In a 3-year study from 2001 to 2003, Spanish researchers documented an 85% increase in organic carbon levels in the top 15 cm (6 inches) of soil in irrigated plots growing vegetables, with amendments of composted animal manure. Andalusia's soils are characterized by low organic matter content, and the plots had been growing cotton prior to the experiment.

Agron. J. 100:136–144 (2008)