Download Chapter 37

Plant Nutrition
37
The Acquisition of Nutrients
• All living things need raw materials from the
environment.
• These nutrients include carbon, hydrogen,
oxygen, and nitrogen.
• Carbon comes from photosynthetic organisms or
from CO2 in the air.
• Hydrogen comes from water.
• Carbon, oxygen, and hydrogen are plentiful, and
enter the living world through photosynthesis.
37
The Acquisition of Nutrients
• Nitrogen is in relatively short supply for plants.
• Nitrogen enters living forms first in bacteria, which can
convert N2 in air to forms that are useful to plants.
• Other mineral nutrients essential for life include sulfur,
phosphorus, potassium, magnesium, and iron.
• Plants take up most nutrients as dissolved solutes in the
water of the soil.
Nitrogenfixing
bacteria
form a
nodule in
clover root.
37
The Acquisition of Nutrients
• Plants are autotrophs. They make
their own organic molecules from
CO2, H2O, and minerals.
• Heterotrophs require organic
compounds as food and depend
ultimately on autotrophs.
• Most autotrophs photosynthesize.
• Plants obtain energy from sunlight,
carbon dioxide from the
atmosphere, and nitrogencontaining ions and minerals from
nutrients from the soil.
37
Mineral Nutrients Essential to Plants
• Plants that are deficient in a
particular essential element show
characteristic deficiency
symptoms.
• These symptoms can be used to
determine which elements are
lacking.
• Appropriate fertilizers can be
applied after diagnosing the
specific deficiencies.
• A fertilizer is an added source of
mineral nutrients.
Effect of iron on
bean folige.
37
• Soil provides the
minerals plants
need.
• Soil provides
water, mechanical
support,
microorganisms,
and oxygen for
roots.
• Soils also contain
organisms that are
harmful to plants.
Soils and Plants
37
Soils and Plants
• Soils are complex mixtures of living and nonliving
components, including bacteria, fungi,
earthworms and other animals, particles of rock,
clay, water, dissolved minerals, air spaces, and
dead organic matter.
• The air spaces are a crucial source of oxygen to
plant roots.
37
Soils and Plants
• The structure of many soils changes with depth,
revealing a soil profile.
• Most soils have two or more horizontal layers.
• Minerals tend to leach, or be carried away by
water from the upper horizons, and sink into
deeper layers.
• Soil scientists recognize three major horizons:
 A, the topsoil
 B, the subsoil
 C, the parent rock
Figure 37.3 A Soil Profile
Most organic matter,
roots, earthworms,
insects, and
microorganisms.
Zone of accumulation
of leached materials
from above.
Parent rock
from which
soil is
derived.
37
Soils and Plants
• Agricultural soils often require fertilizer because
irrigation and rainwater leach minerals from the
soil, and the crop harvest removes nutrients.
• Three elements commonly used in fertilizers are
nitrogen, phosphorus, and potassium.
• N-P-K percentages are often labeled on fertilizer
bags. A 5-10-10 fertilizer has 5% nitrogen, 10%
phosphate (P2O5), and 10% potash (K2O) by
weight.
37
Soils and Plants
• Organic fertilizers, such as manure, release
nutrients slowly, which results in less leaching
than occurs with inorganic fertilizers.
• Organic fertilizers also contain residues of plant or
animal materials that help improve the structure of
the soil.
• Inorganic fertilizers provide an immediate supply
of plant nutrients, and can be formulated to the
requirements of a particular crop or soil type.
37
Soils and Plants
• The availability of nutrient ions is influenced by
soil pH. pH 6.5 is optimal for most crops.
• In the process of liming, compounds such as
calcium carbonate, calcium hydroxide, or
magnesium carbonate are added to acidic soil to
raise the pH.
• The pH of soil can be lowered by adding sulfur,
which soil bacteria convert to sulfuric acid.
37
Nitrogen Fixation
• Earth’s atmosphere is about 78% nitrogen in the form of
N2 gas.
• N2 is very stable. A great deal of energy is required to
break the triple bond.
• Some bacteria have an enzyme that allows them to break
the bond and convert N2 into a more usable form, NH3.
• The process is called nitrogen fixation.
• There are only a few species of these essential nitrogen
fixers - Most nitrogen fixation is done by bacteria.
• Some nitrogen-fixing bacteria live free in the soil.
37
Nitrogen Fixation
• Some nitrogen fixers live in close association with plant
roots in a mutualistic relationship.
• In symbiosis, some bacteria fix nitrogen in association with
fungi as lichens.
• Rice farmers increase fixed nitrogen by growing the water
fern Azolla in their rice paddies.
• Some species of bacteria fixes nitrogen in close association
with plant roots, forming root nodules.
37
Nitrogen Fixation
Nitrogen fixation is the reduction of nitrogen gas by the
stepwise addition of three pairs of hydrogen atoms to the N2.
37
• Most plants take up both
ammonium ions and
nitrate ions.
• Nitrifying bacteria
oxidize ammonia to
nitrate.
• Plants take up nitrate
and reduce it back to
ammonia.
• Denitrifying bacteria
return N2 to the
atmosphere, completing
the biological nitrogen
cycle.
Nitrogen Fixation
37
Carnivorous and Heterotrophic Plants
• Some plants that grow in
acidic, nitrogen-poor
environments trap and digest
insects to help augment
nitrogen and phosphorus
supplies.
• These carnivorous plants
include sundews, Venus
flytraps, and pitcher plants.
• Carnivorous plants can
survive without feeding on
insects, but they grow much
faster in their natural habitats
when they succeed in
capturing insects.
37
Carnivorous and Heterotrophic Plants
• Some plants are heterotrophic
parasites that get nutrients
directly from other plants.
• Parasitic plants can be serious
problems for commercial crops
and timber.
Dodder is a parasitic
plant which obtains all
its food via absorptive
organs which invade
its host’s vascular
tissue.
Tree heavily infected with
mistletoe, a parasitic plant.