pH near 0 is extremely acidic, a pH near 14 is extremely basic, and a pH of 7 is perfectly neutral. In gardening and horticulture, the pH of the soil used for growing plants can have serious effects on the health and growth of the plant. While most plants tolerate a pH of about 6.0-7.5, some grow best under narrow pH ranges, so serious gardeners should learn the basics of soil pH management.<ref name="rf1">https://www.extension.purdue.edu/extmedia/HO/HO-241-W.pdf</ref><ref name="rf2">http://www.organicgardening.com/learn-and-grow/how-lower-your-soil-ph</ref> See Step 1 below to start learning how to lower your soil pH.

#Dig five small holes in the planting area. The pH of your garden soil is easy to determine with a commercial pH testing kit. These are usually sold at hardware stores or garden supply stores and are fairly inexpensive. To begin, you'll want to take a sample from the area you want to test. Dig five small holes (about six to eight inches deep). Select random locations within the plot &mdash; this way, you'll get a sense of the "average" pH of your soil.<ref name="rf3">http://www.lowes.com/cd_Test+and+Improve+Soil_1258058669_</ref> Don't keep any of the dirt from creating the holes.

#Use your testing kit to determine the precise pH level of your soil. Depending on the type of kit you have, the method will vary. For many of the most common test kits, you'll put a small amount of your soil in an included test tube, add a few drops of liquid solution, mix by shaking, and allow the mixture to settle for a few hours.<ref name="rf4">http://www.thegardenhelper.com/acidsoil.html</ref> Eventually, the color of the solution should change &mdash; by comparing the color of the solution to a chart provided with the kit, you can determine the pH of your soil.

#Add organic matter. Many types of organic matter, like compost, composted manure, and acidic mulches (e.g. pine needles) can gradually lower your soil pH over time.<ref name="rf2" /> As organic matter decomposes, bacteria and other microbes grow and feed upon it, creating acidic by-products in the process. Because organic matter needs time to decompose to alter the soil, this option is great for long-term goals, but won't give you dramatic results in the short-term. Many gardeners opt to add organic matter to their soil annually for a gradual, mild pH-lowering effect. However, this will not work for heavy clay soils.

#* Organic matter can also give your soil other benefits &mdash; most noticeably, improving its drainage and aeration.<ref name="rf5">http://www.dummies.com/how-to/content/how-to-improve-garden-soil-with-organic-matter.html</ref>

#Add aluminum sulfate. For rapid pH-lowering, don't rely on the slow, gradual decay of organic matter. Instead, use one of the many acidic soil additives available at your local garden supply store. Of these additives, aluminum sulfate, is one of the quickest-acting options available. Aluminum sulfate produces acidity in the soil as soon as it dissolves, which, for gardening purposes, means that it basically works instantly.<ref name="rf6">http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html</ref> Because of this, aluminum sulfate is a great choice for urgent pH lowering.

#* Depending on the starting pH of your soil, the amount of aluminum sulfate you should use can vary significantly. In ''very'' general terms, you should expect to use about 1.2 pounds of aluminum sulfate to lower the pH of a 10-square foot patch of soil by one on the pH scale (e.g., from 7.0 to 6.0, from 6.0 to 5.0, etc.).<ref name="rf6" /> However, using too much additive can be harmful to your plant, so consult an online resource (like the one [http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html here]) for more precise usage information.

#*Do not use aluminum sulfate for large applications because it can lead to aluminum accumulation and aluminum toxicity in soil.<ref name="rf7">http://web.extension.illinois.edu/cfiv/homeowners/080818.html</ref>

#Add sulfur. Another useful pH-lowering soil additive is sublimed sulfur. Compared to aluminum sulfate, sulfur is generally cheaper, more powerful (in terms of the amount needed) and slower-acting. This is because sulfur needs to be metabolized by bacteria in the soil to be converted to sulfuric acid, which takes time.<ref name="rf6" /> Depending on the moisture of the soil, the amount of bacteria present, and the temperature, sulfur can take up to several months to produce a noticeable effect in the soil.

