Nutrients Plant and Soil - 1
As studied in Biology 211, cells need a variety of atoms and molecules to sustain life.
Animals extract their nutrients, including organic fuel molecules, from the foods they
consume by the process of digestion. Absorbed nutrients are circulated to cells and
tissues, and non-digested materials are eliminated as waste products. Because
animals obtain the vast majority of their nutrients from the foods they eat, obtaining
food resources is a major activity of most animals. (See ecology section).
Most of us are familiar with the nutrient requirements for humans, and we know that if
we fail to make good nutritional choices we may have health problems and nutritional
deficiencies.
Plants, too, have nutrient requirements. Although plants manufacture their own
carbohydrates, proteins and lipids from inorganic materials, they require a number of
minerals and other molecules needed to synthesize their cells' needs. If plants do not
get their needed nutrients, they will have growth problems just like we do.
The ways in which plants obtain their nutrients is quite different from methods used
by animals to obtain food. Plants focus on obtaining inorganic materials from their
local environment rather than searching for appropriate food choices, not surprising
since plants are fixed in location.
Plants are autotrophs. They obtain raw materials from their environment.
Plants need about 18 elements, mostly mineral ions, along with oxygen, water,
and carbon dioxide.
Plants process these nutrients into their needed organic and inorganic molecules
for plant structure and function.
Plants produce few waste products because they have no need to extract
nutrients from pre-formed organic materials (like we do), and their fuel needed
to do cell work is provided by photosynthesis
In this section we will look at what nutrients are needed by plants and how they get
them from their environment. We may also touch on environmental change as it
impacts plants and ways some plants have to augment their mineral uptake.
Plant mineral requirements
In addition to carbon, hydrogen, and oxygen, which comprise about 98% of the fresh
weight of the plant, several other chemical elements, called the essential inorganic
nutrients, are needed for plant growth. Nine of these, including nitrogen, potassium,
calcium, phosphorus, magnesium, and sulfur (along with carbon, hydrogen, and
oxygen), are required in relatively large amounts and are known as macronutrients.
The remaining essential inorganic nutrients: iron, chlorine, copper, manganese, zinc,
nickel, molybdenum, and boron, are required in very small, or trace amounts (as little
as a few parts per million), and are known as micronutrients. The micronutrients
function primarily as cofactors. A chart of plant mineral nutrients and their functions
follows (Oxygen, carbon and hydrogen are often not included in plant mineral requirement lists):
Plant Nutrients and Soils - 2
Plant Nutrients and their Functions
Plant Nutrients and Soils - 3
Mineral Deficiencies in Plants
Plants, just like people, may exhibit a variety of mineral deficiency symptoms. We
should be able to examine some of these first-hand in the greenhouse. The specific
symptoms depend on the function of the nutrient, and the mobility of the nutrient
within the plant. Plants lacking nitrogen or phosphorus often produce anthocyanin
pigments in stems and leaves. Necroses at leaf tips and margins are symptoms of
potassium deficiency. Magnesium, for example, is required for chlorophyll synthesis,
so plants lacking magnesium are chlorotic, or yellowish. Nutrients that are mobile will
be translocated from older parts of plants to newer growth, and deficiency symptoms
of those nutrients will appear in older growth first. Deficiencies of non-mobile
nutrients will appear in new growth. Calcium deficiency, for example, is exhibited in
new growth. The terminal bud usually dies. A key to mineral deficiency symptoms is
reproduced at the end of these notes. (As an aside, the three most common deficiencies in
plant growth are nitrogen, phosphorus and potassium. Micronutrient deficiencies are rare and
localized.)
Mineral deficiency in plants is studied by setting up hydroponic cultures that have
nutrient solutions, each of which is missing one nutrient to be compared with a
complete solution, all grown within the same environmental situation.
Complete
Chlorine deficient
Potassium deficient
Lacking
Complete nutrient solution
Copper deficient
Phosphorus deficient
Zinc deficient
Magnesium deficient
Plant Nutrients and Soils - 4
How Do Plants Obtain Mineral Nutrients?
