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Soil Relations of Plants
Soil structure is a critical factor in water holding capacity and
nutrient availability for plants.
The basic mineral components of soil
are sand, silt and clay. Soil texture is
determined by the relative amounts
of these three components:
What soil texture (and resulting
porosity) determines is water –
holding, oxygen access, and
mineral availability in the soil.
However, we will also need to
consider organic content of soils.
Particle size is critical to porosity.
Sand consists of relatively large, irregular particles. Coarse
texture means that they drain rapidly, and hold water and
minerals poorly. Oxygen, however, is readily available to
plant roots.
Silt particles are also irregular in shape, but of intermediate
size. Therefore, silty soils also have intermediate water and
mineral holding and oxygen access capacity.
Clay particles are far smaller, and have only slow penetration
of water, but can hold high volumes of water and minerals. A
part of the reason has nothing to do with particle size; instead
it is the charged, layered structure of the particles…
Layering means that the surface area of a clay particle (or the
clay in soil) is much larger than you might imagine. In
addition, the surface is charged and holds charged nutrient
ions and ionized water strongly.
That characteristic was also important in the primordial
synthesis of polypeptides…
Porosity and binding properties of soil materials also
influence the rate at which water and nutrients are leached
from soil, i.e. percolate down through soil toward the water
table, accessible only to deeply rooted plants.
Now back to organic content, and the geographic pattern of
soil types…
What differs is the pattern of soil horizons and the pattern of
mineral content and leaching. Horizons:
O horizon(s) – actually above the soil, comprised of litter
(dead bodies of plants that grew in this or previous years) and
partially decomposed plant material lying on the surface.
A horizon(s) – partially and completely decomposed plant
materials forming a humus mixed into surface soil. Minerals
are leached from this layer in a process called eluvation.
Percolating water also tends to carry fine particles (clay)
downward through the profile.
B horizon(s) – these are the layers into which the water,
minerals and soil particles are re-deposited.
C horizon(s) – the layers that are the ‘parent’ materials of the
soils above. This is the rock, broken rock, and gravel that are
slowly broken down to form the horizons above.
Now let’s look at pictures of some of the characteristic soil
types and where they occur…(the names classically used are
Russian, rather than the names from table 4.1 of the text)
Chernozem (or prairie) soil – characteristic
of both American prairies and Russian
steppes. There is low average rainfall, but
occasional storms deliver enough rainfall to
percolate down. It reaches only so far before
evaporation pulls water back up. The
maximum depth of percolation is marked by
a white band of carbonates.
Seasonality is such that litter builds up,
decomposing only slowly. What results is a
thick layer of black soil with high organic
content. In central North America, part of the
richness results from organic silt deposits left
at the bottom of the Mississippi seaway. The
text calls this soil type mollisol.
Podzol (brown earth) – typical beneath
coniferous forests, with their carpet of slowly
decomposing needles. Since the soil is acid
(pH ~4), it is poorly mixed; most mixers, e.g.
earthworms, will not live in acid soil. Water
percolating down through this soil is also
acidic, and dissolves many mineral nutrients
from the lower parts of the A horizon,
carrying them down further. That leaves a
‘bleached’ layer. The minerals are deposited
in the B horizons, making them red and
brown. Podzol means “ash earth” in Russian,
describing the bleached layer that appears
when this soil is plowed. The text calls this
soil type spodisol.
Beneath deciduous forests there is a different sort of podzol.
Mixers are active, and rotting leaf material is mixed into
surface soil, forming a layer 10 – 20cm thick of dark, rich
soil. A bleached layer is not evident, but the percolation of
minerals does colour the B horizon brown; thus the name
‘brown earth’. The transitions between horizons and colours
are much more gradual than beneath boreal forests. The text
calls this alfisol.
Beneath tropical forest a soil type with low fertility develops.
The text calls this soil type oxisol. The more traditional name
is latisol. Beneath tropical forest leaf litter decomposes
rapidly, large amounts of rainfall percolate down through the
soil. So much water percolates that silica is removed leaving
iron and aluminum in surface layers. The iron oxide gives the
soil its characteristic red colour.
Iron oxides are resistant to further
weathering. Nutrients are released into the
surface only by either decomposition or,
when slash and burn agriculture occurs, by
the release of mineral nutrients from the ash
remaining when vegetation is cut and burned.
The surface of laterite soils also hardens
rapidly, forming a layer that is not easily
broken. Therefore, slash and burn agriculture
involves moving to a new area after only a
few years, cutting and burning it to release
nutrients. In addition to large scale
agriculture, increasing population size has
meant loss of more tropical forest area to
small scale, slash and burn destruction.
There is one last group of ‘old’ classification soil types:
lithosols, solonchak soils and desert soils. All are soils of very
dry areas. The ‘new’ classification soil type that encompasses
this group is: aridisols.
There is little leaching of soil minerals within this soil type.
But then the mineral content is low, and so is water holding
capacity, since these soils tend to be dominated by sand.
This soil type is generally limited to warm climates.
You could try to define an arctic tundra soil type, but there is
only a very limited soil layer and permafrost beneath means
that runoff moves a significant fraction of the precipitation
directly into boggy areas or water bodies.
We can put together geographical patterns linking vegetation,
climate, and soils in an abstract way, rather than complex
maps…
In the end, the key how soils impact vegetation is determined
by soil mineral content in the root zone, soil pH (which
determines how accessible those mineral nutrients are) and the
amount and movement of water.
Soil pH is straightforward: the pH is determined by the
concentration of hydrogen ions (H+) in the soil, which is, in
turn, determined by the amount of organic material present.
Organic particles have large surface area and negative charge;
they hold mineral ions needed by plants in the root zone.
