Deficiency symptoms:

Cessation of growth of terminal bud. Short internodes. Young leaves turn light green progressing to yellow colour. Deficiencies in cotton include excess shedding of squares and young boll.

Functions in plant:

Promotes early maturity, affects flower set, fruiting quality and yield. Necessary in conversion of carbohydrates into proteins. Boron affects root growth; it is essential for normal germination of pollen grains and the growth of pollen tubes.

Mobility in plant: Quite immobile, causing temporary deficiencies in terminal growth.

Influence of soil pH: Availability declines between pH 5.5 and 7.5
Factors affecting levels: pH and soils with low organic matter are generally low. Drought or liming can induce boron deficiencies.
Level in soil: 10-100 kg total boron per hectare. 1-10 kg available boron per hectare.
Adequate level in plants: Soybeans 20-80 ppm. Cotton 20-80 ppm (2-6 kg removed annually per hectare. Rice 5-15 ppm. Grape 25-50 ppm.

Corecting deficiencies:

For cotton, apply 1 kg in broadcast fertilizer. For spray applications use 0.5 kg per hectare as foliar spray.

Sensitive crops:

Alfalfa and cotton require more. Soybeans and peanuts least.

Remarks:

The range between deficiency and toxicity is narrow. 1.5 kg per hectare can reduce peanut yields while 0.5 kg per hectare will prevent hollow heart in peanuts. Boron toxicity is characterised by yellowing of new leaves at the tips. Progressive dying of the leaf which shows a burned or scorched appearance follows this.