Microclover - Tall Fescue Lawns in the Mid-Atlantic Region

Amending Soil with Compost to Reduce Stormwater Runoff and Lawn Fertilizer Use

Compost to Reduce Runoff and Lawn Fertilizer Use

Soil Restoration of New Residential Lawn Areas: A House Divided

Mark Carroll, Ph.D., University of Maryland

Recent emphasis on the use of stormwater management practices that incrementally reduce runoff has emphasized the need to retain previous areas in new developments and to restore the hydrologic function of the soil in these areas when it is compromised by construction activity. Soil compaction, while necessary to stabilize the foundation of new homes, dramatically reduces soil infiltration and impedes the growth of plants. Compacted soil frequently results in poorly established lawns, which increases the potential for runoff and may drive some new homeowners

to use more lawn fertilizer than homeowners residing in older, more mature developments.

Soil restoration is a technique that can be used to alleviate soil compaction and improve the nutrient retention capacity of soil. The two primary practices associated with soil restoration are tillage and the addition of organic amendments such as compost. The type of tillage that needs to be performed and amount of compost that needs to be added to restore the hydrologic function of soil are dependent on soil type, the depth of soil compaction and the intended stormwater application of area in question. 

If the area will not receive runoff from adjacent impervious areas and compaction is limited to the top few inches of surface it can be alleviated by simply rototilling the soil. A simple rule of thumb when using compost to improve the infiltration properties of soil high in silt and clay is to add compost at a ratio of two parts soil to one part compost. Since most rototillers only mix soil down to depth of six

inches, this means a general purpose lawn area having compacted soil should be amended with about two inches compost prior to turf establishment. 

When compaction extends to the subsoil a multi-shank ripper is used in combination with rototilling to loosen soil and incorporate compost. The ripper breaks up the subsoil or pan layer residing below the operational depth of the rototiller while the rototiller incorporates the compost into the topsoil. A third implement such as a deep angled subsoiler is sometimes used after ripping to break up the top

soil and upper portion of the subsoil without inverting the two.

Use of a multi-shank ripper and subsoiler are rare in residential lots because of the limited maneuverability of the large tractors that are needed to draw these implements through the soil and the presence of buried utility lines on the lot. Smaller more maneuverable tractors can be used to pull single shank rippers through areas of the landscape where the need for restoration of hydraulic function of soil is high. Lawn areas that serve as a filter strip and rooftop disconnection paths are two examples of such areas.

When high volume additions of compost are made to alleviate soil compaction the amount of nutrients added to soil can be substantial. For example, incorporating two inches of a widely available municipal compost created from yard trimmings will add about 60 pounds of nitrogen (N) and 15 pounds of fertilizer grade phosphorus (P) per 1000 ft2 of treated area. The addition of this amount of N and P will reduce or eliminate the need for lawn fertilizer for several years but has raised concerns about the loss of these two nutrients in stormwater runoff.

Studies that have utilized relatively small field plots have reported that the reduction in runoff resulting from the addition of high volume amounts of compost is large enough to result

in reduced N and P losses from lawns when compared to lawns established in a non-composted amended soil. With impervious surfaces often contributing flow to lawn areas, the dynamics of runoff

from residential lots can be very different small research plots. With this in mind we conducted a project within a new residential development in Clarksville, MD, to compare N and P runoff losses from lots amended with a high volume compost prior to turfgrass establishment with lots that did not receive this treatment. The project was funded by the National Fish and Wildlife Foundation.

The Preserve at Clarksville is a near symmetric development consisting of an east and west side where runoff from the backyards on each side of the development is directed to a swale located just beyond the back property line of each lot. The swale behind the lots on the east side of the development has a configuration of drains that allowed us to isolate and collect runoff from two adjacent lots on this side of the development. The new homeowners on this side of the development

permitted us to amend the top 5.5 inches soil in their backyards with 1.9 inches compost after which the yards were seeded with a tall fescue/microclover seed mixture. Microclover is a small leaf clover

that adds N to the soil and was included in the seed mixture to reduce the long term fertilizer needs of the lawn. The two homeowners agreed not to fertilize or apply herbicides to their lawns for the duration of the project.

The swale on the west side of the development received runoff from seven lots. Six of the lots had previously established lawns on them at the beginning of project. A house was built on the seventh lot during the project with the lawn on this lot being established about a year after monitoring of runoff within the development began. We had no input on how homeowners maintained their lawns on this side of development, but did send them a survey at the end of each year of study asking if and when they fertilized their lawns. Three to four homeowners completed the survey each year with two indicating the lawn was fertilized at least twice each year.

Technical difficulties encountered in the first year of study resulted in little useable N and P data being collected during this time. In the latter half of the monitoring period there was no difference in area adjusted N and P load losses from two sides of the development, although there was a consistent trend of higher P load losses being observed from the compost and microclover treated side of the

development. The opposite trend existed in the N data with lower area adjusted N load losses being seen from the compost and microclover treated side of the development for the last six storm events of the project where the concentration of N and P in runoff was measured.

Due to a recent interpretation of the Maryland Lawn Fertilizer Act of 2011 by the Maryland Department of Agriculture, compost is now treated as a fertilizer source. The 2011 law caps the amount of slowly available fertilizer N that can be added on a single date to 2.5 lbs. of N per 1000 ft2. This means that when conventional tillage practices are used to alleviate surface compaction no more than about 1/8 of inch of compost can be incorporated into the soil in areas where lawns are to be established. This is not enough compost to improve infiltration properties of most soils. If ripping or other deep tillage practices are employed to breakup compacted subsoil it is permissible to incorporate high volume amounts of compost into the soil of areas where lawns will be established.

Soil restoration of lawn areas using compost and conventional tillage practices is a recognized stormwater best management practice in the states of Pennsylvania and Virginia with the latter

state providing explicit reductions in the calculation of total volume of runoff from a site with its use. Thus, there is a potential economic incentive for builders in Virginia to use this practice when it provides a lower cost alternative to reducing stormwater runoff than other approved practices that can used to accomplish this goal.

In contrast, the recent interpretation of the Maryland Lawn Fertilizer Use Act will effectively preempt the practice of rototilling soil to depth of four to six inches and amending it with compost to improve infiltration properties of lawn areas in Maryland. This includes lawn areas that serve as disconnection paths unless deep tillage practices are used to incorporate the compost. Other low nutrient containing organic matter materials could be used in place of compost, however, materials like sphagnum peat moss are difficult to work with and may actually increase the need for fertilizer when incorporated into the soil.

The slow release N properties of yard trimmings compost that was utilized in this project produced a lawn with excellent color and density. Lawns possessing high shoot densities are not only attractive but also reduce runoff. With restrictions on the use of compost now in place in Maryland, landscape contractors will now need to go deep with compost incorporation in order to continuing using it as soil restoration material in lawns areas.