Organic acids from recently decomposed material also alter
ionic conditions, making many of the essential nutrients
readily available.
Water availability and movement is a more complex subject.
Water movement is summarized in this figure from your
text…
What’s important to the plants is the water potential in the
soil, and whether the plants can draw water from the soil.
When soils dry, their water potential becomes more negative,
until the potential drops below that of the roots. When that
happens, plants can’t draw water from the soil; wilting occurs.
Eventually the permanent wilting point is reached, and plants
die.
Since we are looking at the water content of soil, and
evaporative loss varies with temperature, losses and water
potential vary in a diurnal cycle. Leaf water potential declines
during the day and increases again at night. As long as water
potential is lower in the plant than in the soil, plants can draw
water. Over an extended period without precipitation the two
lines draw closer together. Eventually they can cross – wilting
and plant death.
Note also that water potential correlates differently with water
content among different soils as a result of water holding
capacity (due to soil structure…)
Clay, with a greater capacity, has a higher water content (%
by volume) at any water potential, loam is intermediate, and
sandy soils lowest water content for any potential.
Now let’s consider the nutrients themselves. Which nutrients
are necessary for plant growth and reproduction?
Other than carbon, oxygen, and hydrogen (components of
carbohydrates), a few nutrients are described as essential.
They are:
Nitrogen, phosphorus, sulfur (all necessary components of
nucleic and amino acids), potassium (important in pH
regulation and stomate control), calcium (a second messenger
and key in cell division), and magnesium (key in the
functional chlorophyll molecule).
Phosphorus, sulfur, potassium, calcium and magnesium are
absorbed from soil.
Nitrogen is frequently the limiting nutrient for plant growth
and reproduction.
Nitrogen in natural plant communities is mostly obtained
either directly or indirectly as a result of bacterial nitrogen
fixation. The mutualism between Rhizobium and Frankia
bacteria and their plant hosts ranges from facultative to
obligate.
Rhizobium nodules on a bean plant:
Cross section of a nodule:
Frankia is an actinomycete that forms nodules associated with
the roots of ~200 species, most from the tropics and
subtropics. Not all, however. One of the genera most
commonly mutualistic with Frankia are the alders – birch
relatives from north temperate and boreal regions.
Alder with Frankia nodules
There is another association that is important, between
nitrogen-fixing cyanobacteria in the soil and plants. This is a
loose association which provides only small amounts of
nitrogen to plants, but provides root exudate carbohydrates to
the bacteria.
Then there are mycorhizzae – associations between soil fungi
and plant roots. There are two types: ecto- and
endomycorhizzae (ecto – external, endo – internal or within).
The most important endomycorhizzae are arbuscular
mycorhizzae.
Molecular genetics has established that the largest living
organism in the world is a mycorrhizza that ‘occupies’ the
rhizosphere of northern Michigan and adjacent Wisconsin.
Here are pictures of mycorrhizzae:
Ectomycorrhizzae on a pine root tip
Vesicular arbuscular mycorrhizza
1 – visicle
2 - arbuscule
Mycorrhizzae are important in plant uptake of phosphorus in
phosphorus-limited soils. The fungal component secretes acid
phosphatases that release otherwise unavailable phosphorus
from mineral compounds bound to organic matter in soil.
The enormous surface area of the fungal hyphae also can
increase the plant’s ability to obtain water and other nutrients
from soil.
The interaction can be critical to plant survival, for example to
the Ericaceae in acidic arctic and subartic soils.
Vaccimium uliginosum
The text chapter goes beyond how plants obtain nutrients to a
major segment about how efficiently they are used, how
nutrients are related to leaf demography, and nutrient
residence time.
These are not only important concepts, but lead to some very
interesting questions to which answers may not yet have been
developed.
Nutrient use efficiency – is generally measured in terms of
productivity (carbon assimilation, biomass increase,
photosynthetic rate, or another similar measure) per unit
nutrient. An, for example, is growth per unit nitrogen. It varies
among species and also with leaf age.
Nutrient concentration in leaves is dynamic. As leaves age and
approach senescence, nutrients are re-allocated to newly
developing leaves, flowers and storage structures.
How long do leaves survive in different species?
Leaves (or needles) typically last longer on evergreens
(defined by an average leaf lifespan > 1 yr). Nitrogen use
efficiency is also greater on average among evergreens. The
mean residency time for nitrogen is longer for evergreens. In
part this is the result of evergreens living in nutrient-poor soils
and having lower leaf nitrogen concentrations.
Are lifespan, mean residency time and nitrogen use efficiency
the result of less available nitrogen? There is no clear answer.
There are, however, consequences of differences in ‘nitrogenstrategy’. Species (and leaves) that have long nitrogen
residency strategies, long leaf lifespans and high nitrogen use
efficiency have low maximum growth rates, and are at a
disadvantage in nutrient-rich environments (and vice versa).
The relationships are not absolute, but there are decently
strong correlations, as evident from the text figure:
What happens to nitrogen in plants differs in differing
environments. The text figure shows the general pattern…
(Arrows indicate the direction of movement/translocation of
nitrogen from soil or established leaves to newly growing
structures (leaves/flowers). Thickness of lines indicates
relative importance.)
There are some interesting questions you should ponder as we
go forward in the course that relate to the relationships
reflecting nutrient dynamics among and within plants:
1. What is the relationship between competition among plants
and a) leaf demography, b) nitrogen dynamics within plants
and c) nitrogen use efficiency?
2. How do whole plant demography and life history strategies
correlate with leaf demography and nitrogen use efficiency?
3. The physiology of leaves changes with age. Are there also
physiological differences that result from plant interactions?
Keep track of these questions as we consider the relevant
chapters.