Degradation of paint in a sand-based sports turf media
By Josh Lenz and Dr. Nick Christians
Athletic field paint is used worldwide in the sports turf industry to mark boundaries and/or create logos on athletic fields; however, the more it is used the more problems it can cause. Over a period of time, this paint can accumulate in the soil and cause problems with plant growth, sometimes to the point of total turf loss.
Research has been conducted at North Carolina State University evaluating the impacts of athletic field paint on the amount of light a plant receives and the overall effects on photosynthesis. It has been concluded that when a turfgrass canopy is covered by field paint, it alters the amount of light that is available for photosynthesis. Other work has shown that darker paint colors can absorb over 90% of light, reducing the photosynthetically active radiation (PAR) at the leaf surfaces. While the effect paint has on PAR is well understood, there is little information on the accumulation of paint in the soil profile and how it affects the chemical and physical characteristics of the soil.
Paints are made up of four basic elements: pigment, binder, solvent, and additives. Pigments and binders are of particular concern. Pigments are used in paint to ensure a surface is completely coated, and give paint its color by absorbing specific wavelengths of light while reflecting others. Titanium dioxide is the choice pigment when it comes to white paint because it has the highest level of brightness of any pigment. Higher concentrations of titanium dioxide will result in brighter paint, but it will also raise the cost. To keep costs lower, a filler pigment needs to be used.
Calcium carbonate is often used as a filler pigment. It can be used in mixing certain types of cement, so its presence in high concentrations in turf paint can damage grass. Paint is often mixed with water before use. If that occurs and then the paint sits for a long time, the calcium carbonate will separate and harden. This is similar to how it works in the soil. If applied often enough to the same area, it will eventually migrate several inches into the soil and harden; choking off grass plants at their roots, and potentially affecting the chemistry of the soil as well as water infiltration.
Additionally, paints contain a binder or polymer that acts as the adhesion component for bonding to the surface. The level of the binder (resin) will determine how long the paint persists. Common binders used in turf paints are acrylic and latex.
The study began by comparing different rates of a white, water-based acrylic paint from a major supplier of athletic field paint. The objective was to determine the amount of paint that would remain in the soil after a long period of time. Tests were then conducted using three different green-pigmented paints from Sensient Technologies. These paints include acrylic, polysaccharide resin number one (PSR #1) and polysaccharide resin number two (PSR #2). The acrylic paint from Sensient has similarities to water-based acrylic athletic field paints available from other companies with regard to the four basic elements; however, the Sensient product uses less acrylic binder.
The objectives of this study were to determine the effects of green acrylic polymer paint and two green paints with polysaccharide resin technology on the chemical and physical parameters of a sand-based sports field media, and to see if one of the three paints broke down quicker than the other two.
Materials and methods
The study was performed over 6 months in the horticulture greenhouse at Iowa State University. Large trays of sand were placed on greenhouse benches. Trays were arranged in order on the bench and measurements were taken monthly. The soil and paint mixtures in the trays were watered regularly to encourage microbial activity.
The amount of paint mixed with soil was determined from preliminary chemical tests. Several different rates of water-based acrylic paint were uniformly mixed with sand and chemically tested along with a sample containing accumulated paint over the past 8 years from Jack Trice Stadium at Iowa State. The results were used to compare and estimate how much paint should be used in the study. After determining similar results between the mixed samples and the Jack Trice Stadium sample, additional chemical tests were performed on test batches of soil with the Sensient paints to determine the best rate for this study.
Chemical tests were performed monthly on subsamples from each tray, and the samples were submitted to the soil-testing laboratory for soil chemical testing. Samples were collected in small plastic bags and immediately driven to the testing lab so there was no variability among treatments over the 6 months. The lab performed tests for phosphorus (P), sulfur (S), potassium (K), zinc (Zn), sodium (Na), magnesium (Mg), calcium (Ca), pH, buffered pH, cation exchange capacity (CEC), base saturation potassium (K_BSat), base saturation magnesium (Mg_BSat), base saturation calcium (Ca_BSat), base saturation sodium (Na_BSat), and organic matter (OM).
A saturated hydraulic conductivity (Ks) test was completed on additional subsamples taken from the trays monthly. Samples were collected in metal cylinders. The metal cylinders were sealed across the bottom with cheesecloth and taped with electrical tape around the sides. The soil and paint mixture was added to each cylinder by spooning small amounts of sand at a time, leaving about two to three centimeters on top for water to pond. Samples were carefully packed the same way to avoid circumstances that would skew results.
Once all cylinders were filled, the 12 samples were put in a large tub of water and left for 10 minutes until completely saturated. When samples were saturated, they were removed one at a time and placed on a ring and clamp stand. Water was ponded in the top of the cylinder using a Mariotte bottle. The outflow rate was measured with a graduated cylinder and a stopwatch at three different time increments for each replication. Once those measurements were taken, each measurement was recorded along with the ponded depth and the length of the soil sample. These data were used to solve for the Ks in Darcy’s equation. All measurements were taken in centimeters/second. The study was conducted as a split plot in time.
Levels of CEC, K, Zn, Na, Ca, P, and S were affected by the presence of paint in the media.
Cation Exchange Capacity. There were no differences in CEC among paint treatments at any time during the study. However, there were differences in CEC of the media among dates. There was a decrease in CEC as compared to the untreated control in months two and three. By month five, the CEC in all paint treatments exceeded that of the sand in the control. This is likely due to the paint coating individual sand particles in the first few months after initiation of the paint treatments, to the extent that the soil test procedure was not able to properly measure CEC. It is assumed that the paint degraded by the fifth month and then the procedure was able to properly measure this variable. This will be further substantiated by the next few sections on the measurement of cations found in the treated samples. In every case, there was an increase in cation release in the fifth month.
