Features
Nutritional deficiencies in greenhouse vegetables

The following email was received from a greenhouse cucumber grower which encouraged me to write on nutritional deficiencies, especially boron, in this case. 

“Morning Dr Mirza
I’ve got an issue with my greenhouse cucumbers. They have stopped growing, leaves are small and stiff and now turning brown. Water & fertilizer are pH 5.93 and an E.C of 0.93. I did a soil drench — the water coming out at the soil has a pH of 6.54 and EC of 0.20. 

I have spider mites and thrips. Bios for thrips have been here for two weeks and spider mites bios arrive this week but there are no webs or evidence on these leaves that are at the top of the plant. The issue was first detected shortly after putting on the sachetes for the thrips bios. I originally thought water related and only gave water for three days. But am now back to fertilizer& magnesium and calcium alternating days. I am seeing some discolouration on the larger leaves midway up the plant in these past five days. 

Is this a fungus? Virus? Or heat related? Water? Multiple issues?”

Analysis of information from the grower:

• The plants have stopped growing as evidenced in Figure 2. The tops are dead, leaves are small, brittle and brown. 
• The feed pH was 5.93 and EC at 0.93
• Leach pH 6.54 and EC at 0.2 millimhos/cm
• Grower was using a ready-made mix with 20-12-28, with no calcium and no magnesium. It did contain trace elements.
• The EC of 0.2 millimhos in the leach is extremely low, even below the starvation range. EC value of below 0.8 millimhos are starvation range. This means plants were suffering with lack of nutrients. The feed EC was also very low. The grower was using a rate of 0.5 grams/L
• Boron deficiency was diagnosed from the typical symptoms which include browning, brittle leaves and ultimately death of the top part. Figure 1 shows early symptoms of boron deficiency. It can be easily confused with a deficiency of potassium. 

About the boron:
Boron is an essential element for growth of cucumbers and other plants. It is an immobile element which means that it is not transported from lower leaves to upper leaves like nitrogen and potassium. Thus, deficiency symptoms appear in young growth and fruits. Roots can also become short and stubby. It aids in calcium translocation; thus the deficiencies of calcium and boron may be seen together. 

Figure 2: The leaves of a cucumber plant bear evidence of boron deficiency.

Sources of boron:
The term “borax” is often used for a number of closely related minerals or chemical compounds that differ in their crystal water content. Growers must know what they are dealing with. Here are three examples of borax with different contents of water:

• Borax: Anhydrous borax. (Na₂B₄O₇).
• Borax pentahydrate   (Na₂B₄O₇·5H₂O).
• Borax decahydrate   (Na₂B₄O₇·10H₂O).

Anhydrous borax will contain 21.39 per cent boron, borax pentahydrate will contain 14.8 per cent boron and borax decahydrate will contain 11.28 per cent boron. The amount you need for the same ppm concentration will vary depending on the type of borax used. 

Commercial fertilizers formulation will show on the label as to the percentage of boron and from what source. 

What to do to avoid boron deficiency:
The most critical mistake is in the management of fertilizers.When commercial fertilizers are used then the calculations are based on nitrogen percentage or a range in grams per litre.For example, in this case it was recommended to use a rate of 0.5 grams/L.The trace elements are called “micro-elements” so when lower rates are used, then the concentration of these elements is significantly reduced and may approach critically deficient range. 

Boron availability from root zone is stable across the pH range from 5.0 to 7.0 while iron and manganese are more susceptible to lower and higher pH ranges. 

Dr. Mohyuddin Mirza is an industry consultant. drmirzaconsultants@gmail.com