The amount of water in the soil that is available to crops is another soil property that can be improved by increasing organic matter content. B. D. Hudson, in a 1994 paper, presented data on this for sands from Florida and silt loams from Iowa, Kansas, Minnesota, and Wisconsin (Hudson, 1994, as redrawn in Franzluebbers, 2010):
Soil water content is plotted against soil organic carbon (soil organic matter is ~58% carbon). Plant available water is the difference between the upper line, field capacity, and the lower line, wilting point. As the soil carbon (or organic matter) increases by 4x, the plant available water increases by 2.2 to 2.5x.
The good news is that such an improvement is possible, the bad news is that it would take a lot of time and money to quadruple organic matter levels. However, more realistic increases in soil organic matter can still significantly improve water holding capacity; another reason to maintain and add organic matter whenever possible.
Franzluebbers, A. J. (2010). Will we allow soil carbon to feed our needs? Carbon Management, 1(2), 237–251. doi:10.4155/cmt.10.25
Hudson, B. D. (1994). Soil organic matter and available water capacity. Journal of Soil and Water Conservation, 49(2), 189–194.