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Soil carbon: the basics
Soil organic carbon is a complex and varied mixture of materials and makes up a small but vital part of all soils.

What is soil carbon?
Soil carbon, or soil organic carbon (SOC) as it is more accurately known, is the carbon stored within soil.It is part of the soil organic matter (SOM), which includes other important elements such as
Soil organic matter is made up of plant and animal materials in various stages of decay.
Un-decomposed materials on the surface of the soil, such as leaf litter, are not part of the organic matter until they start to decompose.
Different types of soil carbon
Soil organic matter is often reported in soil tests as the percentage of soil organic carbon present in the soil sample.
However, although determining the amount of soil organic carbon in soil is important for understanding soil health, knowing the type of organic carbon present is also important as this can greatly impact soil productivity.
• Crop residues – Shoot and root residues less than 2 mm found in the soil and on the soil surface
• Particulate organic carbon – Individual pieces of plant debris that are smaller than 2 mm but larger than 0.053 mm
• Humus – decomposed materials less than 0.053 mm that are dominated by molecules stuck to soil minerals
• Recalcitrant organic carbon – this is biologically stable; typically in the form of charcoal.

The different types of soil organic carbon not only differ in size but are also composed of different materials with different chemical and physical properties and different decomposition times.
Key functions of the different types of soil carbon
Each fraction of soil carbon has different functions, most of these are due to the relative stability and biological availability of each fraction:
1 Crop residues Readily broken down and provide energy to soil biological processes
2 Particulate organic carbon 1.Broken down relatively quickly but more slowly than crop residues
2.important for soil structure, energy for biological processes and provision of nutrients
3 Humus 1.plays a role in all key soil functions
2.particularly important in the provision of nutrients - for example the majority of available soil nitrogen derived from soil organic matter comes from the humus fraction
4 Recalcitrant
organic carbon charcoal - a product of burning carbon-rich materials. As 'biochar', it is attracting interest as both a carbon sink and, possibly, a source of soil benefits.
usually decomposes very slowly and is therefore unavailable for use by micro-organism

In rainforests or good soils organic carbon can be >10 per cent,
While in many poorer soils or soils which are heavily exploited, levels are typically <1 per cent.

The proportion of some fractions can also vary due to management practices. This is important as different fractions decompose at different rates and contain different quantities of nutrients, which will have an impact on the health and productivity of the soil.
Balancing carbon inputs and outputs
The amount of organic carbon in soil is a balance between the build-up which comes from inputs of new plant and animal material and the constant losses where the carbon is decomposed and the constituents separate to mineral nutrients and gases, or are washed or leached away.
Carbon levels build up: Where water, nutrients and sunlight are plentiful.
Carbon is lost where: Microbial activity is high (such as in warm, moist environments)


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Dear Readers,

"Soil carbon improves the physical properties of soil' .Please throw some light on the statement.



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Humic acid/ Humus

1. Stimulates plant enzymes
2. Acts as an organic catalyst
3. Stimulates growth and proliferation of desirable soil microorganisms as well as algae and yeasts
4. Increases root respiration and formation
5. Increases the availability of micronutrients
6. Increases the permeability of plant membranes, which increases the uptake of nutrients
7. Increases the vitamin content of plants
8. Increases the viability and germination of seed
9. Accelerates cell division and root development
10. Contains a wealth of micro-elements such as Si, Fe, Mg, S, Ba, B, Mn, Co, Ni, Ti, Mo, Cu, Pb, Ag and more
11. Increases photosynthesis in plants
12. Contains soluble silicon
a. Silicon strengthens cell walls
b. Silicon helps block disease invasion at the cell level
c. Silicon helps plants maintain more uniform cell temperature, which increases drought and frost tolerance

1. Increases buffering properties of soil
2. Rich in both organic and mineral substances essential to plant growth
3. Retains water soluble fertilizers in the root zones and releases them to plants when needed
4. Has an extremely high CEC (cation exchange capacity)
5. Promotes the conversion of insoluble nutrients into forms available to plants
6. Reduces or eliminates many soil-related phenomenon, such as dry spots on golf greens

1. Makes soil more friable and crumbly
2. Improves soil workability
3. Increases aeration of soil
4. Reduces thatch build-up in turfgrasses
5. Increases water holding capacity
6. Improves thermal coloring of soil



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Thought this might be useful as well

