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RESEARCH: Make Amends

Features - Turfgrass/Seed/Sod

Longwood Gardens in Kennett Square, Pa., tests compost as a potential fertilizer replacement.

| December 12, 2008

The addition of compost to turf has been shown to improve soil properties, increase microbial activity and supply plant nutrients. These improvements may promote faster turf establishment, increase turf quality, reduce plant disease outbreaks through greater microbial competition and reduce the need for supplemental fertilization.

Compost applications to turf have been used successfully for a number of years. Results have indicated that success depends on the makeup of compost used such as: weed seed and pathogen content, presence of inert material, maturity or degree of decomposition, pH, soluble salt content, metal content, particle size and C:N ratio. Proper application technique and moisture content at time of application are also important factors for success. 

Longwood Gardens produces compost that can be tailored to specific needs. Starting materials come from known sources with low weed-seed and pathogen contents. Woody and herbaceous materials can be mixed carefully in specific proportions.Grinding and screening increase uniformity in particle size, and monitoring oxygen levels and temperature to determine turning frequency ensures consistency of finished products. 

In 2002, 10-by-10-foot plots of tall fescue in full-sun locations were selected for a 4.5-year compost and fertilizer experiment at Longwood Gardens. Treatments included:

1. Compost applications in spring and fall.
2. Fertilization with 2 pounds of IBDU® SGN 230 (31-0-0) per 1000 square feet applied in the fall.
3. No amendment (control). 

Prior to compost application, all plots were core-aerated and compost was applied as a 1/4-inch top dressing. The compost used for this study was composed of one part grass clippings and one part woody material from tub grinding, passed through a 1/2-inch mesh screen. This material was composted for at least 12 weeks prior to application.

At least once a year over a 4.5-year period, each plot was visually evaluated by up to 13 people for turf color, turf density and disease density. Although there were minor differences among treatments throughout the study, no significant trends developed from these analyses. 

Specific soil characteristics were positively affected by both compost and fertilizer applications. Soil tests revealed that compost application nearly doubled the organic matter content from 2.9 percent to 5.5 percent, while organic matter content of plots that received fertilizer only increased 20 percent. Cation exchange capacity (CEC) also increased 46 percent from 10.4 to 15.3 meq/100g in compost-treated plots compared to non-treated plots. Similarly, plots that received fertilizer had only a 21 percent increase in CEC. Higher organic matter content was expected, because compost contains large amounts of this material.

A portion of organic matter is humus, which is highly colloidal and amorphous. These properties of humus translate into an extremely high surface to volume ratio that is even greater than clay particles. These characteristics are responsible for the high adsorptive capacity and CEC of soil organic matter.

Both fertilizer and compost applications increased soil pH from 5.4 to 5.8 and 6.3, respectively. These results were expected with the compost treatment because the pH of the finished compost was always 7 or above. Finished compost typically has a pH of 7 or greater and may be used as an amendment to increase soil pH.

Although application of compost and/or fertilizer had no strong effects of the visual characteristics of turf at Longwood Gardens, there was a positive effect of compost addition on soil characteristics. Increasing soil organic matter and CEC can greatly improve soil quality. Soils with a higher CEC have a greater nutrient holding capacity and therefore can hold on to essential plant nutrients for longer periods of time. These stored nutrients provide turf with a consistent and steady stream of nutrition and reduces the feast and famine effect of a typical fertilization program. Higher soil CEC also increases soil pH buffering capacity, so shifts in pH are more gradual and easier to detect by turf managers.

The higher amounts of N, P, K, Ca and Mg in compost-treated plots compared to all other treatments are presumably due to both the minerals contained within the compost and the increased nutrient holding capacity of the soil (Table 1).  The higher N content in compost-treated plots suggests that compost could serve as a fertilizer substitute. One issue associated with continually applying compost is that soil pH could potentially increase out of the optimum range, which is 6.0 to 7.0 for tall fescue. Additionally, undesirable elements from the compost could begin to accumulate and cause nutrient toxicities. Higher concentrations of heavy metal elements (Pb, Cd and Me) are commonly found in composts made from municipal biosolids. Because Longwood Gardens does not include biosolids as a compost feedstock, this problem is avoided on site.

Based on the results of this study, Longwood Gardens continues to use compost as a turf amendment in conjunction with its fertilizer program. Ongoing soil testing allows turf managers to determine when to apply compost. Nitrogen continues to be applied as IBDU® but when soil tests call for P and/or K, compost is applied in addition to the N fertilizer. Through this type of nutrient management program, Longwood has decreased its dependence on chemical fertilizers, cut fertilizer costs and reduced environmental impacts, all while recycling compost – what others would think of as a “waste” product destined for the landfill.

--Matt Taylor, Shawn Kister, Jim Harbage and Casey Sclar

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