Soil Chemistry and FertilityUnit 2.2 | 29f ) molybdenum (moO 4 2-)
As with boron, molybdenum is needed only in minute quantities. molybdenum is needed for
nitrogen fixation and for reducing nitrate nitrogen to the amine form (plants take up nitrate-nO 3
but use it to create amines -nh 2 for amino acids). Because of this a molybdenum deficiency can
cause a nitrogen deficiency in plants. molybdenum has been found in quantities sufficient to
be toxic to livestock in forage grown in inland desert areas such as the San Joaquin Valley and
nevada. molybdenum levels tend to be low in highly leached soils.
Symptoms of molybdenum deficiency include:
• Stunting and lack of vigor (induced nitrogen deficiency)
• Marginal scorching and cupping or rolling of leaves
• “Whiptail” of cauliflower
• Yellow spotting of citrus
g) Zinc (Zn2+)
Zinc occurs as impurities in crystal structures and in other compounds in the soil. As it is released
by weathering, it is adsorbed onto exchange sites from where it may be taken up by plants or
leached from the soil. Consequently, soils formed from highly weathered materials (such as
quartz and sandstone) may be deficient. Soils formed from igneous rocks tend to have higher
levels of zinc. Zinc is used in the formation of growth hormones; for protein synthesis; and in seed
and grain maturation and production.
A well-aerated soil is necessary for zinc availability.
Symptoms of zinc deficiency include:
• Decrease in stem length and a rosetting of terminal leaves
• Reduced fruit bud formation
• Mottled leaves (interveinal chlorosis)
• Dieback of twigs after first year
• Striping or banding on corn leaves
h) Cobalt (Co2+)
Cobalt occurs as impurities in crystal structures and in other compounds in the soil. As it is
released by weathering, it is adsorbed onto exchange sites from where it may be taken up by
plants or leached from the soil. Consequently, soils formed from highly weathered materials may
be deficient. Cobalt has not yet been shown to be essential to higher plants, but it is essential in
the symbiotic relationship between legumes and Rhizobia.
i) Chlorine (Cl-)
minute amounts of chlorine are essential to some plants. even touching plants with sweaty
hands may be sufficient to provide sufficient chloride. Chlorine is required for photosynthetic
reactions in plants. however, the quantities needed are so small and chlorine is so abundant that
deficiencies never occur in nature.
Symptoms of chlorine deficiency include:
• Wilting, followed by chlorosis
• Excessive branching of lateral roots
• Bronzing of leaves
• Chlorosis and necrosis in tomatoes and barley
Students’ Lecture Outline