Various
types of human activity decrease or increase soil organic matter contents and
biological activity. However, increasing the organic matter content of soils or
even maintaining good levels requires a sustained effort that includes
returning organic materials to soils and rotations with high-residue crops and
deep- or dense-rooting crops.
It is especially difficult to raise the organic matter content of soils that are well aerated, such as coarse sands, and soils in warm-hot and arid regions because the added materials decompose rapidly.
Soil organic matter levels can
be maintained with less organic residue in fine textured soils in cold temperate
and moist-wet regions with restricted aeration.
By
the end of this article, you will be able to:
· Outline
some practices that influence the amount of organic matter in the soil
· Explain
how to avoid practices that will deplete organic matter content of our soils.
Practices that Decrease Soil Organic Matter
Management practices that alter the living and nutrient conditions of soil organisms, such as continuous tillage or burning of vegetation, result in a degradation of their microenvironments.
In turn, this results in a reduction of soil biota,
both in biomass and diversity. Where there are no longer organisms to decompose
soil organic matter and bind soil particles, the soil structure is damaged
easily by rain, wind and sun. This can lead to rainwater runoff and soil
erosion, removing the potential food for organisms, i.e. the organic matter of
the topsoil.
The
factors leading to reduction in soil organic matter in an open cycle system can
be grouped as factors that result in:
a)
A decrease in biomass production
b)
A decrease in organic matter supply
c)
Increased decomposition rates.
a) Decrease in Biomass Production
1. Replacement of Perennial
Vegetation
A
consequence of clearing forest for agriculture is the disappearance of the
litter layer, with a consequent reduction in the numbers and variety of soil
organisms. While many temperate forest species appear to adapt well to
grassland, the effects of deforestation in the tropics appear to be more
marked. Studies have shown that as soil biodiversity declines, adapted species
may take over from the indigenous species and the composition may change
drastically.
2. Replacement of Mixed
Vegetation with Monoculture of Crops and Pastures
The
simplification of vegetation and the disappearance of the litter layer under
grassland and mono-crop production systems lead to a decrease in faunal
diversity. Although root systems (especially of grasses) can be extensive and
explore vast areas of soil, the root exudates from one single crop will attract
only a few different microbial species. This in turn will affect the predator
diversity. The more opportunistic pathogen species will be able to acquire
space near the crop and cause harm. Continuous cultivation and grazing also
leads to compaction of soil layers, which in turn affects the circulation of
air. Anaerobic conditions in the soil stimulate the growth of different
micro-organisms, resulting in more pathogenic organisms.
3. High Harvest Index
One
of the consequences of the green revolution was the replacement of indigenous
varieties of species with high-yielding varieties (HYVs). These HYVs often
produce more grain and less straw, compared with locally developed varieties; the
harvest index of the crop (ratio of grain to total plant mass aboveground) is
increased. From a production point of view, this is a logical approach.
However,
this is less desirable from a conservation point of view. Reduced amounts of
crop residues remain after harvest for soil cover and organic matter, or for
grazing of livestock (which results in manure). Moreover, where animals graze
the residues, even less remains for conservation purposes.
4. Use of Bare Fallow
Traditionally,
a fallow period is used after a period of crop production to give the land some
“rest” and to regenerate its original state of productivity. Usually, this is
necessary in production systems that have drawn down the nutrient supply and
altered the soil biota significantly, such as in slash-and-burn systems or
conventional tillage systems.
Instead
of recovering the soil food web, the soil organic matter is degraded further
and the lack of cover can result in severe erosion and runoff when the rains
start after the dry season.
b) Decrease in Organic Matter Supply
1. Burning of Natural
Vegetation and Crop Residues
Burning
destroys the litter layer and so diminishes the amount of organic matter
returned to the soil. The organisms that inhabit the surface soil and litter layer
are also eliminated. For future decomposition to take place, energy has to be
invested first in rebuilding the microbial community before plant nutrients can
be released. Similarly, fallow lands and bush are burned before cultivation.
This provides a rapid supply of P to stimulate seed germination. However, the
associated loss of nutrients, organic matter and soil biological activity has
severe long term consequences.
2. Overgrazing
There
is a tendency throughout the world to overstock grazing land above its carrying
capacity. Cows, draught animals and small ruminants graze on communal grazing
areas and on roadsides, stream banks and other public land.
Overgrazing
destroys the most palatable and useful species in the plant mixture and reduces
the density of the plant cover, thereby increasing the erosion hazard and
reducing the nutritive value and the carrying capacity of the land.
i) Indicators of
Overgrazing
One
indicator of overgrazing is that the animals run short of pasture. In some
regions of the United States under continuous grazing, overgrazed pastures
promote by short-grass species such as bluegrass and will be less than 2-3
inches tall in the grazed areas.
In
other parts of the world, overgrazed pasture is typically taller than
sustainably grazed pasture, with grass heights typically over 1 meter and
dominated by unpalatable species such as Aristida or Imperata. In all cases,
palatable tall grasses such as orchard grass are sparse or non-existent.
In
such cases of overgrazing, soil may be visible between plants in the stand,
allowing erosion to occur, though in many circumstances overgrazed pastures
have a greater sward cover than sustainably grazed pastures.
ii) Rotational grazing
Under
rotational grazing, overgrazed plants do not have enough time to recover to the
proper height between grazing events. The animals resume grazing before the
plants have restored carbohydrate reserves and grown back roots lost after the
last defoliation.
