Not all composts are alike. The nutrient content, microorganism diversity and population, cation exchange capacity and water holding capacity of compost can be different based on the feedstocks used to make the compost, the process used to make the compost, and the maturity of the compost at the time of application.

Some interesting facts about compost:

• Soil scientists report that for every 1 percent of organic matter content, the soil can hold 16,500 gallons of plant-available water per acre of soil down to one foot deep. That is roughly 1.5 quarts of water per cubic foot of soil for each percent of organic matter. Increasing the organic matter content from 1 to 2 percent would increase the volume of water to 3 quarts per cubic foot of soil.
• Rodale Institute indicates that 1 pound of carbon can hold up to 40 pounds of water. That calculates out to be approximately 38,445 gallons of total water per acre six inches deep.
• A 1994 study by A. Maynard found that a 3 inch layer of leaf compost rototilled to a 6 inch depth increased water holding capacity 2.5 times that of a native sandy soil and provided almost a 7 day supply of plant available water.
• In a 2000 study, Maynard found that increasing the water holding capacity of the soil by adding compost helped all crops during summer droughts by reducing periods of water stress. The amount of water in a plow layer (8 inches) of the compost amended soil increased to 1.9 inches compared with 1.3 inches in unamended soil. Since vegetables require 1 inch of water a week, at field capacity, the compost amended soil held a 2-week supply of water.
• Soil organic matter is built up over time with continuous applications of compost. It is estimated that applying a ton of compost to the acre on a soil with 1 percent organic matter can increase that soil’s organic matter content by 10 percent.

Learn more here 

Also see: Field Guide to Compost Use