Plants obtain their nutrient needs from air and substrate (generally soil). John Baptist
van Helmont demonstrated in the 17th century that increase in plant growth weight is
primarily from water. He grew his famous willow tree for five years and weighed the
soil and the plant at the beginning and at the end of five years. The soil lost just
0.06kg while the willow gained 76.8 kg. Van Helmont concluded that plants grew
mostly from water. A century later, Stephen Hales established that air (CO2) also
provided materials for plant growth.
Obtaining Gases from the Atmosphere
The gases needed for plant growth, CO2 (needed for photosynthesis) and O2, (needed
by all living cells) are atmospheric gases. As we have discussed, CO 2 diffuses into the
plant through open stomata. Excess oxygen produced in photosynthesis will diffuse
out of the plant through stomata as well.
Other Gas Exchange Mechanisms
Since all living cells of plants also require O2 for cell respiration, there are mechanisms
in plants for gas exchange besides stomata. Oxygen produced during photosynthesis
does not diffuse to all parts of the plant. Substances will diffuse along the easiest
route and gradient, and much oxygen is lost through stomata. However, plants have a
number of ways to get oxygen to cells.
Oxygen Routes
Primary growth stems have stomata in the stem epidermis.
Secondary growth stems have lenticels in the cork (bark)
Roots obtain oxygen by diffusion through the root epidermis, which is not
cutinized. Oxygen competes with water for essential soil spaces, and water
saturated soils may cause the roots to suffocate from lack of oxygen.
Submerged aquatic plants must obtain both CO2 and O2 from gases dissolved in
water, and growth rates are generally lower, since amounts of O 2 and CO2 are
less in water than in the atmosphere.
Plant Nutrients and Soils - 5
Obtaining Water and Minerals from the Soil
Water and most minerals are obtained from soil. Hence it is valuable to learn
something about soil when discussing nutrient requirements of plants. Water moves
into roots from the surrounding soil spaces by diffusion. Most minerals must be
absorbed as water-soluble ions, which move into the root dissolved in water. Water
from rainfall percolates through soil spaces and forms a film around soil particles.
Soluble ions (minerals) dissolve into H2O from the surrounding soil
There is a competition for minerals by root absorption versus minerals leaching
through soil as water percolates past root region.
Water and dissolved minerals move into root in the root hair region by
diffusion and active transport
From the root hairs, nutrients move through the cortex, along apoplastic (cell
wall and intercellular spaces) and symplastic pathways (using plasmodesmata
as well as transmembrane passage) and into the stele via the endodermis.
From the stele, most minerals are moved along with water in xylem throughout
the plant.
Recall that mycorrhizae are very important in mineral absorption for many plants,
and accumulate a higher mineral concentration for plants. Mycorrhizae "infection" is
promoted by the secretion of strigolactones by roots that stimulate the growth of
the fungal hyphae (cells)toward the root. Mycorrhizae significantly increase the area
for absorption and actively absorb minerals that are naturally in low concentration in
soil. Both ectomycorrhizae and endomycorrhizae are common.
Recall, too, that nitrogen fixation by bacteria is also critical for nitrogen availability.
(See later with mineral cycling and the Nitrogen cycle.)
Plant Nutrients and Soils - 6
Soil Properties and Mineral Absorption
Soil and its relationship to mineral needs of plants is critical. Soil serves as the:
Reservoir for many mineral ions
Storage for some mineral ions
The rate of mineral absorption is affected by:
Available concentration of the mineral in the soil
o Oxidation may make some minerals more available
o Hydrolysis frees some minerals from parent rock
o Acids such as carbonic acid, free some minerals from parent rock
Solubility of the available form of the mineral in water
The solubility of many minerals can be altered by changes in soil conditions and other
soil substances. Because of this, it is useful to look at soil as it relates to the mineral
requirements of plant.
Soil Components
The origin of any soil is the parent rock, of whatever type, which is "weathered" to
tiny particles by mechanical and chemical processes (mostly involving water). The
mineral component of soil is determined by the parent rock of that area. Living
organisms affect the formation of soil from parent rock, as well. Lichens, mosses and
fungi are especially important in soil formation.