The
result is the same as under continuous grazing: in some parts of the United
States tall-growing species die and short-growing species that are more subject
to drought injury predominate the pasture, while in most other parts of the
world tall, drought tolerant, unpalatable species such as Imperata or Aristida
come to dominate.
As
the sod thins, weeds encroach into the pasture in some parts of the United
States, whereas in most other parts of the world overgrazing can promote thick
swards of native unpalatable grasses that hamper the spread of weeds.
Another
indicator of overgrazing in some parts of North America is that livestock run
out of pasture, and hay needs to be fed early in the fall.
In
contrast, most areas of the world do not experience the same climatic regime as
the continental United States and hay feeding is rarely conducted. Overgrazing
is also indicated in livestock performance and condition. Cows having
inadequate pasture immediately following their calf's weaning may have poor
body condition the following season. This may reduce the health and vigor of
cows and calves at calving.
Also,
cows in poor body condition do not cycle as soon after calving, which can
result in delayed breeding and a long calving season. With good cow genetics,
nutrition, ideal seasons and controlled breeding 55% to 75% of the calves
should come in the first 21 days of the calving season. Poor weaning weights of
calves can be caused by insufficient pasture, when cows give less milk and the
calves need pasture to maintain weight gain.
3. Removal of Crop Residues
Many
farmers remove residues from the field for use as animal feed and bedding or to
make compost. Later, these residues return to contribute to soil fertility as
manures or composts.
However,
residues are sometimes removed from the field and not returned. This removal of
plant material impoverishes the soil as it is no longer possible to recycle the
plant nutrients present in the residues.
Read: Soil Fertile: What Makes a Soil Fertile?
c) Increased Decomposition Rates
1. Tillage Practices
Tillage
is one of the major practices that reduce the organic matter level in the soil.
Each time the soil is tilled, it is aerated. As the decomposition of organic
matter and the liberation of CO2 are aerobic processes, the oxygen stimulates
or speeds up the action of soil microbes, which feed on organic matter. This
means that:
·
When ploughed, the residues are incorporated in the soil together with air and
come into contact with many micro-organisms, which accelerates the carbon
cycle. The decomposition is faster, resulting in the formation of less stable
humus and an increased liberation of CO2 to the atmosphere, and thus a
reduction in organic matter.
·
The residues on the soil surface slow the carbon cycle because they are exposed
to fewer micro-organisms and thus wane more slowly, resulting in the production
of humus (which is more stable), and liberating less CO2 to the atmosphere.
Organic
matter production and conservation is affected dramatically by conventional
tillage, which not only decreases soil organic matter but also increases the
potential for erosion by wind and water.
The
impact occurs in many ways:
·
Ploughing leaves no residues on the soil surface to lessen the impact of rain.
·
Ploughing reduces the quantity of food sources for earthworms and disturbs
their burrows and living space, hence populations of certain species decrease
drastically.
·
Tillage by repeated hoeing or discing smoothes the surface and destroys natural
soil aggregates and channels that connect the surface with the subsoil, leaving
the soil susceptible to erosion.
·
The development of a plough pan or hoe pan, a layer of compacted soil resulting
from smearing action at the bottom of the plough or hoe, may retard both root
penetration and water infiltration.
·
Ploughing or discing under dry conditions exacerbates the pulverization of the
soil, causing the soil surface to crust more easily, leading to greater water
runoff and erosion.
·
Increased runoff during rainstorms may also increase the possibility of drought
stress later in the season, because water that runs off the field does not
infiltrate into the soil to remain available to plants.
In
some circumstances, imbalances of certain soil organisms can disrupt soil
structure and processes, e.g. certain earthworm species in rice fields or
pastures.
2. Drainage
Decomposition
of organic matter occurs more slowly in poorly aerated soils, where oxygen is
limiting or absent, compared with well-aerated soils. For this reason, organic
matter accumulates in wet soil environments.
Soil
drainage is determined strongly by topography - soils in depressions at the
bottom of hills tend to remain wet for extended periods of time because they
receive water (and sediments) from upslope. Soils may also have a layer in the
subsoil that inhibits drainage, again exacerbating waterlogging and reduction
in organic matter decomposition.
3. Fertilizer and Pesticide
Use
Initially,
the use of fertilizer and pesticides enhances crop development and thus
production of biomass (especially important on depleted soils). However, the
use of some fertilizers, especially N fertilizers, and pesticides can boost
micro-organism activity and thus decomposition of organic matter.
The
chemicals provide the microorganisms with easy-touse N components. This is
especially important where the C: N ratio of the soil organic matter is high
and thus decomposition is slowed by a lack of N
Read: Soil Fertility in the Tropics: Definition, Factors & Key Limitations
In
conclusion, various types of human activity decrease or increase soil organic
matter contents and biological activity. Certain management practices that
alter the living and nutrient conditions of soil organisms, such as continuous
tillage or burning of vegetation, result in a degradation of their
microenvironments.
You
have leant that practices that leads to depletion of organic matter anchors on
these three major factors:
· A
decrease in biomass production.
· A decrease
in organic matter supply
· Increased
decomposition rates.
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