Adding organic matter has a small effect in increasing soil's water holding capacity.

What is it about?

Adding organic matter has also been widely promoted for increasing soil's water holding capacity, however there is still no clear consensus on its quantitative effect. We 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. We found that the effect of adding OM to soil enhanced available water capacity only modestly, with an average value of between 15 to 20 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.

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Why is it important?

Compared with reported annual rates of carbon sequestration after the adoption of conservation agricultural systems, the effect on soil available water is negligible. Thus, arguments for sequestering carbon to increase water storage are not well supported. The results also suggest that the gradual loss of organic matter from soil would have a minimal effect on the hydrological cycle.

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