For decades the conventional thinking has been that applications of synthetic nitrogen
fertilizers help enhance soil carbon levels by stimulating soil microbes to feed on organic
matter from crop residues. New research indicates that, in fact, the opposite may be true.
A group of scientists at University of Illinois says that research from the Morrow Plots,
the oldest research plots in the country, is indicating a decline in soil carbon from the use
of synthetic N fertilization.
The Way it Works
Plant residues are left behind in crop production, and various tilling and residue
management methods make use of that residue as a means of adding organic matter to the
soil. What we have understood about synthetic N is that the soil microorganisms are
stimulated by it as an additional food source, which causes these microbes to increase
their activities in breaking down plant residues and adding to the humus/carbon content
of the soil. What’s being discovered now, however, is alarming.
Soil microbes degrade plant residues and reduce their carbon content and nutritional
content into plant available forms and long-term fully degraded carbon, which is the
backbone for forming soil humus. The other result of this microbial activity is that when
microbes feed on carbon in plant residues, and as their bodies die, they release the carbon
as CO2. Volatilization of this rapid cycling carbon from plants may be the larger part of
what happens in this process, so as high inputs of N stimulate the microbes to feed,
eventually the organic matter disappears before it can become humified. The pathway of
humification is interrupted by removal and volatilization of carbon before it reaches the
form of humus. “Over time, the impact of this enhanced microbial appetite outweighs the
benefits of more crop residues” says Tom Philpott of
The last aspect of this carbon loss is explained by taking a look at carbon based organic
acids in the soil: humin, humic acids and fulvic acids. Due to the acceleration of
microbial oxidation of humin by N stimulation, these acids are reduced from the carbonrich
humin, to the less carbon-rich humic acids, and finally to fulvic acids, which have
very little carbon content. This process is natural and healthy, but when the soil is lashed
with excessive amounts of N, the process is accelerated dramatically, allowing less time
for new carbon inputs (from residues) and proper humification of organic matter to occur.
The net effect of this is that soil carbon levels decline, making it even more difficult for
soils to store nitrogen. As the ability of the soil to store nitrogen declines, more N inputs
are needed. Soil tilth, water holding capacity and nutrient retention also suffer causing
compaction and leaching of nutrients. The vicious cycle is born.
New Information
Three university of Illinois professors, Richard Mulvaney, Saaed Khan and Tim
Ellsworth have been raising some eyebrows with new data that testifies to this. In two
recent papers, “The Myth of Nitrogen Fertilization for Soil Carbon Sequestration” (Khan
et al 2007) and “Synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma
for Sustainable Cereal Production” (R.L. Mulvaney et al 2009) the researchers show that
the net effect of synthetic nitrogen use is to reduce soil carbon levels. The proposed
mechanism is stimulation of soil microbes by nitrogen fertilizers, causing the microbes to
consume excessive amounts of organic matter.
To make matters worse, inputs of synthetic N create tough competition for nitrogenfixing
bacteria populations like Rhizobium and Azospirillum. Adding synthetic N is
highly stimulatory to microbes that feed on it. These microbes then outcompete the
nitrogen-fixing bacteria, making N from the atmosphere even less accessible to the crop.
How important are the nitrogen-fixing bacteria? According to the A&L Agronomy
Handbook “It is estimated that 35,000lbs of N are available over a single acre of land. In
return for the supply of food and minerals they get from the plant, these (nitrogen-fixing)
bacteria supply the plant with part of its nitrogen needs, generally not more than 50% to
75% of it”.
What to Do
We know that nitrogen inputs are important in crop and turf production, however we
suggest that nitrogen inputs be managed much, much more carefully and recommend the
use of humates to restore soil carbon in its most active form. Applications of humates
will not only increase soil carbon levels, but will improve water retention, drainage, soil
tilth and nutrient retention. Humates also provide a healthy substrate for beneficial soil
Research has shown that soils with less than 3% organic matter can lose 15% to 40% of
N in a growing season. Anchoring nitrogen in the soil is part of the important job of soil
carbon/humus, as N binds readily with carbon-based acids such as humic and fulvic.
This reduces the need for heavy, expensive inputs of N fertilizers. Moreover, the
stimulation of symbiotic and free-living nitrogen-fixing bacteria by humic substances
adds to availability of N from the atmosphere, supplying, in many cases, up to 75% of a
crop’s needs.
By applying humates to soils where synthetic N and other fertilizers are used, we can
restore soil carbon levels, balance the ratio of carbon to nitrogen and break the vicious
cycle we have created in most conventional production soils. Humates are a direct input
of soil organic matter, providing the most important aspect of a healthy, productive soil.
Joel C. Reid
Mesa Verde Resources
S. A. Khan*, R. L. Mulvaney, T. R. Ellsworth and C. W. Boast, 2007, “The Myth of Nitrogen Fertilization
for Soil Carbon Sequestration” Published in J Environ Qual 36:1821-1832 (2007).
R. L. Mulvaney*, S. A. Khan and T. R. Ellsworth, 2009, “Synthetic Nitrogen Fertilizers Deplete Soil
Nitrogen: A Global Dilemma for Sustainable Cereal Production” Published in J Environ Qual 38:2295-
2314 (2009).
Tom Philpott, 2010, “New Research: Synthetic Nitrogen Destroys Soil Carbon, Undermines Soil Health”
Grist | Environmental News, Commentary, Advice 2010.