Have we overestimated the contribution of organic matter on soil water retention.

From a large number of studies and databases they analysed,
they concluded that the effect of addition of OM on soil-available
water capacity was modest, with average values of between 1.4
and 1.9mm 100mm−1 per 10 g kg−1 increase in OC. Sandy soil
was more responsive to the increase in OC, whereas the effect on
clayey soil was almost negligible. The largest effect of OC was in
large pores, possibly from the formation of macroaggregates, and
its effect decreases with a decrease in size of pores.
The results also suggest that the gradual loss of organic matter
from soil would have a minimal effect on the hydrological cycle.
Global warming might cause a loss in soil carbon, but the effects
on soil water availability to plants and consequent effects on the
hydrological cycle might be less than thought previously. The
negative feedback could be less serious. In addition, suggestions
that sequestering carbon would increase soil water storage are questionable.
They state that even with the best scenario of farm
management practices, available water capacity cannot be increased
meaningfully. Nevertheless, storing carbon in the soil should still be
pursued for improving soil structure, atmospheric CO2 attenuation
and nutrient cycling. Macropores created by organic matter can still
have important effects in increasing infiltration and gas transport.