Features
Springtime production: Preventing toxicity and deficiency

The pictures accompanying this article, were selected for the purpose of emphasizing what can happen if pH is not properly monitored and issues corrected early enough in the season. In many cases, by the sale time, plants will become unsaleable.  

Iron inefficient

It is important to note that plants like petunias, pansies, bacopa and calibrachoa are fast growing, require regular levels of nitrogen above 200 parts per million (ppm) and change the pH of their growing medium towards the alkaline side. Once the pH starts going above 6.2, they begin to show deficiency of iron. That is what is seen in Image 1. This group of plant is called “iron inefficient” because they show iron deficiency quickly in spite of the fact here is enough iron in the feed. 

Most of the time when iron is deficient, manganese is also deficient. A sharp eye, experience, and a tissue test can confirm that. Typical iron deficiency occurs in the youngest leaves: the major veins remain distinctly green and interveinal area, that is between the veins, turns yellow. In case of manganese deficiency, even smaller veins which are spread through the leaves start losing their green colour and a “netting” pattern becomes more visible.  

Many times, I’ve had growers have ask, in reference to Image 1, why the plants on the left side are perfectly green and not showing any signs of deficiency? The reason appears to be the growing medium. 

Sometimes, when lime is added with the peat moss to raise the pH, the mixing may not be adequate and thus the pH of the starting medium is lower to begin with. The growing media manufacturing companies add enough lime to guarantee a certain pH (e.g. 5.0). The added lime also takes a long time – two to three weeks – to react after wetting. I always advise growers to add potassium bicarbonate 1lb/3.2 ft³ bale which will raise the pH to the desirable range of 5.8 to 6.0 

Iron efficient

Geraniums, on the other hand, are iron-efficient, meaning that when pH goes on the acid side, the uptake of iron and manganese increases almost four times, thus resulting in toxicity. 

The speckling seen on leaves (like in Image 2) is an accumulation of iron and likely manganese as well. Once speckling symptoms appear, the damage is done, and the affected leaves have to be removed. The desirable pH range for geraniums is between 5.8 and 6.4. A pH of less than 5.8 the iron toxicity starts, and at more than 6.5, a deficiency of iron and manganese occurs. 

So, here are the steps one can take to correct the pH, which should be the priority. 

Most of the time there is enough iron supplied through irrigation although I have seen some poorly designed fertilizer programs with iron less than 0.5 ppm in the feed program. Some growers tend to foliar feed iron chelate or iron sulfate which may not be effective, and plants can be damaged due to very high rates. Image 3 is an example of where a grower used a rate of one pound/100 gallons of 13 per cent iron chelate based on an internet search. It comes to 130 ppm and that is very high. Consult a professional to help in such matters. This grower contacted me after the damage was done to ask if the plants will recover. When foliar feeding is done, the spray solution also lands on the growing medium and causes damage. 

Key takeaways

• First fact to understand is that the pH (hydrogen ion concentration) is logarithmic, and sometimes it is difficult to visualize this aspect. Simply stated that with one pH unit, the hydrogen ions will change by a factor of 10. pH of 7.0 being neutral, a pH of 6 will have 10-times more hydrogen ions than a pH of 7.0 and a pH of 5 will have 10-times more H+.
• When taking corrective actions to bring pH down or up, it may take two to three weeks before target pH is achieved and that is very important to make the plants saleable or not. Take the example of the geraniums in Image 2. The picture is dated 05/05 and customers are coming into the greenhouse. If you are starting the remedial measures at that time, it will take about 15 to 20 days to bring the pH to the desired levels. So, monitor the geraniums and other plants regularly and watch for any trends in pH up and down before the symptoms appear. 
• For plants deemed “iron inefficient,” petunias and others, bring the pH down by adjusting the pH of the feed solution or water. The lowest you may go is around 5.5. Many growers use ammonium-based fertilizers sometimes which helps to bring the pH down. 
• In case of “iron efficient” plants like geraniums, the pH needs to be brought up. I have seen cases where pH has gone as low as 4.5 and damage is significant. Potassium bicarbonate is commonly used to bring the pH up. When pH is that low then it is recommended to stop fertilizing because plants are not going to take up nutrients and further toxicities may occur. Some precautions with the use of potassium bicarbonate are suggested like wash the leaves after application and potassium can increase significantly in the root zone. Watch the Electrical Conductivity (EC) closely and leach with water if needed. 

Dr. Mohyuddin Mirza is an industry consultant in based in Alberta and can be reached at drmirzaconsultants@gmail.com.

More on bedding plant production from other experts.

Read more from Dr. Mirza in Greenhouse Canada here.

• Tweet

• Greenhouse business sale or transition takes years and specialized knowledge
• KingsOne Farms breaks ground on Kingsville greenhouse