Soil Carbon's impact on water retention

Border Rivers-Gwydir CMA and Grain & Graze ‘Practical clues for pasture cropping’ workshops.
‘Malgarai’ 27 Feb, ‘Gowrie’ 28 Feb and ‘Kyabra’ 1 March 2006

Soil CARBON means WATER to me!!

Christine Jones

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

Changes to groundcover management can have highly significant effects on levels of soil organic carbon, influencing soil surface condition, soil structure, porosity, aeration, bulk density, infiltration rates, water storage potential and the amount of plant available water. An improvement in any of these factors increases 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.

The majority of Australian topsoils have bulk densities in the range 1.2 to 1.8 g/cm3. For this example we will assume a bulk density of 1.2 g/cm3. Within the soil matrix, stable forms of organic carbon, such as humus, can hold up to seven times their own weight in water. To err on the conservative side, let’s assume that one part of soil organic carbon can retain four parts of soil water (Morris, 2004).

How will water storage in the top 30 cm of soil (roughly the top 12” in old terms)
be influenced by changes in the level of soil organic carbon (OC)?

Table 1. Change in the capacity of soil to store water (litres/ha) with changes in levels of soil organic carbon (OC) to 30 cm soil depth. Bulk density 1.2 g/cm3

Change in Change in Extra water Extra water CO2 sequestered
OC level OC (kg/m2) (litres/m2) (litres/ha) (t/ha)

1% 3.6 kg 14.4 144,000 132
2% 7.2 kg 28.8 288,000 264
3% 10.8 kg 43.2 432,000 396
4% 14.4 kg 57.6 576,000 528


The above calculations 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/m3, for every 1% increase in the level of soil organic carbon. That’s 144,000 litres, or about 16,000 extra buckets of water per hectare, in addition to the water-holding capacity of the soil itself.

Factors which 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 ability to store around 432,000 litres of water per hectare.

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 evaporation rates increase … and degraded soils continue to lose their capacity to hold water.

What are we going to do??

Charman and Roper (2000), note that in order to increase soil organic matter levels and develop optimum physical and biological conditions for crop production, the soil needs to be managed in a similar way to a perennial pasture ley.

Landholders now have the opportunity to combine crops and perennial pastures in the revolutionary ‘one-stop-shop’ land management technique known as Pasture Cropping (Seis 2005). What will be YOUR first step to learning about this?

Do you want more SOIL or less? More CARBON or less? More WATER or less?

That decision is entirely in your hands.

Further information

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

References

Charman, P.E.V, Roper, M.M. (2000). Soil Organic Matter. In: Soils: Their Properties and Management. (Eds. P.E.V. Charman and B.W. Murphy) pp. 260-270. 2nd Ed. Oxford University Press.
Jones, C.E. (2006). Aggregate or aggravate? Creating soil carbon. BRG CMA and Grain & Graze ‘Practical clues for pasture cropping’ workshops. ‘Malgarai’, ‘Gowrie’ and ‘Kyabra’
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.
Seis, C. (2005). Pasture Cropping. A land management technique with potential to farm carbon. Proceedings ‘Managing the Carbon Cycle’ Forum, Armidale, NSW, 13-14 Sept. 2005, pp..37 - 39.