By M. Charles Gould, Extension Educator

Compost is an earthy-smelling, humus-like material that is a product of the controlled aerobic decay of organic nitrogen (N) (such as manure) and carbon (such as sawdust, straw or leaves). One advantage of compost is its ability to hold moisture. The focus of this article is to understand how to choose composts that increase the soil’s water-holding capacity.

It is important to understand at the outset that not all composts are alike. For example, composts made from manure are not the same as composts made from leaves. The nutrient content, microorganism diversity and population, cation exchange capacity (CEC) 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. 

Therefore, it is important to understand the quality of a compost before using it to ensure you get the intended benefit you are seeking. Further information on compost quality can be found in the following publications: Field Guide to Compost Use, AAPFCO Soil Amendment/Compost Uniform Product Claims and Compost: Matching Performance Needs with Product Characteristics.

Water-Holding Capacity of Soil Organic Matter

Soil scientists report that for every 1% of organic matter content, the soil can hold 16,500 gallons of plant-available water per acre of soil down to 1 foot deep. That is roughly 1.5 quarts of water per cubic foot of soil for each percent of organic matter, according to Sullivan in “Drought Resistant Soil. Agronomy Technical Note. Appropriate Technology Transfer for Rural Areas” at the National Center for Appropriate Technologies in 2002. 

Increasing the organic matter content from 1% to 2% would increase the volume of water to 3 quarts per cubic foot of soil. Rodale Institute presenters, on the other hand, assume 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 6 inches deep. 

The point here is that organic matter holds a lot of water, thus, the amount of organic matter in a soil directly influences the availability of water to a crop over time. However, organic matter in droughty soils breaks down so rapidly that getting above 2% or 3% is difficult to do, but getting to 2-3% can have major positive impacts.

How Effective is Compost at Holding Water?

A 1994 study by A. Maynard found that a 3-inch layer of leaf compost roto-tilled 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.

Reduce Water Application

The U.S. Compost Council (2008) has stated that the frequency and intensity of irrigation may be reduced because of the drought resistance and efficient water use characteristics of compost. Compost reduces soil crusting, which helps with water absorption and penetration into the soil. Recent research suggests that the addition of compost in sandy soils can facilitate moisture dispersion by allowing water to more readily move laterally from its point of application.

How Much Compost Must Be Added to Soil to Increase Organic Matter Content?

The limiting factor for compost application in Michigan is soil phosphorus (P) levels. In the Generally Accepted Agricultural and Management Practices for Nutrient Utilization (Nutrient GAAMPs) it states when soil P levels exceed 300 pounds per acre, no source of P can be applied. That means no compost can be applied to soils that exceed 300 pounds per acre. 

When soil P levels are between 150 and 299 pounds per acre, compost is to be applied based on the P removal rate of the crop. When soil P levels are less than 150 pounds per acre, compost is to be applied based on the N requirements of the crop.

For most cropland in Michigan, this means that low amounts of compost will be applied, so choosing composts high in organic matter is critical if increasing soil water holding capacity is your goal.

According to the U.S. Compost Council’s Field Guide to Compost Use, farmers should choose composts that have an organic matter content between 50-60% and a water holding capacity of 100% or higher.

 When purchasing compost, ask to see an analysis to verify organic matter content and water-holding capacity. Commercial sources of compost in Michigan can be found at the FindAComposter.com website.

Soil organic matter is built up over time with continuous applications of compost. Some farmers in Michigan’s Thumb area have found that applying 1-2 tons of compost per acre per year on field crops makes a difference in the soil’s ability to grow a crop. It is estimated that applying a ton of compost to the acre on a soil with 1% organic matter can increase that soil’s organic matter content by 10%.

Compost spread evenly over 1 acre at a depth of 1 inch equals about 135 cubic yards or 54 tons, assuming the compost has 60% organic matter and a bulk density of 800 pounds per cubic yard at 30% moisture.

Michigan State University Extension educators are available to assist farmers with compost use and application. To locate an educator, go to Michigan State University Extension Bioeconomy page and click on the “MSUE Find an Expert” button on the left side of the page. When the “Find an Expert” page is displayed, type the word “compost” in the line next to “Educator Area of Specialty” and names of individuals you can contact will be displayed.