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Soils 4: Organic Matter

Course: Sustainable Soils | Last Updates: 26th November 2015

 
Alice Midmer
Project coordinator
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Humus, the dark-coloured organic material in the final stages of decomposition, is relatively stable reservoir of plant nutrients and water.
OM content can have a large influence on the soil structure and this in turn can impact compaction and drainage issues. A soil rich in OM is also preferred by much soil biodiversity as it provides a healthy living environment.
Similarly, soils rich in OM are intrinsically healthy and nutritious for crops. In this way, OM is an integral part of soil health but can often be overlooked in favour of more obvious soil qualities. Soils that are rich in OM are generally dark brown/black in colour; light brown is generally sign of lower OM content.
This video provides useful guidance:

Measuring OM

Testing for organic matter percentage can be undertaken in a laboratory. It can also be assessed visually using the table below as a reference.

Indicator Poor (0) Medium (1) Good (2)
Nutrient balance and exchange
Soil organic matter status Organic matter levels are low, soil is crusty, cloddy, hard. Light brown in colour. Organic matter levels are moderate, some crusting and clods. Brown in colour. Organic matter levels are high. Soil is friable, with good soil structure. Dark brown in colour.

Sandy soil has inherently low organic matter while peaty soils have naturally higher levels and in some situations rich organic peat is used to help boost the content. Clay and loamy soils are somewhere in between, depending on individual composition, use and management. Land under grass will often have higher organic matter levels than arable land with comparable soil type. Despite these variances, OM is important in all types of soils and with the appropriate soil management can be improved.

Improving OM

Despite variations in soil type, there are a number of measures that can help enhance the organic matter status of your soil and its availability to crops and grassland. Adjusting soil management practices appropriately can have a large influence on the soil quality and can particularly influence the long term sustainability of your soil. These farm case studies illustrate some of the options.

Test OM %

Few farmers measure the percentage of OM in their soils because it is unlikely to change significantly from year to year. However gaining a specific measurement can help track whether its increasing and where it varies across the farm.
An organic matter measurement should be added to regular soil testing every five years or so to get a clearer picture and assess impact of practices.

Increase organic additions

Addition of OM as farmyard manure (FYM) or slurry is widely adopted on farms and often the turning point in a soil’s performance. Different types of organic matter additions provide different benefits to the soil but all can help improve the content and general soil health. Slurry and FYM are largely added for their nutrient value while box muck and compost are valued for their benefits to soil structure.
If organic materials are not readily available, there is always the possibility of swapping materials with neighbouring livestock farms. Application method of organic matter could have an influence on impact. While most farmers broadcast manures, Low Field Farm was investigating umbilical cord spreading to minimise the impact of tractors, while manure at Llysun was drip fed to minimise volatilisation.

Ploughing straw

Ploughing straw back into the land can provide a really important supply of organic matter.
Since the straw burning ban, this has formed an essential constituent to many soil management plans.
 

Cover crops

Cover crops have many benefits. They can help reduce erosion as well as aid drainage but they can also form be an important source of organic material. Instead of leaving stubble or your soil bare, consider the benefits of including a winter cover crop.

Grass leys

Putting land into grass, or as a break crop, is a highly effective way to build up organic and restore soil structure and health. Introducing livestock on the farm to graze pasture also provides valuable FYM benefits.
This video shows the positive impact on yields and reducing run off:

Min-till

Not inverting the soil and disturbing it less via min-till is another technique to help minimise OM loss via oxidation and promote its build-up. Many arable farmers carry out some form of min-till during their rotation; strip-, min- or zero-till because of the benefits to soil quality and organic matter content.

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