Adding organic matter to soil has a limited effect on water holding capacity
Sequestering carbon in the soil via the addition of organic matter has been widely promoted for the mitigation of climate change. Enhancing soil organic matter can improve soil quality, i.e., increasing nutrient retention, improving soil structure, enhancing soil biotic activity and improving soil moisture and temperature regimes. Adding organic matter has also been widely promoted for increasing soil's water holding capacity. It was suggested as a means to buffer yields against future variable weather conditions.
While the positive effect of OM on soil water retention is much studied and widely promoted, there is still no clear consensus on its quantitative effect. The increase in the amount of water that is available to plants with an increase in organic matter is still uncertain and maybe overestimated. To clarify this issue, researchers from the Sydney Institute of Agriculture at the University of Sydney conducted a critical review now published in the European Journal of Soil Science. They evaluated data from 60 published studies and analysed large databases of soil water (more than 50,000 measurements globally) seeking relations between organic carbon (OC) and water content at saturation, field capacity, wilting point and available water capacity.
From this first comprehensive review, the authors found that the effect of adding OM to soil enhanced available water capacity only modestly, with an average value of between 1.5 to 2.0 mm per m with 1 percent mass increase in organic carbon. Sandy soil was more responsive to the increase in OM, whereas the effect on clayey soil was almost negligible. The largest effect of OC was in large pores, possibly from the formation of large aggregates, and its effect decreases with a decrease in the size of the pores.
Read more at: https://phys.org/news/2017-10-adding-soil-limited-effect-capacity.html#jCp
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