Is your substrate-pH too low?
For iron-efficient crops (such as geraniums, marigolds, lisianthus, and pentas), iron-manganese toxicity is a common problem when substrate-pH drops below 6.0. Once plants show toxicity symptoms of necrotic spots and marginal burn, the affected leaves do not completely heal. The only options become shipping lower-quality plants, taking additional time to produce healthy new growth that will cover the older damaged foliage, or to throw them away. Test corrective actions on a small group of plants first, and rely on prevention rather than cure.
Symptoms from excess fertilizer
Test substrate-pH and substrate-EC to confirm the problem is nutritional, using a standard protocol and a calibrated meter. Symptoms from excess fertilizer (high EC) may look similar to low substrate-pH. There are many other causes of necrotic spots and chlorosis in older leaves of geraniums and marigolds other than low substrate-pH, for example damage from pests and diseases, or pesticide phytotoxicity. Check pH and EC before applying a basic drench, and evaluate crop history.
Steps to rapidly correct low pH levels
- When growing geraniums and other iron-efficient crops, and after you have tested that the substrate-pH is below 6:
- If acidifying irrigation water, stop acidification.
- Change to a basic reaction (high-nitrate) fertilizer (such as 13-2-13) at 150 ppm N or a lower fertilizer rate.
- If plants are beginning to show toxicity symptoms
- Apply flowable lime at 1:100 in cool weather (to slow rate of drying on the foliage) with high volume (ideally through a hose and breaker, leaching out half a soil volume of solution). Wash foliage with water immediately after application to remove residue, and clean irrigation equipment.
- Or apply potassium bicarbonate (KHCO3) or potassium carbonate (K2CO3) at 2 lb/100 gal (2.4 g/L). Rinse foliage to remove residue. Leach the media heavily with a basic-reaction (high nitrate) fertilizer solution that contains calcium and magnesium (such as 13-2-13 or 14-0-14) one day after application in order to remove the high levels of potassium and to restore the nutrient balance.
- Check substrate-pH again after 3 days. Reapply flowable lime or potassium bicarbonate/carbonate if pH remains below 6.0 after 3 days.
- Use potassium bicarbonate or potassium carbonate for flood floors, low-volume drippers, hard-to-reach corners. Otherwise, use flowable lime.
Measure applied water volume and uniformity
- Place a collection tray under your mist area, and calculate applied volume based on water collection or run time of the irrigation nozzles.
- For comparison, the recommendation from Clemson University based on surveying multiple growers in the U.S. is to apply up to 1 quart/square foot (11 L/m2) over 10 days of mist for most annuals and geraniums during peak late winter propagation. If you are applying more than this amount of water, check irrigation frequency and duration settings, and the mist droplet size and distribution.
- Test mist uniformity by laying out trays across benches or booms. More uniform = fewer dry spots.
Use best management rractices (BMPs) in your fertilizer program for a stable pH
- Insufficient pre-plant lime, or the wrong lime type (either too slow-acting or very fast-acting without a residual effect).
- A fertilizer high in ammoniacal nitrogen (more than 25% of all nitrogen is in the ammonium form) used on a continual basis has an acidic reaction, especially when combined with low water alkalinity (60 ppm CaCO3 or below).
- Over-acidification of irrigation water (100-120 ppm alkalinity, with water-pH around 6.0, is the usual target when acidifying a high-alkalinity water).
- Certain plant species, especially geranium, can interact with the media to reduce substrate-pH.