Soil Organic Matter Status of Bangladesh and it’s Role

in Soil Health

Dr M. Altaf HossainPSO

Survey Section

Soil organic matter (SOM) is the organic matter component of soil, consisting of plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by soil organisms.

SOM exerts numerous positive effects on soil physical and chemical properties, as well as the soil’s capacity to provide regulatory ecosystem services.

Particularly, the presence of SOM is regarded as being critical for soil function and soil quality.

What is Soil Organic Matter?

The pie chart represents organic matter in soil before cultivation. After land has been cultivated for one or two decades, much of the active fraction is lost and stabilized organic matter makes up more than half of the soil organic matter.

The positive impacts of SOM: SOM's effects on soil functioning includes improvements related to soil structure, aggregation, water retention, soil biodiversity, absorption and retention of pollutants, buffering capacity, and the cycling and storage of plant nutrients. SOM increases soil fertility by providing cation exchange sites and acting as reserve of essential nutrients, especially nitrogen (N), phosphorus (P), and sulfur (S), along with micronutrients, which are slowly released upon SOM mineralization.

As such, there is a significant correlation between SOM content and soil fertility.

SOM also acts as the major sink and source of soil carbon.

Soil represents one of the largest C sinks on the planet and plays a major role in the global carbon cycle.

Therefore, SOM/SOC dynamics and the capacity of soils to provide the ecosystem service of carbon sequestration through SOM management have received considerable attention in recent years.

The mass of SOM in soils as a percent generally ranges from 1 to 6% of the total topsoil mass for most upland soils.

Soils whose upper horizons consist of less than 1% organic matter are mostly limited to desert areas, while the SOM content of soils in low-lying, wet areas can be as high as 90%.

Soils containing 12-18% SOC are generally classified as organic soils.

A good soil should have an organic matter content of more than 3.5 per cent. But in Bangladesh, most soils have less than 1.7%, and some soils have even less than 1 % organic matter.

Organic matter status of Bangladesh SoilsClass of SOM

AEZ No. Main Locations Totalarea(Mha)

% ofNCA

Very low(<1.0%)

1, 7, 8, 10, 11, 16, 25, 26, 27, 28, 29, 30

Dinajpur, Sherpur, Jamalpur,Rajshahi, Pabna, Kushtia, Bogra,Naogaon, Rangpur, Khagrachari,Bandarban, Chittagong

4.05 44.5

Low(1.0%-1.7%)

2, 3,17, 18,19

Adjoining areas of Tista, DharlusChandpur, Lakshimipur,Noakhali, Bhola, BarisalPatuakhali, Narsingdi, Dhaka

1.56 17.1

Medium(1.7%-3.5%)

4, 12,9, 20,22, 23

Sirajganj, Mymensingh,Kishoreganj, Sherpur, Jamalpur,Sylhet, Moulvibazar, Feni, Cox'sbazar

1.94 21.3

High(> 3.5%)

5, 6, 1,13,14,15, 21

Panchagaher, Natore, Naogaon,Khulna, Satkhira, Madaripur,Gopalganj, Munshiganj, HabiganjSunamganj, Netrokona

1.56 17.1

Source: Land Degradation Situation in Bangladesh, Soils Division,BARC, 1999

The Changing Forms of Soil Organic Matter

Additions. When roots and leaves die, they become part of the soil organic matter. Transformations. Soil organisms continually change organic compounds from one form to another. They consume plant residue and other organic matter, and then create by-products, wastes, and cell tissue. Microbes feed plants. Some of the wastes released by soil organisms are nutrients that can be used by plants. Organisms release other compounds that affect plant growth. Stabilization of organic matter. Eventually, soil organic compounds become stabilized and resistant to further changes.

Role of Soil in carbon cycling

Soil plays a major role in the global carbon cycle, with the global soil carbon pool is estimated at 2500 gigatons. This is 3.3 times the size of the atmospheric pool (750 gigatons) and 4.5 times the biotic pool (560 gigatons).

The pool of organic carbon, which occurs primarily in the form of SOM, accounts roughly 1550 gigatons of the total global C pool, with the remainder accounted for by soil inorganic carbon (SIC).  

The pool of organic C exists in dynamic equilibrium between gains and losses; soil may therefore serve as either a sink or source of C, through sequestration or greenhouse gas emissions, respectively, depending on exogenous factors (Lal, 2004).

What Does Organic Matter Do In Soil?

Of all the components of soil, organic matter is probably the most important and most misunderstood.

Organic matter serves as a reservoir of nutrients and water in the soil, aids in reducing compaction and surface crusting, and increases water infiltration into the soil.

Yet it's often ignored and neglected.

What Are the Benefits of Organic Matter?

Nutrient Supply: Organic matter is a reservoir of nutrients that can be released to the soil. Each percent of organic matter in the soil releases 20 to 30 pounds of nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3 pounds of sulfur per year.

The nutrient release occurs predominantly in the spring and summer, so summer crops benefit more from organic-matter mineralization than winter crops.

Water-Holding Capacity: Organic matter behaves somewhat like a sponge, with the ability to absorb and hold up to 90 percent of its weight in water. A great advantage of the water-holding capacity of organic matter is that the matter will release most of the water that it absorbs to plants. In contrast, clay holds great quantities of water, but much of it is unavailable to plants.

