Archive for Thursday, January 24, 2013

Garden Calendar: Soil pH plays important role in plant growth

January 24, 2013


Gardeners love talking about soil.

We lament high clay content, brag about bountiful fertility and express concern for conservation. We talk about adding compost, insulating the soil with mulch, and about how and when to add water.

We are often missing the important topic of soil pH, though. Although indistinguishable by sight and touch, soil pH affects the way plants take up nutrients and the beneficial organisms that live in the soil. Paying heed to it will result in healthier, more fruitful plants.

Soil pH is technically a measure of acidity or alkalinity. The scale goes from 0 to 14, with a pH of 7 being neutral. Soils with a pH below 7 are acidic and above 7 are alkaline. Most plants and beneficial microorganisms that live in the soil prefer a slightly acidic pH of 6 to 7, although they may tolerate a wider range. A few — like azaleas and blueberries — prefer an even more acidic environment.

Problems in plant growth from high or low pH are typically related to nutrient availability. Ever see a yellow pin oak? The yellowing is most likely a condition called iron chlorosis, the result of an iron deficiency in the tree. In most cases, there is plenty of iron in the soil, but it cannot be taken up by the tree when it is present in soil with high alkalinity. The tree needs iron and will decline with repeated years of deficiency.

For healthier plants, you will need to determine both the pH and the optimum pH for the plant(s) you want to grow. Then you can decide whether to change your plant choices or your soil. You may have to do a combination of the two.

The only way to determine pH is to test soil through a chemical procedure. Home test kits are available at most garden centers and offer a quick, general indication of soil pH. Soil can also be submitted to professional soil testing laboratories for more accurate analysis and recommendations for remediation. Douglas County residents can test up to 10 samples per year for free through K-State Research and Extension-Douglas County, thanks to a grant from the Douglas County Conservation District.

Other area residents can check with their local Extension offices, but soil testing is typically less than $10 per sample.

Testing kits typically contain some information about preferred pH, or you can typically find the information through a Web search or your local Extension office. If you test through a lab, this information will be provided for you as long as you tell the lab what you want to grow.

Of the hundreds of lawn and garden samples that come through my office, almost all have a pH above 7, indicating alkalinity. For fescue lawns and tomatoes, that is acceptable. But for a pin oak or a blueberry plant, that high pH can lead to a slow, torturous death for the plant.

Some examples of plants that are especially pH particular are blueberries, azaleas, rhododendrons, pin oaks, river birch, white pine, strawberries and grapes.

If your soil is alkaline and you want to lower the pH to keep that white pine alive, you will need to add sulfur. You could also use iron sulfate, aluminum sulfate, or sphagnum peat moss, but there are some issues associated with each of these that make them less desirable for use.

Sulfur is available at most garden centers and farm supply stores. Avoid adding lime or wood ashes to alkaline soils as it will raise the pH even more.

If your soil happens to be one of the unusual ones on the low side, lime can be added to raise the pH. In other parts of the country and in many farm fields in our area, lime has to be applied to each year to keep the pH up. In other regions, this is because of the soil’s parent material. Here, it is a result of application of certain forms of fertilizer and leaching of calcium out of the soil from irrigation water.

Even though irrigation water can leach calcium from soil, this is unlikely to help if you are watering from a municipal water supply or water district. In fact, because the pH often runs high in these water sources, it can actually contribute to soil alkalinity (high pH).

One last bit of bad news: Plants are also less tolerant of drought when growing in alkaline soils. If the drought continues and gardeners continue to water with high-pH water, our soil alkalinity problems have just begun.

Information about how to collect and submit a soil test is available from K-State Research and Extension-Douglas County, 2110 Harper St., Lawrence, and on their website,

— Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County. She can be reached at 843-7058.


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