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Soil Chemistry & Fertility

Unit 2.2 | Part 2 – 73

As with boron, molybdenum is needed only in minute quantities and is toxic at levels
above what plants require. 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.

g) Zinc (Zn2+)

Zinc activates enzymes that run photosynthesis, helps regulate and combust
carbohydrates, and is part of the synthesis of the plant hormone auxin. It is also key for
seed and grain maturation and production.

Symptoms of zinc deficiency include:

i. Decrease in stem length and a rosetting of terminal leaves

ii. Reduced fruit bud formation

iii. mottled leaves (interveinal chlorosis)

iv. Dieback of twigs after first year

v. Striping or banding on corn leaves

Soils formed from highly weathered materials may be deficient in Zn, while soils formed
from igneous rocks tend to have higher levels of Zn. Warm soil temperatures improve Zn
availability, as does a well-aerated soil. high levels of available P can cause Zn deficiency
in plants.

h) others

other micronutrients that may be of importance are:

i. Cobalt (Co2+)

Cobalt has not yet been shown to be essential for plants, but is generally beneficial.
however, it is essential in the symbiotic relationship between legumes and their
associated Rhizobia bacteria.

ii. Chlorine (Cl-)

Chlorine is required for photosynthetic reactions in plants. however, the quantities
needed are so small that deficiencies are rare and usually in places with high rainfall
and sandy soils, where Cl anions would leach out.

iii. Silicon (Si)

While not an essential plant nutrient, Si gives plants mechanical strength and may
help minimize water loss and increase disease resistance. Plant pathologists are
especially interested in the potential for improving disease resistance.

Lecture 2: Plant Nutrient Requirements & Nutrient Cycles