Chapter 37 Plant Nutrition
Outline: A Nutritional Network
• Every organism is an open system linked to its environment by a
continuous exchange of energy and materials.
• In ecosystems, plants and other photosynthetic autotrophs
perform the crucial step of transforming inorganic compounds
into organic ones.
• Plants need sunlight as the energy source for photosynthesis.
• They also need inorganic raw materials such as water, CO2,
and inorganic ions to synthesize organic molecules.
• Plants obtain CO2 from the air. Most vascular plants obtain
water and minerals from the soil through their roots.
• The branching root and shoot systems of vascular plants allow
them to draw from soil and air reservoirs of inorganic nutrients.
• Roots, through fungal mycorrhizae and root hairs, absorb
water and minerals from the soil.
• CO2 diffuses into leaves from the surrounding air through
Concept 37.1 Plants require certain chemical elements to complete
their life cycle
• Early ideas about plant nutrition were not entirely correct and
• Aristotle’s hypothesis that soil provided the substance for plant
• van Helmont’s conclusion from his experiments that plants grow
mainly from water.
• Hale’s postulate that plants are nourished mostly by air.
• In fact, soil, water, and air all contribute to plant growth. • Plants extract mineral nutrients from the soil. Mineral nutrients are
essential chemical elements absorbed from soil in the form of
• For example, many plants acquire nitrogen in the form of nitrate
• However, as van Helmont’s data suggested, mineral nutrients
from the soil contribute little to the overall mass of a plant.
• About 80–90% of a plant is water. Because water contributes most of
the hydrogen ions and some of the oxygen atoms that are
incorporated into organic atoms, one can consider water a nutrient.
• However, only a small fraction of the water entering a plant
contributes to organic molecules.
• More than 90% of the water absorbed by a field of corn is lost
• Most of the water retained by a plant functions as a solvent,
provides most of the mass for cell elongation, and helps
maintain the form of soft tissues by keeping cells turgid.
• By weight, the bulk of the organic material of a plant is derived not
from water or soil minerals, but from the CO2 assimilated from the
• The dry weight of an organism can be determined by drying it to
remove all water. About 95% of the dry weight of a plant consists of
organic molecules. The remaining 5% consists of inorganic
• Most of the organic material is carbohydrate, including cellulose
in cell walls.
• Carbon, hydrogen, and oxygen are the most abundant
elements in the dry weight of a plant.
• Because some organic molecules contain nitrogen, sulfur,
and phosphorus, these elements are also relatively
abundant in plants. • More than 50 chemical elements have been identified among the
inorganic substances present in plants.
• However, not all of these 50 are essential elements, required
for the plant to complete its life cycle and reproduce.
• Roots are able to absorb minerals somewhat selectively, enabling the
plant to accumulate essential elements that may be present in low
concentrations in the soil.
• However, the minerals in a plant also reflect the composition of
the soil in which the plant is growing.
• Some elements are taken up by plant roots even though they
do not have any function in the plant.
Plants require nine macronutrients and at least eight micronutrients.
• Plants can be grown in hydroponic culture to determine which mineral
elements are actually essential nutrients.
• Plants are grown in solutions of various minerals in known
• If the absence of a particular mineral, such as potassium,
causes a plant to become abnormal in appearance when
compared to controls grown in a complete mineral medium,
then that element is essential.
• Such studies have identified 17 elements that are essential
nutrients in all plants and a few other elements that are
essential to certain groups of plants.
• Elements required by plants in relatively large quantities are
• There are nine macronutrients in all, including the six major
ingredients in organic compounds: carbon, oxygen, hydrogen,
nitrogen, sulfur, and phosphorus.
• The other three macronutrients are potassium, calcium, and
• Elements that plants need in very small amounts are micronutrients. • The eight micronutrients are iron, chlorine, copper, zinc,
manganese, molybdenum, boron, and nickel.
• Most of these function as cofactors, nonprotein helpers in
• For example, iron is a metallic component in cytochromes,
proteins that function in the electron transfer chains of
chloroplasts and mitochondria.
• While the requirement for these micronutrients is modest (e.g.,
only one atom of molybdenum for every 60 million hydrogen
atoms in dry plant material), a deficiency of a micronutrient can
weaken or kill a plant.
The symptoms of a mineral deficiency depend on the function and
mobility of the element.
• The symptoms of a mineral deficiency depend in part on the function
of that nutrient in the plant.
• For example, a deficiency in magnesium, an ingredient of
chlorophyll, causes yellowing of the leaves, or chlorosis.
• The relationship between a mineral deficiency and its symptoms can
be less direct.
