Plant Peril: Soil Too Acid or Alkaline
When a plant is ailing, the soil it’s in may be the culprit.
The soil’s degree of acidity or alkalinity--it’s pH--is one of the last things gardeners check, when it should be the first.
“Many plant health problems are not caused by disease, insects or nutritional deficiencies, but rather by soil that is too acid or too alkaline,” said Bob Denman, co-owner of Denman & Co., a gardening tool store in Orange.
Soil pH is vital to plant health. “If it is too low or too high, many nutrients cannot be released to the plants,” said research plant physiologist Darren Haver.
“A common example of this is phosphorus, which needs a pH near neutral to be available,” said Haver, who is with the Department of Environmental Sciences at UC Riverside. “Without this essential nutrient, plants can’t perform variety of important functions, such as photosynthesis, and root and flower growth.”
Another common problem is chlorosis in citrus.
Though this is actually due to iron deficiency in the plant, it’s not always a lack of iron in the soil that leads to it. The soil may be too alkaline, and iron is best absorbed in acidic soils.
Other important nutrients such as calcium, magnesium and nitrogen can also be tied up if pH isn’t correct.
Soil pH can also have an effect on the activity of soil microorganisms such as fungi and bacteria. A pH reading that is too high or low will lead to a loss of these microorganisms, which will result in a less healthy soil overall.
In addition, pH affects the solubility and potency of certain toxic chemicals, such as aluminum, which can be taken up by plants if the pH is off.
Burn in plants is another by-product of a pH problem, added Joe Sweazy, technical services associate with Environmental Test Systems Inc., in Elkhart, Ind., which carries Accugrow soil-testing kits.
“A lot of people will look at tip burn and think lack of water or overfertilizing, but it could be the pH,” he said.
Soil pH is a scale of acidity-alkalinity that ranges from 0 to 14, with the most common levels found between 4 and 8. Seven is neutral.
Readings above 7 show alkalinity; readings below, acidity. In general, most plants do best between 6 and 6.5. Some plants, such as azaleas, like more acidic conditions, and others, such as some California natives, require alkalinity.
Each full point up or down the scale represents a tenfold increase or decrease in the degree of soil acidity or alkalinity.
For example, a soil with a pH of 6 has 10 times more acid than one with a pH of 7 and one with a pH of 5 is 100 times more acidic than the 7 soil.
“A full point change can mean the difference between life or death for certain plants,” Denman said.
In Orange County, as with much of the western United States, soils are generally alkaline, Denman said.
“PH conditions between 7.1 and 8.5 are the rule, and higher readings are not uncommon,” he said. “This is because we don’t get enough rainfall, which is slightly acidic, to flush naturally occurring alkaline salts out of the soil.
“In areas of the country where rainfall is abundant, such as the Pacific Northwest and the East and Southeast, soil tends to be acidic.”
Testing Your Soil
Knowing that your soil is probably alkaline isn’t enough. Experts suggest testing your soil’s pH on a regular basis.
This is easily done with a testing kit or a meter.
Kits tend to be sufficient if you have a small yard and don’t plan on testing very often. Each kit can do multiple tests and costs $6 to $30, depending on the number of tests available and if it checks for additional soil components, such as nitrogen.
A pH meter, however, can be used indefinitely, requiring only occasional calibration. Experts suggest staying away from less expensive meters because they can be off by as much as 1.5 points. Accurate models generally cost $50 to $70.
No matter what you use, it’s important to take representative samples of the soil you’re testing.
Take three or four samples, two to six inches deep.
Tests are performed by mixing the soil with water. It’s important to use water that is neutral-pH or it will skew your results. Distilled water tends to be neutral. If you are unsure, check the water’s pH before testing.
Keep in mind that soil pH will change over the year, especially when soil temperature changes. It will also be different throughout your yard.
Areas near concrete, for instance, will tend to be more alkaline. Fertilizer will also alter the soil pH, as will water.
Tap water in Orange County tends to be alkaline.
Adjusting Your pH
If your pH needs adjusting, chances are you want to lower it. Generally, moving down a pH point (for instance from 7.5 to 6.5) is relatively easy, Haver said.
“Changing the pH by more than 1 point can be more challenging though,” he said.
If you have alkaline soil and want to grow top-form azaleas, which require constant low pH, consider growing them in containers with an acidic potting soil or replace the soil where you want to plant them with peat moss.
A variety of amendments will acidify the soil, but the best is soil (agricultural) sulfur, Denman said.
“Soil sulfur works fairly quickly, lasts for a while and is pure sulfur, so you aren’t adding other nutrients that you may not need or want. In general, one pound of sulfur added to a 10-by-10 area will take it down a full pH point,” he said.
Keep in mind, however, that the makeup of the soil will also affect how the soil changes. Heavy clay or lots of organic material in the soil tend to act as buffers, causing the soil to resist a change in pH.
Lighter, sandier soil will more readily allow a change in pH. Experts recommend treating the area and retesting in two weeks, a month and two months. If the reading is still unsatisfactory, add more sulfur.
Use caution when applying sulfur during hot weather, as it can throw plants into shock. Denman suggests adding one-fourth of the recommended dose at one-week intervals.
During the cooler months of spring and fall, add the entire amount at once.
Before applying other amendments or fertilizers, check their pH levels, as they will also affect the soil’s pH.
If you have a complete soil test done and it determines your soil is deficient in certain nutrients, such as calcium, potassium or magnesium, consult a certified nursery professional as to what amendments you should use to correct the deficiency.
If you happen to have acidic soil and need to make it more alkaline, use finely ground agricultural lime, according to package directions.
Organize Your Garden
One of the best things you can do for your garden in terms of managing pH is to place plants with like pH needs together.
“Don’t try to grow cymbidium orchids, which like acidity, and alyssum, which likes alkalinity, next to one another,” Denman said. “Neither will be happy.”
* Denman & Co., 401 W. Chapman, Orange. (714) 639-8106. Open Tuesday through Friday, 10 a.m. to 6 p.m.; Saturday, 9 a.m. to 5 p.m.
* For Accugrow soil-test kits, call (800) 589-5551 or check the Web: https://www.Accugrow.com.
* Soil and Plant Laboratory, 1594 North Main St., Orange, (714) 282-8777. Conducts complete soil tests and offers recommendations, including organic suggestions.
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What Can You Grow?
The following plants tend to like alkaline conditions:
The following plants require acidic soil: