Joey Shaw and Beth Guertal

People, golf carts, and lawnmowers—all are present on a golf course, and all contribute to soil compaction. When a soil is compacted, the turf also receives wear, and these combined effects lower the visual quality of the grass, hurt the turf’s ability to obtain soil nutrients, and make it harder for roots to grow and obtain water. Often you can see the effects of soil compaction: think of the worn sidelines on a football field or the path through your lawn cut by pedestrian traffic. Catching the compaction before it is visible is important because it gives turf managers time to apply treatments to alleviate the compaction or to map alternative routes for the traffic.

One relatively new method for detecting stress in crop and turf systems is through remote sensing of the effect of the sun’s energy on plants. Energy from the sun can be either absorbed, transmitted, or reflected by plants. Most remote sensing applications measure reflected energy, referred to as spectral reflectance. The amount of energy reflected by plants can be affected by stresses, such as drought or soil compaction, placed on the plant. Either a small hand-held unit (see Figure 1) or an aircraft-mounted sensor is used to evaluate the spectral reflectance of the turf in the different wavelengths of the electromagnetic spectrum. This pilot study, funded by the Alabama Space Grant Consortium, was conducted to evaluate the potential of using spectral reflectance to determine if turf areas are compacted, and to see if the severity of the turf compaction could be measured.

Figure 1. Spectral reflectance of the turf is evaluated by a small hand-held unit.

The study was conducted on a five-year-old stand of Tifway (C. dactylon × C. transvaalensis) bermudagrass maintained as a golf course fairway (mowed to a height of one inch). Tifway bermudagrass is the most common hybrid bermudagrass used on Alabama golf course fairways, athletic fields, and home lawns. Traffic treatments were applied with a golf cart. Traffic was applied to simulate golf courses receiving 0, 10,000, 20,000, 30,000, or 40,000 rounds of play per year. This resulted in levels of traffic grouped as “No Traffic,” “Slight Traffic,” “Moderate Traffic,” “High Traffic,” and “Extreme Traffic.” Periodically, researchers collected measurements of spectral reflectance (with a radiometer), soil compaction (with a penetrometer), and soil bulk density (zero to six inches). The radiometer measures reflectance in specific wavelengths (expressed as nm, nanometers, which is the unit of wavelength measurement). For these analyses, 507 nm (visible range-VIS), 559 nm (VIS), 661 nm (VIS), 706 nm (near infrared-NIR), 760 nm (NIR), and 935 nm (NIR) were the wavelengths analyzed for turf stress.

Figure 2. Soil strength (with depth) as affected by traffic, June 1999.

Soil strength data from the penetrometer revealed that driving the golf cart over the plots compacted the soil (Figure 2). Spectral reflectance measurements in November 1998 and July 1999 in the VIS range had significant positive correlations to soil penetrometer readings, while readings in most of the NIR portion of the spectrum did not correlate with soil strength. Best correlations between percent reflectance and soil compaction occurred at the 506, 661, and 706 nm wavelengths, regardless of the time of year in which measurements were obtained.

Thus, spectral reflectance readings in the VIS portion of the electromagnetic spectrum did separate differences in soil compaction.

In almost every case the VIS part of the spectrum tended to separate into three distinct groups: (1) “No Traffic” data points had the lowest reflectance, ( 2) “Slight” and “Moderate” traffic points grouped in the middle, and (3) “High” and “Extreme” traffic points were grouped with the highest reflectance in particular points of the of the electromagnetic light spectrum. Much of the turfgrass reflectance is controlled by the chlorophyll concentrations in the leaves. Many studies have shown that as plant stress increases, destruction of chlorophyll and other factors often lead to relatively increased reflectance in the regions where chlorophyll most strongly adsorbs. Thus, it was not surprising to see higher reflectance in portions of the spectrum under the intense trafficking.

Two years of traffic did not result in compaction differences great enough to distinctly separate the soil compaction measurements and the reflectance of the turf into five separate treatment levels. However, it did show that the hand-held radiometer is capable of separating compaction stresses into “None,” “Light,” and “Heavy” compacted turf. This may help turfgrass managers apply core-aerification treatments to relieve compacted turf when compaction is only at the “Light” stage, and before turf is damaged by heavy traffic.

Two years of research on remote sensing of stress in turfgrass suggest that multispectral radiometers have the ability to detect differences in turfgrass wear and stress. However, this ability is better at certain wavelengths, and, in this study, month-to-month variability affected the quality of relationships between percent reflectance and soil compaction as measured by bulk density and soil penetrometer readings.

It also appears that the grass species affects the quality of readings, as percent reflectance measurements when the turf was overseeded with perennial ryegrass produced stronger correlations with level of traffic than did readings on bermudagrass turf. Finally, detection of turf stress with the hand-held radiometer appears useful for reading big differences in soil compaction, such as none vs. intense, but it could not detect slight differences between the five different levels of compaction.