Potassium. There were no differences in extracted K among paint treatments for five of the six months of the experiment. In the fifth month, sand treated with acrylic paint and with PSR #2 were both much lower than the sand treated with PSR #1. The K level found in sand treated with PSR #1 was 313.5 ppm while the acrylic and PSR #2 measurements were 92.3 and 22.6 ppm respectively.
Potassium levels did vary by month. During the first three months, the release of K from all treatments was similar to the untreated control. It was in the fourth, fifth, and sixth months that K levels were higher in the paint containing treatments than in the control. This was likely a result of the degradation of paint that was coating the CEC sites by the fourth month.
Zinc. There were differences among dates and paint treatments in extracted Zn throughout the duration of the study. In months one, two, and three, all treatments were lower compared to the untreated control. In month three, there was a difference among paint treatments. The sands treated with PSR #1 and PSR #2 were lower than the sands treated with the acrylic paint, but the two polysaccharide treatments did not vary from one another. In the fifth month, sand treated with acrylic paint released more Zn than the untreated control. At that time, the sand treated with acrylic paint tested at 0.7 ppm, which was higher than the PSR #1 at 0.2 ppm. The PSR #1 was lower than PSR #2 at that date.
The greatest release of Zn occurred in the fifth month. This was consistent with the increase in CEC during that month and again is likely due to the degradation of paints by month five.
Sodium. There were no differences in extracted Na among paint treatments for five of the six months of the experiment. In the fifth month, sand treated with acrylic paint and with PSR #2 were both much lower than the sand treated with PSR #1. The PSR #1 treatment had a value of 149.6 ppm, which was much higher than the other two paint treatments. It is assumed that the PSR #1 contains Na.
Sodium levels did vary by month. In the first month, the release of Na from all treatments was similar to the untreated control. In the second and third months, all treatments were lower than the untreated control. It was in the fourth, fifth, and sixth months that Na levels in painted treatments had exceeded the control. This was likely caused by the paint coating the CEC sites during the first few months and by the fourth month it had started to degrade.
Calcium. There were no differences in Ca among paint treatments at any time during the experiment. The Ca levels found in the paint treatments in month one were not different than the untreated control. In month two and three, all Ca levels were lower than the control. As was the case with the other cations, there was a release of Ca in the fifth month.
Phosphorus. Similar to the cations, P availability in the sands treated with paint was comparable to the control for the first three months. However, in months four and five, P levels generally exceeded those in the untreated control. The P level found in sand treated with PSR #2 in month four was higher than the other two treatments. In month five, the acrylic and PSR #2 measurements were 12.6 ppm and 14.8 ppm and the PSR #1 was 0.7 ppm.
Sulfur. Like P, the release of S from the treated sands began in month four. There were no differences in extracted S among paint treatments for four out of the six months of the experiment. There was a difference in the S levels in the fourth month between sand treated with PSR #2 measuring 374.2 ppm compared to sand treated with PSR #1 and acrylic with 209.6 ppm and 167.8 ppm respectively. In the sixth month, PSR #2 was also higher than PSR #1 and acrylic.
Saturated Hydraulic Conductivity.The Ks results are measured in centimeters per second (cm/sec). Each treatment was adjusted for the control by corresponding dates. The values show a range from -0.01 to 0.04 cm/sec. When a value is negative, the movement of water through the profile is slower in the paint treatment than it was in the untreated control. When a value is positive, the movement of water through the profile is faster in the paint treatment than it was in the untreated control.
The Ks values demonstrate a trend just opposite of the chemical variables. There were no differences in Ks among paint treatments for five of the six months of the experiment. In the second month, sand treated with acrylic paint and with PSR #1 were both faster than the sand treated with PSR #2. The average Ks in sand treated with acrylic paint and PSR #1 was 0.04 cm/sec and 0.04 cm/sec respectively compared to PSR #2 at 0.02 cm/sec.
Saturated hydraulic conductivity levels did vary by month. Rates were accelerated in months one and two. By month three and four, the Ks began to decrease but it was still faster than the untreated control. This is the opposite result of what was expected. It was anticipated that the presence of paint would slow infiltration. It is likely that the paint coated the sand particles and this coating repelled water resulting in an increased Ks. By the fifth month, the paint had degraded to the point where Ks was similar to the untreated control.
The paint evidently coated the sand particles, thereby reducing CEC and the release of cations for the first four months of the experiment. By the fifth month, the paint had degraded to the extent that CEC sites were exposed and cations were released into the soil solution. The anions, P and S, showed an increase by month four, thirty days before the cations were released. The reason for this is unclear, although it may be due to other unknown components in the paints.
This is also the case for Ks. The acrylic paint does appear to speed up water movement, followed by PSR #1 and then PSR #2. In all paint treatments, the first three months the Ks is quicker than the untreated control. By the fourth month, the paint begins to degrade and as a result the Ks gets slower. Then in the fifth month, all treatments are slower than the untreated control. This substantiates the hypothesis that the paint was coating the sand particles and degraded over a four to five month period.
It cannot be determined from the chemical or physical results that one of the paints broke down quicker than the other two. The paints did affect each treatment, but it took six months for each of them to break down chemically. The same is true for the Ks results. Although the PSR #2 had less of an effect on the Ks over the first four months, the difference between the three treatments is very minimal. Then by month five, all three treatments show no differences compared to the control.
Further research on the effects of paint on the chemical and physical properties of the soil should include an expanded number of paints, including those commonly used on sports fields. The number of physical tests performed on the samples should also include particle size analysis (PSA) and physical performance evaluation (PE), which would include information about infiltration rate, porosity, bulk density, particle density, and organic matter.
Josh Lenz is a graduate student at Iowa State University; Nick Christians, PhD is a professor in the Department of Horticulture at Iowa State.