• For example, chlorosis can also be caused by iron deficiency
because iron is a required cofactor in chlorophyll synthesis.
• Mineral deficiency symptoms also depend on the mobility of the
nutrient within the plant.
• If a nutrient can move freely from one part of a plant to another,
then symptoms of the deficiency will appear first in older
• Young, growing tissues have more “drawing power” than
old tissues for nutrients in short supply.
• For example, a shortage of magnesium will initially lead to
chlorosis in older leaves. • If a nutrient is relatively immobile, then a deficiency will affect
young parts of the plant first.
• Older tissue may have adequate supplies, which they can
retain during periods of shortage.
• For example, iron does not move freely within a plant.
Chlorosis due to iron deficiency appears first in young
• The symptoms of a mineral deficiency are often distinctive enough for
a plant physiologist or farmer to make a preliminary diagnosis of the
• This can be confirmed by analyzing the mineral content of the
plant and the soil.
• Deficiencies of nitrogen, potassium, and phosphorus are the
most common problems.
• Shortages of micronutrients are less common and tend to be
geographically localized due to differences in soil composition.
• The amount of micronutrient needed to correct a
deficiency is usually quite small. Care must be taken,
because a nutrient overdose can be toxic to plants.
• One way to ensure optimal mineral nutrition is to grow plants
hydroponically on nutrient solutions that can be precisely regulated.
• This technique is practiced commercially, but the requirements
for labor and equipment make it relatively expensive compared
with growing crops in soil.
• Mineral deficiencies are not limited to terrestrial ecosystems or to
• Photosynthetic protists and bacteria can also suffer from mineral
• For example, populations of planktonic algae in the southern
oceans are limited by iron deficiency. • In a trial in relatively unproductive seas between
Tasmania and Antarctica, researchers demonstrated that
dispersing small amounts of iron produced large algal
blooms that pulled carbon dioxide out of the air.
• Seeding the oceans with iron may help slow the increase
in carbon dioxide levels in the atmosphere, but it may
cause unanticipated environmental effects.
Concept 37.2 Soil quality is a major determinant of plant distribution
Soil texture and composition are key environmental factors in
• The texture and chemical composition of soil are major factors
determining what kinds of plants can grow well in a particular
• Texture is the general structure of soil, including the relative
amounts of various sizes of soil particles.
• Composition is the soil’s organic and inorganic components.
• Plants that grow naturally in a certain type of soil are adapted to its
texture and composition and are able to absorb water and extract
essential nutrients from that soil.
• Plants, in turn, affect the soil.
• The soil-plant interface is a critical component of the chemical cycles
that sustain terrestrial ecosystems.
• Soil has its origin in the weathering of solid rock.
• Water that seeps into crevices and freezes in winter fractures
rock. Acids dissolved in soil water also help break down rock
• Organisms, including lichens, fungi, bacteria, mosses, and the
roots of vascular plants, accelerate the breakdown by the
secretion of acids and the expansion of roots in fissures. • This activity eventually results in topsoil, a mixture of particles from
rock; living organisms; and humus, a residue of partially decayed
• Topsoil and other distinct soil layers, called horizons, are often visible
in a vertical profile through soil.
• Topsoil, or the A horizon, is richest in organic material and is thus the
most important horizon for plant growth.
• The texture of topsoil depends on the size of its particles, which are
classified from coarse sand to microscopic clay particles.
• The most fertile soils are loams, made up of roughly equal
amounts of sand, silt (particles of intermediate size), and clay.
• Loamy soils have enough fine particles to provide a large
surface area for retaining minerals and water, which adhere to
• Loams also have enough course particles to provide air spaces
that supply oxygen to the root for cellular respiration.
• Inadequate drainage can dramatically impact survival of many
• Plants can suffocate if air spaces are replaced by water.
• Roots can also be attacked by molds that flourish in soaked
• Topsoil is home to an astonishing number and variety of organisms.
• A teaspoon of soil has about 5 billion bacteria that cohabit with
various fungi, algae and other protists, insects, earthworms,
nematodes, and the roots of plants.
• The activities of these organisms affect the physical and
chemical properties of soil.
• For example, earthworms aerate soil by burrowing and add
mucus that holds fine particles together.
• Bacterial metabolism alters the mineral composition of soil. • Plant roots extract water and minerals. They also affect soil pH
by releasing organic acids and reinforce the soil against
• Humus is the decomposing organic material formed by the action of
bacteria and fungi on dead organisms, feces, fallen leaves, and other
• Humus prevents clay from packing together and builds a
crumbly soil that retains water but is still porous enough for the
adequate aeration of roots.