#* As noted above, compared to aluminum sulfate, you will generally need a relatively small amount of pure, sublimed sulfur to produce an equivalent pH change. In general, you'll need about 0.2 pounds of sulfur to lower the pH of a 10-square foot patch of soil by one whole number on the pH scale.<ref name="rf6" /> Consult an online resource (like the one [http://www.clemson.edu/extension/hgic/plants/other/soils/hgic1650.html here]) for more precise usage information.

#Add sulfur-coated urea. Like sulfur and aluminum sulfate, soil additives containing sulfur-coated urea can increase the acidity of soil over time (lowering its pH). As an additive, urea is fairly quick-acting, producing some effects as soon as a week or two after being introduced to the soil.<ref name="rf8">http://www.extension.umn.edu/agriculture/nutrient-management/nitrogen/fertilizer-urea/</ref> Sulfur-coated urea is a common ingredient in many fertilizers, so if you were already planning on fertilizing your plants, you may want to save yourself the trouble of finding a separate soil additive by simply picking a fertilizer containing this type of urea.

#Add another acidic additive. Besides the additives listed above, many other substances can lower soil pH. Many of these substances are often included in certain fertilizer mixes, while others are sold on their own. The time and amount needed can vary greatly for each, so consult the additive packaging or talk to an experienced employee at the gardening supply store. Additives that can lower your soil pH:<ref name="rf1" />

#Grow alkaline-tolerant plants. If your soil is too alkaline (basic) to grow plants which require acidic soil, raising alkaline-loving plants can gradually lower the pH of your soil over the life of the plant. As the plant grows, matures, and decays, the organic matter that returns to the soil will promote the growth of bacteria and gradually lower the pH level of the soil (similar to how adding organic matter in the form of mulch or manure). This method is generally one of the slowest ways to lower your soil pH, as the plant must grow to begin depositing organic matter in the soil. A few examples of alkaline-tolerant plants:<ref name="rf9">http://www.hortmag.com/headline/plants-for-alkaline-soil</ref>

#Lower soil pH for shrubs like rhododendrons and azaleas. Certain types of flowering shrubs, like rhododendron and azalea plants, require fairly acidic soil to grow properly. These plants are often native to areas with a high amount of rainfall, like the Pacific Northwest region of the United States (frequent rainfall generally lowers soil pH).<ref name="rf10">http://www.rhododendron.org/v46n2p77.htm</ref> For these types of shrubs, a pH range of about 4.5 - 5.5 is optimal. However, pH levels as high as 6.0 are usually acceptable.<ref name="rf10" />

#Lower soil pH for flowers like begonias and hydrangeas. Many brightly-colored flowers, like petunias and begonias, grow best in acidic soils. For some of these flowers, changing a soil acidity from ''slightly'' acidic to ''very'' acidic can produce a visible change in the bloom color. For instance, growing a hydrangea in soil with a pH level of about 6.0 - 6.2 will produce a plant with pink flowers, while lowering the pH  level to about 5.2 - 5.5 will produce a plant with purple/blue flowers.<ref name="rf11">http://www.hydrangeashydrangeas.com/colorchange.html</ref>

#* The blue color of low-pH hydrangeas comes from the chemical aluminum. When the pH of the soil is low, it's easier for the hydrangea to absorb aluminum from the soil, which shows in the flower petals.<ref name="rf11" />

#Reduce the soil pH for certain berries. Perhaps the most well-known acid-loving plant is the blueberry, which thrives in highly acidic soils (generally about 4.0 - 5.0 is ideal). However, several other species of berry also favor acidic soil. For instance, cranberries grow best at a level of about 4.2 - 5.0, while gooseberries, currants, and elderberries grow best at a level of about 5.5 - 6.5.<ref name="rf12">http://www.offthegridnews.com/2013/06/19/25-fruits-and-vegetables-to-grow-in-acidic-soil/</ref>

#Drop the pH to just below neutral for ferns. Most varieties of garden ferns prefer soil pH levels below 7.0 &mdash; even those that prefer alkaline soils can usually tolerate slightly acidic soils. For instance, Maidenhair Ferns prefer soil pHs of about 7.0 - 8.0 but can grow even in soils with levels of about 6.0.<ref name="rf13">http://www.whiteflowerfarm.com/growing-and-planting-garden-ferns.html</ref> Some ferns can even tolerate soils with a pH as low as 4.0.<ref name="rf13" />