Improving soil organic matter and water holding capacity

Environment and climate
David Exwood

David Exwood

NFU Deputy President

NFU Vice President David Exwood talks about the ways he is working to increase SOM (soil organic matter) and the water holding capacity of the silt-clay soil on his mixed farm in West Sussex.

David manages a mixed-farming business, located south of Horsham, West Sussex. The first generation farm has been in his family since 1990 and has a total land area of 2,500 acres.

Some 1,800 acres is in a long, six-year rotation of winter wheat, winter oats, winter oilseed rape, spring beans and spring maize. The remaining land area consists of temporary leys and permanent pasture grazed by sheep and beef cattle.

Soil characteristics

The soil texture is approximately 50% silt, 30% clay and 20% sand. It is classed as a silty-clay, characterised by a clay content greater than 50% which is highly productive but challenging to manage.

Silty-clay soils are advantageous due to their high cation-exchange capacity, good water holding capacity and ease of workability. However, silty-clay soils are notorious for poor water infiltration and tendency to crust, and can become compacted.

The soil type is categorised as a planosol, defined as soils with stagnating water and abrupt textural discontinuity over a sub-surface layer of clay accumulation.

The undulating topography of the land also means diffuse pollution and runoff can be an issue in the area.

The climate and soil type can create two extremes: waterlogged soils but alternately incredible periods of dryness. Therefore, soil management is crucial for crop productivity and environmental sustainability.

Soil management

A full soil test is carried out every three years to assess the health of the soil and whether management can be changed to improve sustainability.

David has a particular interest in SOM (soil organic matter) and how it can be used to increase the water holding capacity of soil and overall health.

SOM can be defined as all the organic materials found in a soil which are part of or have been part of living organisms.

The optimal agricultural soil contains 45% minerals and 5% SOM, with the remaining 50% being water and air-filled pore space.

David actively carries out practices with the aim of increasing the SOM content of his soils. The practices listed below can all contribute to increasing the SOM  percentage in the long term:

  • long, diverse rotations
  • cover cropping
  • grass leys
  • organic manures
  • straw incorporation
  • maintenance of drains
  • careful timings.

Increasing SOM content

Crop rotations – the use of long and diverse crop rotations is known for increasing SOM because the quality and quantity of crop residues incorporated into the soil increases, which then becomes available to microbial communities to stabilise SOM.

Cover crops – cover crops are considered a ‘green manure’ and are essential in increasing SOM. Cover crops are beneficial for overall soil health because they are grown over the periods when soils would otherwise be bare, protecting soil from degradation and promoting environmental resilience.

Drains – maintenance of drains enables improved soil health because a functioning drainage system allows soil to spend less time in an anaerobic state, aiding nutrient uptake, root growth and greater ability of soil to retain structure and withstand compression.

Timings – careful timings, such as only ploughing when dry also benefits soil health.

Environmental and productivity benefits

Since focusing on SOM improvement, David has experienced a SOM increase of 1% in five years.

The current SOM percentage across the farm is between 3-4%.

As a result, the farm has seen dramatic improvements in yield, workability, less cultivations and reduced diesel usage.

Andy Neal at Rothamsted Research states that for a silty-clay loam ‘every 1% increase in soil organic carbon equates to a water holding capacity increase of 354,000 litres a hectare to a depth of 30cm’.

Crucially, soil texture has a great influence upon this, and the relationship is not linear.

The overall benefits associated with increasing SOM, and subsequently the water holding capacity of soil, primarily involve greater environmental resilience to extreme wet and dry periods.

Overall, by increasing the water holding capacity of soil, crop yield and environmental resilience will be improved, promoting economic and environmental sustainability.

Resilience to flooding and drought

During dry periods, soils will hold more water and be able to withstand long periods of drought.

During wet periods, soils will have a greater capacity to store water, reducing flooding and erosion.

Moreover, soils with a greater water holding capacity require less water from rainfall or irrigation, increasing their resilience and reducing the cost of irrigation and associated infrastructure.

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