Micronutrients function in plants mainly as cofactors,
Iron: Is a component of cytochromes, proteins in the electron transport
chains of chloroplasts and mitochondria.
Micronutrients can generally be recycled repeatedly, so plants need
very little quantities of these elements from the soil. Yet a deficiency
from a micronutrient can weaken or kill the plant.
Molybdenum: There is only one atom of this rare element for every
60 million atoms of hydrogen in dried plant material.
23.8: The Soil
Fertile soil supports plant growth. It can support abundant plant
growth by providing conditions that enable plant roots to absorb
water and dissolved nutrients.
Three distinct soil layers. The A horizon, or topsoil is subject to
extensive weathering (freezing, drying and erosion). Topsoil is a
mixture of rock particles of various sizes, living organisms and
humus, the remains of partially decayed organic material produced
by the decomposition of dead organisms and fungi. The rocks
provide a large surface area where it retains water and minerals
while also forming water spaced containing oxygen for the roots.
Topsoil is rich in organic materials and is therefore most important
for plant growth.
The soils B horizon, contains fewer organisms and much less
organic matter than topsoil and is less subject to weathering. Fine
clay particles and nutrients dissolved in soil water drain down from
the topsoil and often accumulate in B horizon.
The C horizon is composed mainly of partially broken-down rock
that serves as the “parent” material for the upper layers of soil.