Boron (B)

Function:

Boron's (B) major function is in cell division and/or enlargement and carbohydrate transport.

Deficiency:

Deficiency symptoms are expressed as death of the growing point, resulting axillary (secondary) shoot growth. New leaves and stem growth may be distorted. It is important to correct B deficiency when symptoms first appear because growing point death or distorted leaves can not be reversed. B deficiency symptoms will first appear on the upper, young leaves. Excessive levels of K or Ca can have an antagonistic effect on B availability.

Excess:

Symptoms vary, but often appear as interveinal necrosis (often spotty at first) on older leaves.

Confirm your actual B levels by conducting a plant tissue analysis. Root substrate (medium) tests are less reliable for determining B levels.

Monitoring and Management Strategy for Boron (B)

Irrigation Water:

Irrigation water may contain adequate or even excessive levels of B. Test the irrigation water to determine available levels. Supplement with additional B in your fertilization program if B is <0.3 ppm. No additional B may be needed if B levels are between 0.3 to 1.5 ppm. Select fertilizers that do not contain B if irrigation water levels are >1.5 ppm.

Fertilization Rate:

0.3 to 1.5 ppm constant liquid fertilization (see Irrigation Water, above)

Interactions:

Excessive levels of K or Ca can have an antagonistic effect on B availability. B is less soluble in soilless substrates and when the pH is above 6.5.

Tissue Concentration:

Normal range is 25 to 75 ppm. Levels below 15 ppm are considered deficient and above 100 are considered toxic.

Options:

Preplant:

Incorporation of a micro-nutrient mix into your soilless substrate, if required.

Irrigation Water:

Irrigation water may contain adequate or even excessive levels of B. Test the irrigation water to determine available levels. Supplement with additional B in your fertilization program if B is <0.3 ppm. No additional B may be needed if B levels are between 0.3 to 1.5 ppm. Select fertilizers that do not contain B if irrigation water levels are >1.5 ppm.

Continual Fertilization:

1. Most commercial fertilizers that provide micronutrients including B. Remember to add supplemental B applications if using calcium nitrate (Ca(NO₃)₂) + potassium nitrate (KNO₃) and your irrigation water lacks B.
2. Excessive levels of K or Ca can have an antagonistic effect on B availability. B is less soluble in soilless substrates and when the pH is above 6.5.

Corrective Fertilization:

1. Corrective applications of borax or solubor at 6.25 ppm of B (see mixing rates below) for soilless substrates. Visible improvements with the use of the corrective B fertilization will be very slow. It is important to correct B deficiency when symptoms first appear because growing point death or distorted leaves can not be reversed. For Corrective Application: Add 0.75 oz. (21.3 g) of borax (11% B) per 100 gallons of water or 0.43 oz.(12.2 g) of solubor (20% B) per 100 gallons of water.

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