Practices that Decrease the Amount of Organic Matter

 

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.

 

Read: Natural Factors Influencing the Amount of Organic Matter

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.

 

Read: What is Soil Organic Matter? – Importance & Sources

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.

 

Read: Types of Roots and Root Systems

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.