Any soil
"community" includes:
Mineral particulates
Living organisms
Air and water spaces (30 - 60% of soil volume)
Humus (decayed and decaying organic material) component
Plant Nutrients and Soils - 7
The availability and concentration of minerals is critical for growth. If a needed
mineral is absent from the soil, the plant cannot grow properly, if at all. Mineral
cycling, which involves the entire food chain, is an important process for ecosystems.
(See ecology section). Disturbed ecosystems may lose minerals in many ways,
diminishing plant growth. Soil erosion or other soil loss also depletes available
minerals.
Soil particle size is very important for plant growth, and particularly for water and
mineral absorption and for oxygen availability. We generally have three particle size
ranges:
Clay (less than 2 um
Silt (2 20 um)
Sand (20 - 400 um)
Sand, the largest particle, makes soils that are porous, with good air spaces. Water
and minerals leach through sand very well, however, so sandy soils are often dry and
mineral poor. Sandy soils can often be saline when found in hot dry areas because
water evaporates faster than it can perk, leaving mineral deposits on the soil surface.
Clay particles are tiny, and clay soils have few air spaces. The clay particle size
attracts water and many of the needed minerals, especially those with positive
charges, because the surface of clay particles is negatively charged. Clay soils can
easily become water logged.
Mineral Uptake
Plants secrete H+ to release positive ions from soil particles, particularly clay, a
process called cation exchange, making the ions available for uptake by roots.
Negative ions dissolve directly in soil water for uptake, but are more easily
leached from soils when water perks through the A horizon, and may be less
available to the plant than positive ions.
Soil pH will also affect ion uptake. Acid soils may inhibit ion solubility.
Absorption of water and minerals
Cation exchange with clay soils
Most soils have a mixture of particle sizes. Loam for example, has nearly equal
proportions of all three particle types. Since liquids displace gases, water-logged soils
are oxygen poor. One of the most common causes of house plant death is root rot,
which results when roots suffocate.
Plant Nutrients and Soils - 8
In addition, soil is layered; not all soil layers provide growth conditions for plants.
The topsoil, or A horizon, contains living organisms, partially decayed organic
material and nutrients, which are readily available through nutrient cycling.
Water perks through the A horizon. The depth of the A horizon is important
for plant growth.
The B horizon contains less much organic material, but is a region of
deposition of materials that leach from the topsoil.
The C horizon is the soil base above the parent bedrock.
The major agricultural areas of the world are characterized by having soils called
Chernozem soils, which are calcium-rich with lots of humus and loamy texture. Acid
soils, such as the podzols of many conifer forests and the iron or aluminum-rich
laterite soils of the tropics are not well suited to cultivation and form "Hard-Pan"
with disturbance. High aluminum content in soils is also toxic to many roots.
Plant Nutrients and Soils - 9
Mineral cycling is affected by agricultural practices that leave soil open to wind
and water erosion. Many feet of top soil can be washed away in floods or by wind,
as documented in the 1930s dustbowl era in United States dry land farming. Most
agriculture relies on the use of added fertilizers to supply minerals that are not
returned by nutrient cycling. (See ecology section for nutrient cycles in the ecosystem.)
Fertilizers may be organic or inorganic. There are benefits and detriments to both.
Any added fertilizer is potentially a water pollution problem since minerals leached
from soil by rain or irrigation may wind up in waterways. In spite of adding fertilizers,
soil depletion is a serious problem in agriculture.
As discussed earlier, nitrogen-fixing bacteria are an important source of nitrogen for
plants. The Rhizobium bacteria that form root nodules in legumes are the best known
nitrogen-fixing associates. Such bacteria can convert atmospheric nitrogen into
ammonia, the first in a series of bacterial reactions that provides soluble nitrate ions
for plant use. Each legume has a host-specific Rhizobium. Chemical signals
(flavenoids) from legume roots stimulate Rhizobium to activate a set of bacterial Nod
(nodule) genes that produce enzymes that affect root hair growth and formation of
the plasma membrane infection thread as a means for the Rhizobium to invade the
root cortex cells. Once within the cortex cells, Nod factors promote dedifferentiation
and cell division to initiate the nodule formation.
Rhizobium root nodules
Nodule, xs
Plant Nutrients and Soils - 10
Nitrogen fixation is just one step in making nitrogen available for plants. A series of
soil bacteria are involved with providing plants with the soluble form of nitrogen
needed for plant growth.
