Compost can increase the water holding capacity in droughty soils
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EmailCompost has the ability to increase water holding capacity of soils and can be a beneficial soil amendment in agricultural applications.
Compost is an earthy-smelling, humus-like material that is a product of the controlled aerobic decay of organic nitrogen (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. 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. 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 Composting Handbook.
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 one foot deep. That is roughly 1.5 quarts of water per cubic foot of soil for each percentage of organic matter, according to Sullivan in Drought Resistant Soil. Rodale Institute researchers assume that one 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 Hudson showed that a silt loam soil with 4% organic matter holds more than twice the water of a silt loam with 1% organic matter. 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% to 3% can have major positive impacts. A 50-acre farm that improves its soil organic matter from 1% to 3% will hold millions of gallons of water after each rain. The numbers get big quickly.
How effective is compost at holding water?
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.
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 levels. In the Generally Accepted Agricultural and Management Practices for Nutrient Utilization (Nutrient GAAMPs) it states when soil phosphorus levels exceed 300 pounds per acre, no source of phosphorus can be applied. That means no compost can be applied to soils that exceed 300 pounds per acre. When soil phosphorus levels are between 150 and 299 pounds per acre, compost is to be applied based on the phosphorus removal rate of the crop. When soil phosphorus levels are less than 150 pounds per acre, compost is applied based on the crop's nitrogen requirements.
- 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.
Organic matter in soil is built up over time with continuous applications of compost. Some farmers in Michigan’s Thumb area have found that applying 1 to 2 tons of compost/acre/year on field crops makes a difference in the soil’s ability to grow a crop. It is estimated that applying one ton of compost to an acre of soil with 1% organic matter can increase that soil’s organic matter content by 10%. Compost spread evenly over one acre at a depth of one-inch equals about 135 cubic yards or 54 tons, assuming the compost has 60% organic matter and a bulk density of 800 lbs./cubic yard at 30% moisture.
As we continue to face changes in our weather and climate, it’s important to understand how to adapt your own agricultural practices. Compost application may help in reducing water usage on farms, and support crops throughout droughts and dry spells. For questions on compost application or production, you can contact MSU Extension’s compost systems educator, Eliza Hensel, at hensele1@msu.edu.
Additional information
- MSU Extension’s Drought Resources