Soil Structure Aggregation: Organic matter causes soil to clump and form soil aggregates, which improves soil structure. With better soil structure, permeability (infiltration of water through the soil) improves, in turn improving the soil's ability to take up and hold water.

Erosion Prevention: This property of organic matter is not widely known. Data used in the universal soil loss equation indicate that increasing soil organic matter from 1 to 3 percent can reduce erosion 20 to 33 percent because of increased water infiltration and stable soil aggregate formation caused by organic matter.

How Much Organic Matter Is in the Soil?

An acre of soil measured to a depth of 6 inches weighs approximately 2,000,000 pounds, which means that 1 percent organic matter in the soil would weigh about 20,000 pounds per acre.

Remember that it takes at least 10 pounds of organic material to decompose to 1 pound of organic matter, so it takes at least 200,000 pounds (100 tons) of organic material applied or returned to the soil to add 1 percent stable organic matter under favorable conditions.

Measuring soil organic matterThe most common methods for measuring soil organic matter in

current use actually measure the amount of carbon in the soil.

This is done by oxidising the carbon and measuring either the amount of oxidant used (wet oxidation, usually using dichromate) or the CO₂ given off in the process (combustion method with specific detection).

Laboratories these days generally report results as soil organic carbon. Those that report as soil organic matter have usually measured carbon and converted to organic matter by multiplying by 1.72.

However, this conversion factor is not the same for all soils, and it is more precise to report soil carbon rather than organic matter.

How Can We Maintain or Improve Soil Organic Matter Levels?

Building soil organic matter is a long-term process but can be beneficial. Here are a few ways to do it.

Reduce or Eliminate Tillage: Tillage improves the aeration of the soil and causes a flush of microbial action that speeds up the decomposition of organic matter. Tillage also often increases erosion. No-till practices can help build organic matter.

Reduce Erosion: Most soil organic matter is in the topsoil. When soil erodes, organic matter goes with it. Saving soil and soil organic matter go hand in hand.

Soil-Test and Fertilize Properly: You may not have considered this one. Proper fertilization encourages growth of plants, which increases root growth. Increased root growth can help build or maintain soil organic matter, even if you are removing much of the top growth.

Cover Crops: Growing cover crops can help build or maintain soil organic matter. However, best results are achieved if growing cover crops is combined with tillage reduction and erosion control measures.

What Determines Soil Organic Matter Levels?

The amount of organic matter in soil is the result of two processes: the addition of organic matter (roots, surface residue, manure, etc.), and the loss of organic matter through decomposition.

Five factors affect both additions and losses.

1. Management: Practices that increase plant growth on a field (cover

crops, irrigation, etc.) will increase the amount of roots and residue added to the soil each year.

Intensive tillage increases the loss of organic matter by speeding decomposition. While tillage primarily burns younger organic matter, older, protected organic compounds can be exposed to decomposition if small aggregates are broken apart. In addition to changing the amount of soil organic matter, tillage practices affect the depth of soil organic matter.

2. Soil texture: Fine-textured soils can hold much more organic

matter than sandy soils for two reasons. First, clay particles form electrochemical bonds that hold organic compounds. Second, decomposition occurs faster in well-aerated sandy soils. A sandy loam rarely holds more than 2% organic matter.

3. Climate: High temperatures speed up the degradation of organic

matter. In areas of high precipitation (or irrigation) there is more plant growth and therefore more roots and residues entering the soil.

4. Landscape position: Low, poorly-drained areas have higher organic matter

levels, because less oxygen is available in the soil for decomposition. Low spots also accumulate organic matter that erodes off hill tops and steep slopes.

5. Vegetation. In prairies, much of the organic matter that dies and is

added to the soil each year comes from grass roots that extend deep into the soil. In forests, the organic matter comes from leaves that are dropped on the surface of the soil. Thus, farmland that was once prairie will have higher amounts of organic matter deep in the soil than lands that were previously forest.

Differences in organic matter appear in the color changes across a bare field. Less organic matter is produced on the drier hilltops, and some is lost to soil erosion and deposited in low spots.

How do organic matter levels change?

Most organic matter losses in soil occurred in the first decade or two after land was cultivated.

Native levels of organic matter may not be possible under agriculture, but many farmers can increase the amount of active organic matter by reducing tillage and increasing organic inputs.

An illustration of soil organic matter losses and gains in response to tillage.

Does more carbon mean that crops grow better?

This is a hard question to answer, because there are so many things that affect how crops perform.

We have looked at this in two ways: how crop yields and soil properties vary in different parts of the same paddocks, and secondly, how crop yields and soil properties vary from paddock to paddock.

Results comparing different poppy paddocks show no clear relationship between soil carbon and yield. This is perhaps not surprising because other important factors like water, fertilizer, pest and disease management also varied from paddock to paddock.

Summary

Organic matter is an important soil property. It is associated with a variety of other important soil physical, chemical and biological characteristics.

Cool climate favours the retention of organic matter to a greater extent than is the case in warmer regions.

It is not easy to show a clear link between soil organic matter concentrations and crop yield and quality. Many other factors affect yield and quality to a greater extent.

Nevertheless, there is sufficient evidence linking organic matter and yield to suggest that farmers should strive to maintain or increase organic matter in their soils.

Maximising inputs of organic matter by incorporating crop residues and including green manures and pastures in the rotation, where practical, should be a goal for all farmers.

Thanks