Many other plants have nitrogen-fixing associates. To provide adequate nitrogen for
rice paddies, an aquatic fern, Azolla, is grown in the rice paddies because it houses a
cyanobacterium, Anabaena, in its cells that fixes nitrogen. Alder trees in the Pacific
Northwest have bacterial associates (Actinobacteria) for nitrogen fixation, and there
are a number of free-living nitrogen fixing bacteria, as well.
Azolla sporophyte
Anabaena (arrows) in
Azolla female gametophyte
Available water source is also critical to agriculture. The Columbia River
Hydroelectric Power System originated as a means to provide irrigation water for dry
land farming. The benefit of generating electricity came later, and the detriment to
salmon was not addressed for several decades. This doesn't even touch on the loss
of habitat from flooding caused by dams. Drawing of irrigation water from
underground aquifers has seriously depleted water reservoirs. The use of the
Colorado River for irrigation has led to severe pollution and insufficient water for its
terminus in Mexico. In many parts of the world, parasitic infections abound in
irrigation areas. Siltation in dam reservoirs is another concern.
Plant Nutrients and Soils - 11
Use of pesticides and herbicides to minimize crop loss is a whole other issue, not to
be addressed at this time.
In spite of these issues, or perhaps because of these issues, there is much emphasis
today in agriculture to use practices for sustainable agriculture, practices that
minimize erosion such as minimal tillage, contour planting and windbreaks are used.
Soils are monitored for acid/base balance and mineral needs to use fertilizers more
judiciously, along with crop rotation using crops that require different mineral
proportions on different years. Research is conducted on how best to irrigate for
plant needs, conserve water and reduce leaching.
Genetic Modifications to Enhance Plant Survival
As discussed in Biology 211, genetic modification of plants to improve plant growth
within a given environmental condition or to improve a plant for human purposes is
now common. We will discuss a few examples of genetic modification to enhance
plant survival in environments not conducive to plant growth, and sometimes not
conducive to human survival, too.
Flood or Waterlogged soil tolerance
Most plants cannot survive long periods in waterlogged or flooded soils. Oxygen
deprivation results poor growth from inadequate ATP production, exacerbated
by the toxic alcohol accumulation from fermentation in root cells. Some strains
of rice have a gene that inhibits fermentation in the root cells, prolonging the
time the plant can survive flooded conditions.
Salt tolerant plants, called halophytes, are being studied for their potential to
be used to breed crop plants that have salt tolerance. Most plants cannot grow
in saline soils because of osmotic imbalance. The sodium content of the soil
inhibits water uptake, and in many cases, is simply toxic to the plants. Some
salt tolerant plants, such as Atriplex, have salt glands on their leaf surfaces for
the excretion of salt absorbed and translocated. Others have sodium pumps in
root cells to pump absorbed sodium out of roots while retaining needed
potassium.
Atriplex
Salt glands on leaf surface
Plant Nutrients and Soils - 12
Aluminum Tolerance.
As mentioned earlier, many soils have a high aluminum content, which is toxic to
plants. Agricultural geneticists have developed plant strains whose roots
secrete organic acids, such as malic or citric acid, which can chelate aluminum in
the soil, reducing its toxicity. Both tobacco and papaya have had genetic
modification for this purpose.
Smart Plants
Research using Arabidopsis has produced a mutant strain that produces a blue
pigment when the plant's phosphorus supply is low, but prior to the plant
exhibiting deficiency symptoms. The plant is communicating that it needs
phosphorus!
There is a genetically modified plant that can be sown in areas with land mines
that serves as an indicator of land mine location.
Modification for Phytoremediation
Plants are even used in some environments to remove toxic substances from soils, a
process called phytoremediation.
Plants that can absorb potentially toxic minerals
are planted in landfills. In some cases, the plants can degrade harmful chemicals;
other plants accumulate the substance, and the plants can be harvested, dried, and
removed to a safe storage site. Test sites are in use today for removing TCE, a
carcinogenic organic solvent, TNT, mine sludge contaminants and a number of heavy
metals.
