Production Practices and Quality Assessment of Food Crops. Vol. 1

1) High rainfall conditions where Bo is readily leached from soils as B(OH) 3.
2) At increasing soil pH especially in calcareous soils and soils with a high clay
content presumably as a result of the formation of B(OH) 4 and anion absorption.
3) Under drought conditions, probably because of both a decrease in boron mobility
by mass flow into the roots (Kluge, 1971) and polymerization to boric acid:
3B(OH) 3 B 3O 3(OH) 4 + H 3O + H 2O.
Plant species differ characteristically in their capacity for Bo uptake when grown
in the same soil. This is shown in Table 7 below.
Table 7. Boron content of the leaf tissue of plant species from the same location.
Plant species Boron content (mg/kg dry weight)
Brussel sprouts 050.2
Carrots 075.4
Sugar beet 102.3
Effect of Preharvest Factors 17
Based on Gupta (1979) adapted from Marschner (1989).
The critical deficiency range, expressed as milligrams boron per kg dry weight is
about 5–10 in monocots, 25–60 in red clover, 30–80 in carrots and 40–100 in
sugar beet (Bergmann, 1983). Differences in boron requirements are most likely
related to differences in cell wall composition. High light intensities seems to
increase sensitivity to boron deficiency, by raising the requirements for boron in
the tissue.
Symptoms of B deficiency in the shoots are noticeable at the terminal buds or
youngest leaves, which become discoloured and may die. Internodes are shorter,
giving the plants a bushy or rosette appearance (witches broom). Interveinal chlorosis
on mature leaves may occur and misshaped leaf blades are also symptoms of B
deficiency in leaves. An increase in the diameter of petioles and stems is common
and may lead to symptoms such as ‘stem crack’ in celery. Buds, flowers and
developing fruits drop. In heads of vegetable crops (e.g. lettuce), water soaked areas,
tipburn and brown or blackheart may occur.
In storage roots of celery or sugar beet, necrosis of the growing areas lead to heart
rot. With severe deficiency the young leaves also turn brown and die, subsequent
rotting and microbial infections of the damaged tissue being common. Nodule
number was lower in B – deprived plants of Vigna subterranean Reduction or
even failure of seed and fruit set are also common B deficiency symptoms. There
was also reduced levels of crude fat and protein in soybean seed grown in B
deficient soils (Eguchi, 1999).
The application of boron either to the soil or as a foliar spray, different sodium
borates, including borax or sodium tetraborate can be used. Boric acid can also be
used as foliar sprays. The amount of boron applied varies from 0.3 to 3.0 kg/ha
depending on the requirement and sensitivity of the crop to Bo toxicity.
Boron toxicity may occur when large amounts of municipal compost are applied
(Purves and Mackenzie, 1974), and it is of much concern in semi-arid and arid