Sunday, April 02, 2006

Carbon Stops Salt

Jones C. E. (2006) Australian Farm Journal May 2006

Balancing the Greenhouse Equation – Part V

Soil carbon stops salt

Balancing the agricultural greenhouse equation is not just about reducing emissions. As Christine Jones has explained in previous articles in this series, it is also about restoring soil health. One of the most visible and pervasive symptoms of poor soil health is dryland salinity.

Since the time of European settlement there have been significant losses of humic materials and other forms of organic carbon from Australian soils. This has had a dramatic impact on landscape health in many ways, not the least of which is an increased incidence of dryland salinity.

Humic substances play three vital roles in reversing dryland salinity:

i) increased soil water storage potential
ii) stimulation of biological activity
iii) chelation and inactivation of salts

The influence of soil humus on the storage and subsequent movement of water in the landscape is the most important of these roles.

Why is the way water moves important?

When moisture rises in the soil profile, it is often accompanied by salts, which concentrate on the soil surface through the process of evaporation. The key factor in reversing dryland salinity is to always have a small amount of fresh water slowly moving downwards, flushing salts from the root zone. Fresh water has a lower density than salt water and will sit above the salt, provided the soil is capable of retaining sufficient moisture for this purpose.

Water movement in soil depends on many factors, including rainfall, slope, soil depth, soil texture and the clay minerals present. We cannot control rainfall, slope or soil type. But we certainly CAN influence the capacity of the soil to store water.

Carbon and water

Humic substances and other forms of soil organic carbon have highly significant effects on soil structure, surface condition, porosity, aeration, bulk density, infiltration rates, water storage potential and the amount of plant available water. Improvements in any of these factors increase the effectiveness of the rain that falls, enhancing productivity as well as reducing rates of erosion, dispersion, waterlogging and dryland salinity.

Sounds good, but what does it all mean??? Let’s use an example.

Glenn Morris extensively researched the water holding capacity of humus for his Masters Degree at the University of Sydney. Glenn concluded that within the soil matrix, one part of soil humus can, on average, retain four parts of soil water.

From this we can calculate how water storage in the top 30 cm of soil (roughly the top 12” in old terms) will be influenced by changes in the level of soil organic carbon. The majority of Australian topsoils have bulk densities in the range 1.2 to 1.8 g/cm3. We will assume a bulk density of 1.2 g/cm3.

The calculations in Table 1 show that an increase of 14.4 litres (almost two buckets) of extra plant available water could be stored per square metre in the top 30 cm (12”) of soil with a bulk density of 1.2 g/cm3, for every 1% increase in the level of soil organic carbon. That’s 144,000 litres, or about 16,000 extra buckets of water that could be stored per hectare, in addition to the water-holding capacity of the soil itself.

The flip side is that the same amount of water will be lost when soil carbon levels fall. Low soil moisture and low levels of soil organic carbon go hand in hand.

Soil water storage

Factors that reduce soil organic carbon levels and therefore reduce the ability of soil to store water, include

i) Loss of perennial groundcover
ii) Intensive cultivation
iii) Bare fallows
iiv) Stubble burning and pasture burning
v) Continuous grazing

Most conventional agricultural practices include one or more - or all - of the above. Over the last 50 to 100 years, soil organic carbon levels in many areas have fallen by about 3%. This represents the LOSS of the soil’s ability to store around 432,000 litres of water per hectare.

One inch (25mm) of rain delivers 250,000 litres of water per hectare, while two inches (50mm) delivers 500,000 litres per hectare.

If the soil has lost it’s porosity due to structural changes, millions of litres of water move across the landscape as run-off - taking soil and nutrients as well – to cause both recharge and discharge problems in lower landscape positions. Reversing dryland salinity will therefore first and foremost require that water be held where it falls.

The emphasis needs to be on rapid infiltration and increased water storage in topsoil in all parts of the catchment, rather than on high water use after the water has run off. Attempts to use accumulated water represent bandaid solutions that fail to address fundamental causes. High water-use plants such as lucerne and trees often exacerbate dryland salinity by over-drying soil profiles and drawing salts closer to the surface.

The end result is the de-watering of saltier and saltier soils.

Greenhouse emissions

In addition to water losses from the landscape, a 3% reduction in soil organic carbon represents almost 400 t/ha extra carbon dioxide (CO2) emitted to the atmosphere, contributing to increased levels of greenhouse gases and the possibility of climate change. With global warming, rainfall levels could fall even further …. while degraded soils continue to lose their capacity to hold water … and evaporation rates continue to increase … leading to more and more dryland salinity.

Re-balancing the soil water equation and re-balancing the greenhouse equation have many factors in common. Both processes require soil building, which in turn requires that carbon dioxide from the atmosphere be sequestered in soil as organic carbon.

Building soil carbon

If organic carbon begins and ends its journey as a gas, carbon dioxide (CO2), how does it get into soil?

