Seed and zonal geraniums are iron-efficient crops that are prone to iron/manganese toxicity when the media-pH gets too low. Once plants show toxicity symptoms of necrotic spots and marginal burn (Figure 1), the affected leaves do not completely heal. The only options become shipping lower-quality plants - to take additional time to produce healthy new growth that will cover the older damaged foliage - or throwing plants away.
Therefore, the best approach is to prevent iron/manganese toxicity (and low media-pH) from occurring. Here are some pointers for growing geraniums:
- With normal fertilization practices, the acceptable pH range for iron-efficient crops like geraniums is 6.0 to 6.6.
- Pre-test your root medium to determine its acceptability for growing geraniums. Often, moistening the media to near container capacity and giving it three to seven days to incubate allows the limestone a chance to react and gives a more realistic starting pH measurement than sampling the medium right out of the bag. Indicators that iron/manganese toxicity is more likely to arise for a given medium include:
a. Media-pH that is below 6.0 using any soil test method. If pH is 5.5 or below, plan on applying flowable lime at the second watering of the crop after planting.
b. Media-EC that is above 1.0 (2 water: 1 soil test), 3.0 (SME test) or 3.5 (pour-through).
c. Iron concentrations above 3 ppm (SME test from a laboratory).
- Test water quality at a commercial laboratory at least once a year (more frequently if the well is new, shallow or if it has been an exceptionally wet or dry year). Be particularly attentive to alkalinity concentrations because high water alkalinity (rather than the water pH) has a large effect on increasing media-pH.
- Use the water-quality test results to decide whether to acidify and the type of fertilizer you will use. Be cautious about using acidification with geraniums or other iron-efficient crops. Only acidify the water on geraniums:
a. If high pH and lack of iron in plant tissue tends to be a problem in your greenhouse based on past years of growing geraniums, marigolds, etc.
b. And you are already set up with the appropriate injector system.
c. And you plan to test alkalinity each week through the season to ensure total alkalinity remains at 120 ppm.
d. And water alkalinity before acidification is above 120 ppm.
- There are many situations when water alkalinity is below 120 ppm. For example, it may be impractical to separate the acidified water going to geraniums from all the other crops in the greenhouse. The natural alkalinity of the irrigation water may already be below 120 ppm. There is no question quality plants can be grown at alkalinity levels below 120 ppm. However, the lower the alkalinity level, the more critical fertilizer selection becomes because media-pH may drop over time.
- Do not over-fertilize geraniums. High rates of blended fertilizers increase the concentration of macronutrients and micronutrients such as iron and manganese. Keep media-EC within the moderate range (2:1 test equals 0.4 to 1.0; saturated media extract equals 1.0 to 2.0; pour-through equals 1.5 to 3.0). If media-EC is:
a. 1.0 mS/cm above the recommended maximum, leach with clear water.
b. 0.5 mS/cm above the recommended maximum, reduce fertilizer concentration (for example, by 50 ppm).
c. Within the recommended range, continue with current fertilizer concentration (but be aware of trends of rising or falling EC over time).
d. Below the recommended maximum, increase fertilizer concentration (for example by 50 ppm).
- With standard fertilization practices, a media-pH of 6.0 is the general threshold below which iron and manganese solubility can reach a level where toxicity can occur in geraniums (and other iron-efficient plants). Media-pH can drop below 6.0 because of insufficient lime in the root medium, low water alkalinity combined with a high ammonium fertilizer, over-acidification of the irrigation water, and also the ability of certain plant species such as geraniums to drive the media-pH down over time. Once media-pH is below 6.0, toxicity symptoms can occur in as little as one to two weeks.
- The aggressiveness of your reaction to low media-pH depends upon how low the media-pH has become.
a. If the media-pH is between 5.8 to 6.0;
- If EC is above the recommended range, leach twice immediately with clear water to remove excess salts, including micronutrients.
- If acidifying water, stop acidification.
- Change to a basic fertilizer (e.g. 13-2-13) at 150 ppm N or a lower fertilizer rate, regardless of water alkalinity.
b. If the media-pH has fallen below 5.8, then more drastic measures are needed to raise the media-pH to a safe level (above 6.0). We have seen phytotoxicity when applying potassium bicarbonate above 2 pounds/100 gallons (Figure 2), and leaf-spotting with lime residue after flowable lime applications (Figure 3).
It is up to you to balance the risk of phytotoxicity against the damage caused by low media-pH. As with any chemical application, it is advisable to run a test application on a small number of plants to check for phytotoxicity before applying to the entire crop. Given that caution, we recommend the following:
- Apply flowable lime or potassium bicarbonate immediately (see tips for applications below) and check media-pH again after three days.
- Reapply flowable lime or potassium bicarbonate if pH remains below the target range (pH 6.0-6.6 for the geranium group) within three days.
Tips For Applying Flowable Lime
1. Do not use with low-volume drippers.
2. Dilute solution in stock tank to at least one-half strength.
3. Use the injector at the lowest dilution rate or use a simple proportioner (e.g. Hozon) that is easy to clean.
4. Keep stock tank agitated (one person stirring or place a circulating pump into the stock tank).
5. Apply a generous volume with moderate to high levels of leaching (50 mL per 806 cell, 75 mL per jumbo cell, 100 mL per 4-inch pot, 150 mL per 6-inch pot).
6. Immediately wash foliage before lime dries using clear water with a backpack sprayer or boom to eliminate residue.
Tips for Applying Potassium Bicarbonate
1. Use no more than 3 pounds per 100 gallons at a single irrigation.
2. Apply a generous volume with moderate to high levels of leaching.
3. Apply 50 mL per 806 cell, 75 mL per jumbo cell, 100 mL per 4-inch pot, 150 mL per 6-inch pot.
4. Immediately wash foliage before lime dries using clear water with a backpack sprayer or boom to eliminate residue or tissue damage.
5. The next day, apply a basic fertilizer (preferably without micronutrients) with moderate to high levels of leaching to remove the extra potassium and to reestablish adequate nutrient levels in the media.
- If media-pH is too high, geraniums can show iron deficiency symptoms, especially when combined with low media-EC (Figure 4). When media-pH is above 6.6, fertilize with an acidic fertilizer (30-50 percent of nitrogen in the ammonium form, e.g. 20-10-20) in order to reduce pH. Acidify water to below 120 ppm total alkalinity if this is practical for your operation. If plants appear chlorotic, or pH is not below 6.7 within two weeks, send tissue and soil samples to an analytical lab. The problem may be related to iron deficiency or some other nutrient imbalance.
Simply increasing fertilizer concentration may green up foliage, but check first that media-EC is not already high, or the chlorisis may be due to a problem other than iron deficiency such as poor root health.