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 water logging 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 1.4 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 1.4 g/cm3
|
Change in |
Change in |
Extra water |
Extra water |
CO2 sequestered |
|
OC level |
OC (kg/m2) |
(litres/m2) |
(litres/ha) |
(t/ha) |
|
1% |
4.2 |
16.8 |
168,000 |
154 |
|
2% |
8.4 |
33.6 |
336,000 |
308 |
|
3% |
12.6 |
50.4 |
504,000 |
462 |
|
4% |
16.8 |
67.2 |
672,000 |
616 |
The above calculations show that an increase of 16.8 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 1.4 g/cm3, for every 1% increase in the level of soil organic carbon. That’s 168,000 litres, or almost 20,000 extra buckets of water per hectare, in addition to the water-holding capacity of the soil itself.
With thanks to Christine Jones, PhD Founder, Amazing Carbon www.amazingcarbon.com
