Thursday, 29 July 2010

Dairy Farmers Need to Regularly Test for Soil Organic Matter

Soil Organic Matter is a very good measure of how sustainable agriculture really is today.
Soil Organic Matter (SOM) is usually highest under permanent pasture & grazing. Pasture based farmers have NOT been given the credit they are due!
Pasture based dairy farming is very beneficial to the environment.

So UK pasture based dairy farmers should be measuring & monitoring SOM% in their regular soil tests. Test for SOM% every time you soil test.Organic matter is critical for soil health and for soil productivity. It:
Provides energy for soil microbes
Supports and stabilises soil structure
Increases water storage
Stores and supplies nutrients
Builds soil biodiversity
Stores carbon
Buffers chemical behaviour such as pH
But, what is it?Organic matter derives from the growth and death of organisms.
Soil organic matter is:
The living component of the soil (roots, micro-organisms, animals and plants);
Exudates from living organisms; and
Dead, decaying and highly decomposed materials.
Organic matter is constructed from cellulose, tannin, cutin, and lignin and various proteins, lipids and sugars. These are all based on chains of carbon molecules which mean that a measure of soil organic carbon can give an indirect measure of soil organic matter.Decomposed organic matter has a black or dark brown colour and will darken soil colour.How does it get into soil?Plant growth is the primary source of soil organic matter. Photosynthesis converts sunlight, carbon dioxide and water into plant material. On death, the plant material is steadily decomposed and progressively incorporated into the soil. What is its fate in soil?Organic matter is a dynamic component of soil. Plant and animal debris is regularly added and carbon dioxide is routinely lost as soil organisms use organic matter as an energy source. This is the soil carbon cycle. If the rate of incorporation is low, or the rate of respiration is high, soil organic matter levels will decline. Thus the level of organic matter in soil is dependant on the balance between inputs and losses of soil carbon.Soil organisms rely on organic matter as their food source. In doing this some of the carbon chains of the organic matter are converted to carbon dioxide (termed respiration). Organic matter is consequently decomposed. How is it measured?
The amount of organic carbon present in the soil is used to estimate organic matter. Various experiments have shown that organic matter contains about 58% carbon. On this basis, the following relationship can be used to estimate levels of organic matter.
Organic matter(% by weight)
Organic carbon(% by weight)
Why does it matter to soil health?
The primary value of organic matter to soil health is in providing the mechanism for fuelling the soil with energy and nutrients. It provides a reservoir of metabolic energy that enables biological processes to occur.As plant remains steadily decompose in this part of the carbon cycle, secondary benefits occur. Nutrients are mobilised, soil mixing occurs, and soil structure is improved and strengthened as decomposition products adhere to physical soil particles and build aggregation.

Every tonne of Carbon in the soil is equivalent to 3.67 tonnes of CO2.
Soil Organic Matter is 58% Carbon.
On a well managed dairyfarm pasture (say 10% SOM) every hectare contains approx 260 Tonnes of Carbon in the top 30cm of soil.(Assuming a bulk density of 1.5Tonnes per cubic metre of soil). The International soil carbon stocks are measured to an agreed (IPPC) depth of 30cm.
The UK like every other country MUST protect & maintain (if not increase) their "Carbon Stocks". So Soil Carbon is vitally important.
Pasture based dairy farming is crucial to a future healthy environment.
Total C fixed through photosynthetic processes in pasture plants equates to approximately 40% of total dry matter (DM). In a pasture with an annual yield of e.g. 10t DM ha-1 yr-1 (i.e. 10t DM harvested as intake by animals), the amount of C harvested is therefore c. 4t C ha-1 yr-1. The total amount of C fixed from the atmosphere in photosynthesis is considerable, and has been measured e.g. as c. 16t C ha-1 yr-1, of which some 40-50% (6.4-8t C ha-1 yr-1) is returned to the atmosphere in plant respiration. This includes the respiration involved in the synthesis of shoot and the maintenance of shoot tissues. This figure also includes respiration from the synthesis and
maintenance of roots (some of which is expended via the shoot). Of the remaining c.8t C ha-1 yr-1 in new shoot and root tissues, only about 50% is typically harvested (hence the 4tC ha-1 yr-1 harvested), and the remainder of the plant tissues turnover and senesce to form shoot and root litter. Shoot and root litter contribute C ultimately to either respiration from the soil (and soil surface), from the microbes that consume the litter, or contribute to a potential increase in C sequestered in the soil.
Management (both fertiliser inputs and/or changes in grazing intensity) alter all of these fluxes. In general, increasing the intensity of utilisation (e.g. increasing stocking rate per se) will reduce all the fluxes, simply because it reduces vegetation cover (leaf area and so photosynthesis) although there is an optimum grazing.
Why Soil Organic Matter matters
Soil organic matter contributes to a variety of biological, chemical and physical properties of soil and is essential for good soil health.
Soil health is important to optimise productivity in agricultural systems.
Healthy, productive soil is a mixture of water, air, minerals and organic matter.
In turn, soil organic matter is composed of plant and animal matter in different stages of decay, making it a complex and varied mix of materials.
Functions of soil organic matter
Soil organic matter (SOM) is a key indicator of soil health because it plays a role in a number of key functions. These functions can be divided into three types:
biological functions of SOM
-provides nutrients and habitat for organisms living in the soil
-provides energy for biological processes
-contributes to soil resilience (the ability of soil to return to its initial state after a disturbance, for example after tillage).
chemical functions of SOM
-measure of nutrient retention capacity
-provides resilience against pH change
- main store of many key nutrients especially nitrogen and potassium.
Soil organic matter is a key indicator of soil health because it plays a role in a number of key functions.
physical functions of SOM
-binds soil particles into aggregates improving soil structural stability
-enhances water holding capacity of soil
-moderates changes in soil temperature.
There are often strong interactions between these different functions. For example, the biological function of providing energy that drives microbial activity also results improved structural stability and creates organic materials that can contribute nutritional capacity and resilience to change.
Optimising the benefits of soil organic matter
Managing soil organic matter for a maximum contribution to soil health and resilience can present a conundrum.
Decomposition and mineralisation of organic matter are required for functions such as provision of energy and nutrients. However, the maintenance or increases in organic matter help to maintain its positive effects on soil chemical and physical properties.
So, when managing soil organic matter the never-ending turnover and the need to replace and rebuild is a constant demand of good agricultural practice.
When selecting management scenarios to optimise the benefits of soil organic matter the following needs to be considered for each particular site:
- what are the most important functions that organic matter provides?
- how big is the contribution of organic matter to soil health and resilience?
Management actions that optimise the provision of these functions and maintain the contribution to soil health and resilience will ensure maximum benefit from soil organic matter.
How can we maintain or improve SOM% under dairy cow grazing in the UK?
1. Maintain permanent pastures
2. Minimize cultivation & use direct drill technology
3. Reduce Soil compaction & encourage earthworms
4. Return stored slurry to all pastures
5. Increase pasture production & the clover content & deep rooting grasses
6. Ensure sufficient S & P fertilizers are being used
7. Consider deferred pasture/sabbatical farming options including the USA concept of “Tall Grazing” at least on parts of the farm each year.
8. Soil test regularly (same time each year) & monitor SOM%.

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