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When micros become macros

Diagnosing and correcting iron and manganese and magnesium deficiencies in greenhouse plants

June 10, 2024  By Dr. Mohyuddin Mirza


1. A hanging basket of petunias shows the early signs of iron deficiency.

Iron and Manganese deficiencies are the most common in greenhouse crops and did you know that the pH is the major reason for that? 

When we talk about plant nutrients which are considered “essential” they are divided in two categories: micros or trace elements and macros or major elements. This classification of micros and macros is based on the quantities needed by plants for optimum growth and development. For example, nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are” macros” because they are needed in 50 to 300 mg/L in the root zone while all the “micros” range from 0.12 to 3 mg/L. Micros include iron, manganese, copper, zinc, boron and molybdenum. Researchers are constantly adding more micros to the list of “essential” nutrients like silicon, chloride and recently iodine. 

The point is that plants are non-selective in root uptake of elements and their presence in plant tissues does not indicate they are essential for growth. Furthermore, any of the essential nutrients can become a limiting factor for growth and growers must be aware of visual symptoms so that corrective measures can be taken quickly rather than waiting for a growing medium test and leaf analyses. 

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What are typical symptoms of iron and manganese and magnesium deficiencies?
In Image 1, of a hanging basket of petunias, symptoms appeared on young leaves in the top four inches of shoots as leaves showing what is called “interveinal” chlorosis. Loss of colour in between the green veins. The degree of colour loss is progressive and if not corrected they will turn whitish in colour. The plant energy balance changes towards reduced photosynthesis up to 50 per cent and more. Even the flowers start showing loss of colour and they appear duller. This is typical iron deficiency and manganese may also be deficient. 

Images 2 and 3 are from a pumpkin seedling crop destined for field planting. Image 2 shows symptoms of interveinal chlorosis but on more mature leaves. Veins are green and interveinal areas are yellow. So just that the symptoms are present on recently mature leaves, it is in the sphere of magnesium deficiency. Magnesium is part of chlorophyll and photosynthesis is reduced. In image 3, younger leaves show iron and manganese deficiency. The netting pattern where smaller veins are green and the area around them is yellow indicates manganese deficiency as well. 

Images 4 and 5 highlight the differences between magnesium and iron deficiencies in cucumber plants. Image 4 shows a field crop of cucumbers grown in a greenhouse and then planted outside and Image 5 is a picture of magnesium deficiency in regular long English cucumber crop with serious deficiency of magnesium. The reasons are different in both cases. The field cucumber crop showed magnesium deficiency symptoms due to very high potassium in soil, about four-times more potassium than magnesium and an antagonism occurred, while the greenhouse cucumbers fertilizer program had magnesium levels were less than half of that required. 

Reasons and measures to correct these deficiencies
Once a tentative diagnosis is made from the visual symptoms the next step is to investigate why this is happening. That is where your expertise and experience is very important. Here are some steps to take to correct the problem:

Review your fertilizer program and check if magnesium, iron and manganese have been miscalculated to prepare the stock solution. I have seen mistakes happening when ‘magnesium’ and ‘manganese’ get confused and reversed the amounts added. For example, in one case, the person who prepared the stock solutions added manganese at amounts calculated for magnesium. Magnesium is required around 50 to 70 ppm while manganese needed is around 0.8 to 1.0 ppm. In this case, manganese became toxic and magnesium became deficient. Good management practices include keeping written records of amounts added for each batch of stock solution prepared. 

Know the facts about antagonism between calcium, potassium and sodium against magnesium, between iron and manganese. Proper ratios of elements are important from an uptake by the roots. 

The major factor in creating deficiencies and toxicities is the pH of the root zone where roots are functioning to absorb the nutrients. If the fertilizer program is perfect, delivery through irrigation is proper, climate is good, then remember that with commercial soilless growing media pH values going over 6.4, iron and manganese deficiencies will start showing up. The pH will start creeping up when plants are growing fast in the vegetative phase and more roots are developing.

On the other hand, pH will start creeping downward when the plant is setting fruit and root health is not perfect. So, focus on monitoring and managing pH on a regular basis and make adjustments as quickly as possible. The fact is that iron and manganese uptake is reduced when pH starts going over 6.4 and uptake is significantly higher when the pH starts going below 5.5. 

Foliar feeding to correct magnesium deficiency does help although the major focus should be root zone adjustments. For iron and manganese deficiencies may also benefit but use proper rates of application.  


Dr. Mirza is a greenhouse industry consultant in Alberta and can be reached at drmirzaconsultants@gmail.com.


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