It’s often misdiagnosed, but one common problem growers experience with their geranium crop is improper pH level. When soil pH is not in the correct range for the crop you are growing, it affects the plant’s ability to absorb essential elements from the soil media. This may lead to either major and/or minor element deficiencies and toxicities, which all have unique, visual symptoms in geraniums.
What is pH?
pH is a measure of the alkalinity or acidity of a growing medium. A pH of 7.0 is neutral, below 7.0 is acid, and above 7.0 is alkaline. Most soiless media has a pH of between 5.6 to 6.2 out of the bag. Optimum pH levels for ivy geraniums should be grown at 5.5, zonal geraniums at 5.8 to 6.2, and Stardom floribunda geraniums at 6.2 to 6.5. When the soil pH is maintained within these ranges, problems with deficiencies and toxicities are minimized, assuming proper fertilization schedules are followed.
If a grower begins with a soiless mix with a pH between 5.6 and 6.2, what factors will cause this pH to move out of this range?
The most common culprit is your irrigation water. You should know the pH of your water, and even more important its alkalinity. If your water’s alkalinity is high, an alkalinity of 120 or higher, acid forming fertilizers should be used. If the alkalinity of your water is low, 80 or lower, use a calcium/magnesium fertilizer to raise the alkalinity and pH. A second reason soil pH changes is due to your choice of fertilizer.
Some fertilizers will raise your pH, others will lower your pH. How do you know which fertilizer is right for you?
In order to answer this, you first need to know three things: the pH requirement of the crop you are growing; what the pH of the media you are using; and, the pH and alkalinity of your water. Most fertilizer companies offer water and media testing services to help you determine the correct fertilizer to purchase from them.A third reason soil pH might change is the length the crop growing cycle. All, peat-lite, mixes have lime added to raise the pH of peat which is naturally acidic. After 6 to 8 weeks, the lime starts to be used up. As the available lime diminishes, your soil pH will start to fall. In order to counteract this, you can add more lime to each pot, which is very time consuming, or you can liquid feed a calcium/magnesium fertilizer, which will help keep the pH up.So now you know the pH and alkalinity of your irrigation water, your soil’s pH in the beginning and the pH requirement of the crop you are growing.
What is the best way to keep your pH within the acceptable range during your growing cycle?
TEST your soil once every two weeks. By testing often, you can see what direction your pH is going, up or down. You can graph your test results for easy analysis.. The idea behind frequent soil testing is that potential problems are detected in the soil before the problem manifests itself in the plant. Most greenhouse supply companies sell very good pH test kits, for about $200. Most of these kits will also allow you to test EC and salt levels. You do not have to lose very many 4" zonal geraniums for the test kit to pay for itself.In closing, to maintain your soil pH in the optimum range, please know the following: the pH and alkalinity of your irrigation waterpH requirements of the crop you are growing the initial pH of your media length of your crop growing period
You can then select the proper fertilizers or irrigation water additives to help maintain the ideal pH range. Last and most important, TEST your soil pH every two weeks. This will help you solve potential problems before they start costing you money! If you are interested in a more detailed explanation of minor/major element deficiencies and the symptoms of toxicities, "Geraniums IV," published by Ball Publishing is a recommended source.