The ‘way in’ for soil carbon is the process of photosynthesis in green leaves. The cheapest, most efficient and most beneficial form of organic carbon for soil is exudation from the actively growing roots of plants in the grass family, which includes many crop plants. The decomposition of fibrous roots is also an important source of carbon in soils. Organic carbon additions are governed by the volume of plant roots per unit of soil and their rate of growth. The more active plant roots there are, the more carbon is added. It’s as simple as that.

Yearlong Green Farming

It is important that soil always be covered and that green plants be present for as much of the year as possible to sequester atmospheric carbon and translocate it to soil as organic carbon. This builds organic matter and develops optimum physical and biological conditions, irrespective of agricultural enterprise, environment or landscape position.

Yearlong Green Farming (YGF) has two main principals:-

i) roots of actively growing green plants transfer carbon into soil
ii) in non-growth periods soil must remain covered to prevent carbon losses

Variations on the Yearlong Green theme are limited only by human creativity.

One approach is to double crop grain and forage species, so that soil building continues all year. For example, a direct drill winter cereal could be followed by direct drill forage sorghum. The summer forage crop will not only prevent losses of soil carbon, but will be more profitable than maintaining a bare summer fallow.

Alternatively, a summer grain crop could be followed by mixed species winter forage (eg oats, triticale, legumes). Yearlong Green Farming practices are most beneficial when they include livestock, because strategic grazing maximises the sequestration of soil carbon.

Pasture Cropping

The quickest and most cost effective way to restore degraded cropland is through a grazed perennial pasture ley. Ironically, the good work is undone when conventional cropping resumes. Thanks to the brilliant insight and visionary thinking of innovators Darryl Cluff and Colin Seis, landholders wishing to build soils through a Yearlong Green technique now have the opportunity to combine annual crops and perennial grasses in the revolutionary ‘one-stop-shop’ land management technique known as Pasture Cropping.

Many of the benefits of Pasture Cropping can be attributed to having perennial grasses and cereals together, side by side in space and time. The ongoing carbon additions from the perennial grass component evolve into highly stable soil aggregates, significantly improving soil structure, while the short-term, high sugar forms of carbon exuded by the cereal crop stimulate microbial activity.

In this positive feedback loop, CO2 respired by plant roots and soil microbes, slowly moves upwards through the topsoil and increases the partial pressure of CO2 beneath the crop/pasture canopy, enhancing photosynthetic potential. As money makes money, so carbon makes carbon – but only when the management is right.

The need for change

Under conventional cropping regimes, the stimulatory exudates from crop roots are negated by cultivation, bare earth and harsh chemicals. Over time, soil carbon levels fall to levels where the soil is essentially ‘dead’ and has little ability to store water. The prime purpose of bare fallows - water storage - becomes self-defeating. Bare soil is also an open invitation to weeds.

With grazing enterprises, soils continually lose organic carbon under set-stocking regimes if insufficient root biomass is present in the soil. This is particularly evident under annual pastures. Forms of grazing management designed to build soil and restore healthy, perennial grasslands are absolutely essential (David Marsh, Australian Farm Journal, March 2006).

Reversing dryland salinity requires a whole of landscape approach. The bottom line is that soils low in humic substances and biological activity cannot effectively store water. The water moves off-site, removing soil and nutrients and transforming the most precious of our natural resources into salinity, sedimentation and eutrophication ‘problems’.

A ‘no salt’ result

A ‘no salt’ result depends on regenerative groundcover management designed to increase levels of organic carbon.

This will enable soil to:-

i) absorb moisture as it falls and
ii) hold water for sufficient length of time for slow percolation to occur

It is extremely important for future generations that these processes be rekindled.

Moisture moving slowly down the soil profile provides clear, filtered water to maintain perennial base flow to springs and streams. When the water runs on the top of the ground, or on top of the subsoil, we witness the all too familiar flash flood syndrome, with rivers carrying too much and then too little water, while freshwater aquifers continue to decline. Many once ‘perennial’ streams are now ephemeral, simply due to losses in soil carbon in the catchments that feed them.

Increased levels of humic materials in agricultural soils will not only reverse the incidence and severity of dryland salinity – they will optimise farm productivity and significantly improve the quality of our air and water.

Take a photo of your ‘salt patch’ to show the grand-children what it used to be like – then find out how you can start Yearlong Green Farming today!!

Further information

Morris G. D. (2004). ‘Sustaining national water supplies by understanding the dynamic capacity that humus has to increase soil water-holding capacity.’ Thesis submitted for Master of Sustainable Agriculture, University of Sydney, July 2004. Contact

‘Managing the Carbon Cycle’ Forums - Horsham, VIC, 26-27 July; Katanning, WA, 2-3 August and Kingaroy, QLD, 25-26 October 2006. See or contact