• Humus is also a reservoir of mineral nutrients that are returned
to the soil by decomposition.
• After a heavy rainfall, water drains away from the larger spaces of the
soil, but smaller spaces retain water because of water’s attraction for
the electrically charged surfaces of soil particles.
• Some water adheres so tightly to hydrophilic particles that
plants cannot extract it, while water that is bound less tightly to
the particles can be taken up by roots.
• Many minerals, especially those with a positive charge, such as
potassium (K+), calcium (Ca2+), and magnesium (Mg2+), adhere by
electrical attraction to the negatively charged surfaces of clay
• Clay in soil prevents the leaching of mineral nutrients during
heavy rain or irrigation because of its large surface area for
• Minerals that are negatively charged, such as nitrate (NO3?),
phosphate (H2PO4?), and sulfate (SO42?), are less tightly
bound to soil particles and tend to leach away more quickly.
• Positively charged mineral ions are made available to the plant when
hydrogen ions in the soil displace the mineral ions from the clay
particles. • This process, called cation exchange, is stimulated by the
roots, which secrete H+ and compounds that form acids in the
Soil conservation is one step toward sustainable agriculture.
• It can take centuries for soil to become fertile through the breakdown
of soil and the accumulation of organic material.
• However, human mismanagement can destroy soil fertility within just
a few years.
• Soil mismanagement has been a recurring problem in human history.
• For example, the Dust Bowl was an ecological and human disaster
that occurred in the southwestern Great Plains of the United States in
• Before the arrival of farmers, the region was covered with hardy
grasses that held the soil in place in spite of long recurrent
droughts and torrential rains.
• In the 30 years before World War I, homesteaders planted
wheat and raised cattle, which left the soil exposed to wind
• Several years of drought resulted in the loss of centimeters of topsoil
that were blown away by the winds.
• Millions of hectares of farmland became useless, and hundreds
of thousands of people were forced to abandon their homes
• To understand soil conservation, we must begin with the premise that
agriculture is not natural and can only be sustained by human
• In natural ecosystems, mineral nutrients are recycled by the
decomposition of dead organic material.
• In contrast, when we harvest a crop, we remove essential
• In general, agriculture depletes minerals in the soil. • To grow 1,000 kg of wheat, the soil gives up 20 kg of
nitrogen, 4 kg of phosphorus, and 4.5 kg of potassium.
• The fertility of the soil diminishes unless minerals are replaced
• Most crops require far more water than the natural vegetation
for that area, making irrigation necessary.
• The goals of soil conservation include prudent fertilization, thoughtful
irrigation, and prevention of erosion.
• Complementing soil conservation is soil reclamation, the return of
agricultural productivity to damaged soil.
• A third of the world’s farmland suffers from low productivity due to
poor soil conditions.
• Farmers have been using fertilizers to improve crop yields since
• Historically, these have included animal manure and fish
• In developed nations today, most farmers use commercial
fertilizers containing minerals that are either mined or prepared
by industrial processes.
• These are usually enriched in nitrogen, phosphorus, and
potassium, the macronutrients most often deficient in farm and
• Fertilizers are labeled with their N-P-K ratio. A fertilizer marked
“10-12-8” is 10% nitrogen (as ammonium or nitrate), 12%
phosphorus (as phosphoric acid), and 8% potassium (as the
• Manure, fishmeal, and compost are “organic” fertilizers because they
are of biological origin and contain material in the process of
• The organic material must be decomposed to inorganic
nutrients before it can be absorbed by roots. • However, the minerals that a plant extracts from the soil are in
the same form whether they came from organic fertilizer or from
a chemical factory.
• Compost releases nutrients gradually, while minerals in
commercial fertilizers are available immediately.
• Excess minerals are often leached from fertilized soil by
rainwater or irrigation and may pollute groundwater, streams,
• Genetically engineered “smart plants” have been produced. These
plants produce a blue pigment in their leaves to warn the farmer of
impending nutrient deficiency.
• To fertilize judiciously, a farmer must maintain an appropriate soil pH.
pH affects cation exchange and influences the chemical form of all
• Even if an essential element is abundant in the soil, plants may
starve for that element if it is bound too tightly to clay or is in a
chemical form that the plant cannot absorb.
• Adjustments to soil pH of soil may make one mineral more
available but another mineral less available.
• The pH of the soil must be matched to the specific mineral
needs of the crop.
• Sulfate lowers pH, while liming (addition of calcium carbonate
or calcium hydroxide) increases pH.
• A major problem with acidic soils, particularly in tropical areas, is that
aluminum dissolves in the soil at low pH and becomes toxic to roots.