101 Questions answered
Organic Matter and Water Holding: Small Gains, Big Benefits for Soil Health
The short answer is yes, improving soil organic matter (SOM) can improve soil water holding capacity (WHC). How much and how quickly it can do this is another matter.
Good news first. Soil organic matter helps improve soil structure, stabilise aggregates and pores, and improves water infiltration. On the whole, it helps make the soil ready to accept and hold on to more water.
Soil organic matter can also hold several times its own weight in water. In low rainfall areas and when the crop is stressed, even an extra millimetre of water will help.
But how much of an improvement in WHC is realistic? One rough guideline is that increasing SOM from 1–2% (an increase in SOC from 0.58–1.16%) increases soil water holding capacity by 2–5 mm (Hoyle and Murphy, 2018).
Another guideline is a 1% increase in SOM leads to a 2% increase in WHC. If the soil has a WHC of 150 mm, this is an additional 3 mm of water.
An extra 3–5 mm of WHC capacity sounds great, but how realistic is it to increase SOM by 1%?
Increases in SOC in dryland farming are usually in the range of 0.01 to 0.03% per year (0.02–0.05% SOM) and are not linear. In drier years, for example, SOM levels can drop down again. It’s going to take a long time to get a 1% boost in SOM and the associated boost in soil water holding capacity, if it’s even possible in your environment. See Question 89. ‘How much carbon can I add?’ for some realistic numbers.
Work by DEW (Tim Hermann, see graph ‘Grain yield needed to maintain SOC’ below) suggests that for a wheat crop, the yield required to maintain SOC at a specific level, e.g. to maintain 1% SOC, you would need to yield 1.7t/ha. This considers mineralisation and turnover. So, to increase OM there needs to be OM additions or yield above that, which is a big ask in low rainfall areas.
What if you add organic matter? Calculations from WA suggest you’d need to add an extra 100 t/ha of organic matter on top of normal additions from cropping, pasture, etc. to get a 1% increase in SOM.
There’s also some disparity between where the SOM is in the soil and where the crops need the water. In the Mallee, SOM tends to mostly exist in the top 10 cm of soil, so any increase in water holding capacity won’t be much help later in the season when roots are much deeper.
Overall, improving SOM through good farm management will improve soil quality and resilience. While the absolute increases in water holding capacity may not be high, the multiple other benefits such as improved soil structure, erosion resistance, and fertility create a more productive and resilient soil. It can also mean better root systems that can explore further to access soil water.
