Plant Care

High pH & Alkalinity

A Cause of Mineral Deficiencies in Hibiscus

Water - hibiscus need it, and we give it to them, usually straight out of the faucet. Whether it comes from your own well or from pipes managed by your water district, the quality of water can vary quite a bit and still be deemed drinkable and safe for people. A few areas of the country have very pure water - mostly rainy areas like the Pacific Northwest. The rest of us use water that has a variety of minerals floating in it or dissolved in it. These minerals can even be good for us, as for example, the extra calcium you might get from drinking water. However, we are talking about plants here, and water is rarely if ever managed so that it is ideal for plants.

We think of water as something we pour into our plants, but water is actually INSIDE plants too. Roughly 90% of a plant's tissue is water. The water we give our plants needs to dissolve the correct amount of nutrients, and then circulate those nutrients through all parts of the plant and right into each individual cell. Two factors that are critically important to the water both outside and inside a plant are pH and alkalinity. Let's take a closer look at both of these.

People sometimes use the word "alkaline" to describe a high pH, which makes these two terms very confusing. Alkalinity can sometimes cause high pH, but they are still two very different things. It helps to understand what each of these terms really means.

What is pH?

In its simplest terms, pH is a measure of the concentration of hydrogen ions in the water. We describe pH as high or basic when the measurement is above 7, neutral when the measurement is right around 7, and low or acidic when the pH is less than 7. Hibiscus can grow well in pH between 5.0 and 7.5, but like it best when the pH is between 5.5 and 7.0 or slightly acidic. If the pH is outside this range, problems with plant nutrition often occur.

How does pH Affect Hibiscus?

The nutrients in soil and plant fertilizers dissolve best and stay dissolved longer in water that is more acidic. Most of us have heard that we should take our daily calcium tablet with a glass of orange juice or some other mildly acidic drink in order to fully dissolve the calcium and make it available for our bodies to absorb. This is exactly how pH works in plant nutrition too. Each type of plant has evolved to absorb the correct level of nutrients from the soil in the specific range of pH that it is accustomed to in its native habitat. Varying the pH levels of a plant makes it absorb either too few nutrients, causing malnutrition, or too many nutrients, causing toxicity. So for optimal nutritional levels, it's important to keep your water within the pH range that works best for the plants you grow. For hibiscus, this is a pH from 5.5-7.0.

What is Alkalinity?

Alkalinity is a measure of the buffers in water, or the ability of the water to neutralize acid and resist becoming acidified. The most common buffers in highly alkaline water are carbonates picked up by water as it passes through sedimentary rock like limestone. Certain types of soil in certain geographic locations are know to be full of alkaline buffers that dissolve easily in water, and the more of these buffers water contains, the more alkaline it is said to be. The test that measures this usually appears on water quality reports as "ppm of calcium carbonate (CaCO3)." The best range for most plants is between 30 and 60 parts per million (ppm), but anywhere from 0-100 ppm is acceptable for hibiscus. Above 100 ppm problems can occur.

How does Alkalinity Affect Hibiscus?

Alkalinity is a fairly common problem for gardeners and hobbyists, particularly in deserts and very arid climate zones where water tables and channels flow through long distances of sedimentary rock and soil. Alkaline water can cause malnutrition for hibiscus by preventing the water the plant uses from acidifying to the 5.5-7.0 pH hibiscus need to properly absorb nutrients. Remember, alkalinity is the ability of the water to RESIST acidifying. So even if you use a good hibiscus fertilizer with the exact pH hibiscus need, the alkaline buffers in your water can stubbornly maintain the wrong pH for your plants, giving your hibiscus a case of malnutrition that slowly starves it.

The worst situation for hibiscus is when the water has both high pH and high alkalinity. This type of water can prevent your plants from absorbing all kinds of nutrients, including essential minerals like iron. Without these minerals, your hibiscus can't make enough chlorophyll, and leaves will turn yellow (chlorotic) no matter how many minerals and fertilizers you use on them. Highly alkaline water can simply prevent the minerals from dissolving in water, so your plants can't use them. As a general rule of thumb for hibiscus, alkalinity of 1-100 ppm is optimum, 100-200 ppm is a mild-moderate alkalinity problem, and above 200 ppm is a high alkalinity problem.

How do I Find out the pH and Alkalinity Levels of my Water?

So, how do you find out the pH and alkalinity of your own water? The easiest way to find out is to call the local Water District that supplies your water. They are required to test the water regularly and should be able to supply you with the results, including the pH and alkalinity. If you use well water for irrigation, you will either need to send a sample of the water to a lab for testing (costs less than $100) or obtain test equipment to test it yourself (costs more than $100 for reliable equipment).

How to Treat Mild-Moderate Water Alkalinity or Slightly High pH

By far the most common problem with water is mild-moderate alkalinity (100-200 ppm) or slightly high pH (7.5). Both these problems can be corrected simply and easily with an acidifying fertilizer. These fertilizers are designed to acidify hard-to-acidify water, and work very well for hibiscus. If your alkalinity levels hover just above 100 (like ours here in Southern California), you will want to use an acidifying fertilizer several times a year. The higher your alkalinity levels are, the more you'll need to use the acidifying fertilizer. If your alkalinity levels average closer to 200, as is sometimes the case in places like Arizona and New Mexico, you may need to use acidifying fertilizer all the time. In drought years, alkalinity can increase in water. So if you are in an area that is currently going through a drought, you may need to increase your use of acidifying fertilizer until the drought ends.

What do I do if I Find my Water is has Other Problems with pH or Alkalinity?

It's not possible to describe all the many possibilities that can occur and what to do about them, so we won't attempt to do that here. If you learn that the pH and alkalinity of your irrigation water are far outside normal ranges, and if your hibiscus are having problems despite receiving good fertilizer, then you can conclude that your situation is more challenging than most and can start finding out what you can do about it. Commercial growers commonly inject acids such as phosphoric or nitric acid into high alkaline water to counter the effects of the alkalinity. This is less practical for home gardeners, but special fertilizers are available that are "acid forming" or that can correct specific nutritional deficiencies. The good news is that if you have water with high alkalinity or low pH most of your neighbors will have it, too. Nurseries and the county Department of Agriculture will be familiar with the situation and most likely will be able to advise you on what to do about it. The Extension Service at the county Dept. of Agriculture is often very helpful to anyone coming to them with questions. Master Gardener groups that exist in many areas are other good sources of local tips on growing successfully in difficult local conditions.

If you need more help than this with a difficult pH or alkalinity problem, or just want to discuss it further, don't hesitate to ask a question or bring up the topic in the Hibiscus Care section of our Forum. We'll do our best to help you there, and you may find other hibiscus lovers with similar problems who can help you too.