Plant Nutrients and Soils - 13
Plants and Air Pollutants
While plants may be good candidates for soil remediation, they are very sensitive to
air pollutants. Many plants show great damage to when exposed to toxic levels of air
pollutants, such as sulfur oxides and many heavy metals. Growth is reduced when
stomata close in response to ozone, which damages mesophyll cells. Other
components of photochemical smog can reduce photosynthetic rate by 66%.
Sulfur dioxide damaged leaf
Ozone damaged
Although it first seems that elevated levels of CO2 in our atmosphere would promote
photosynthesis and plant growth, controlled studies show mixed results, in part
because increased rate of photosynthesis requires more enzymes and proteins to
support increased growth rates, and available nitrogen soon becomes a limiting factor.
Plants become nitrogen poor. Herbivores have lower quality feed, and this problem
escalates as we go through the food chain.
Epiphytes, Parasites, Saprophytes and Carnivores
To conclude our discussion on plants and their mineral requirements, lets look at bit
at plants that do not use a soil substrate and a bit at the non-photosynthetic plants
saprophytes and parasitic plants.
Epiphytes
Many plants are epiphytes, particularly in tropical moist environments, with adequate
humidity. Such plants simply use a host plant for attachment, but obtain their own
nutrients and perform photosynthesis. Much of their nutrient absorption is from
rainfall. It is common in tropical rainforests for a single tree to have well over 100
different epiphytes attached. In our area, licorice fern is a common epiphyte on bigleaf maple trees, as are many, many mosses and leafy liverworts.
Parasites, Saprophytes and Carnivores There are some species of plants that
have lost the ability to photosynthesize and use root haustoria to penetrate into host
plant roots or shoots to obtain the carbohydrates they need for growth and
development. Other parasitic plants do photosynthesis, but use hosts for mineral and
water uptake. Some plants rely on decaying material for their nutrient source and are
saprophytes. As discussed with modified structures, carnivorous plants, which
evolved in nitrogen-poor environments, degrade animal protein to provide needed
nitrogen
Plant Nutrients and Soils - 14
Dodder a parasitic plant
Coral Root Orchid
Indian Pipe
Mistletoe in tree
Pine Sap
Leaves are not the Orchid
Carnivorous plants are often perceived as heterotrophs, but they simply rely on an
organic source of nitrogen to supplement their nitrogen needs. The native habitats of
most carnivorous plants are nitrogen poor, low pH bogs, and once they trap their
prey, secrete protein-digesting enzymes to provide additional nitrogen.
Nepenthes
Venus Flytrap
Bladderwort
Plant Nutrients and Soils - 15
Key to Plant Nutrient-Deficiency Symptoms
I.
Effects general on whole plant or localized on older, lower leaves...............................................2
2.
Leaves light green. Uniform chlorosis of older leaves, which may die and turn
brown. Abnormal production of anthocyanins in stems and leaves. Stems with
greatly reduced terminal growth................................................................................Nitrogen
2
Leaves dark green. Stunted growth. Abnormal production of anthocyanins
resulting in red and purple colors. Death of older leaves. Stems weak and
spindly...............................................................................................Phosphorus
II.
Effects mostly localized on older, lower leaves.............................................................................3
3.
Older leaves chlorotic, initially interveinal, beginning at tips of leaves.
Margins and tips of leaves may turn or cup upward. If severe, all leaves
become yellow or white. Older leaves may drop off...........................................Magnesium
3.
Older leaves mottled, with necrosis of leaf tips and margins. Leaves may
curl and crinkle. Internodes abnormally short and stems weak, sometimes
with brown streak.................................................................................................Potassium
III.
Effects localized on new leaves ..............................................................................................4
4.
Terminal bud dies. Tips and margins of youngest leaves necrotic and then buds.
Initially young leaves pale green with hooked tips, as well as being
deformed..............Calcium
4.
Terminal bud remains alive.............................................................................................5
5.
Leaves light green (never yellow or white), beginning with younger ones.
Veins lighter than interveinal areas. Necrotic spots may appear but not
common.....................................................................................................Sulfur
5.
Leaves chlorotic, beginning with younger ones. Veins remain green,
except in case of prolonged, extreme deficiency.......................................................Iron