Efficient Irrigation and Fertilization Practices for Urban Landscapes
John Sloan and Milt Engelke
Background
Turfgrass is by far the most common plant in the urban landscape. It is very important to the quality of urban life. Benefits of turfgrass include its aesthetic appeal, the provision of safe surfaces for recreational activities, control of soil erosion, and enhancement of air quality by reducing the absorption and storage of solar radiation. The majority of irrigation used in urban landscapes is applied to turfgrass. Therefore, it is important to use irrigation and fertilization practices that promote turfgrass vigor, yet also conserve water and protect surface water quality.
Turfgrass irrigation for soils with
high clay contents poses unique challenges due to very slow water infiltration
and percolation rates. Irrigation and
fertilization practices recommended for coarser-textured soils, such as silty
and sandy loams, are not appropriate for high clay soils because they can
result in excessive runoff. The
conventional wisdom for proper irrigation is to apply water infrequently, but
to add enough to wet the entire rooting zone.
This is not appropriate for high-clay soils because the water
application rate quickly exceeds the infiltration rate into the soil, resulting
in water runoff from the landscape into streets where it quickly enters the
stormwater sewer system. Surface runoff
from urban landscapes commonly contains high levels of soluble fertilizer
nutrients and pesticides. The overall
result is a waste of potable city water and increased contamination of surface
waters which supply a major portion of municipal water supplies in the Northern
Blackland Prairies Resource Area. The
objective of our research project is to develop irrigation practices that are
appropriate for the clay textured soils that are common to major metropolitan
areas in
Project
Accomplishments and outcomes
Background: The study area
for this project was an existing 2.2 acre linear gradient irrigation system
(LGIS) at the Texas A&M University Research and
Equipment acquisition and installation: The first year of this project was devoted
primarily to obtaining and installing the technical equipment for monitoring
moisture levels in the soil profiles of the LGIS. We chose the Diviner 2000® soil
moisture probe from Sentek Pty. Ltd,
We installed the PVC access tubes into the Tifway 419 Bermudagrass plots because Bermudagrass is a very common warm-season grass in the southern US. Tubes were installed at locations that received irrigation to replace 0, 50, or 120 percent of Class A pan evaporation. Three tubes were installed at each monitoring location. The soil moisture values for these three tubes were averaged to create a single data point for that location. Tubes were installed in 4 field plot replications of Tifway 419 Bermudagrass. A total of 36 PVC access tubes were installed in the LGIS (i.e., 3 irrigation levels x 3 tubes/irrigation level x 4 reps = 36 tubes).
Soil Moisture Monitoring:
We began to monitor soil moisture on a daily bases starting on
Soil moisture varied widely depending on rainfall and irrigation (Fig. 3). The most dramatic changes in soil moisture occurred in the upper 10 cm of the soil profile, with changes becoming less dramatic with increasing depth in the soil profile. Rain was relatively regular and plentiful prior to July 1, so irrigation was applied infrequently. However, there were clear differences in soil moisture levels among the three irrigation levels. In general, the 120% Ep irrigation level maintained a greater amount of soil water at the 10, 20, and 30 cm depths than the 50% and 0% Ep irrigation levels. However, at depths greater than 30 cm, there was no difference in soil moisture for the different irrigation levels. Starting in June and continuing through mid September, rainfall became less abundant. The driest period was between June 18 and August 30 when total rainfall was only 3.4 inches. During that period, soil moisture levels began to decrease for the 50% and 0% Ep irrigation levels. There was little difference in soil moisture between the 50% and 0% Ep irrigation levels in the upper 20 cm of the soil, but at depths greater than 20 cm, the 50% Ep irrigation level maintained a higher level of soil moisture than the 0% Ep irrigation level. The 120% Ep irrigation level maintained volumetric soil moisture content near 40% at every depth for the entire period.
Figure 4 provides a snapshot of soil moisture levels in the upper 100 cm of the soil profile on May 8, August 8, and October 6, 2003. May 8 corresponds to the beginning of the irrigation season when natural rainfall was beginning to decrease. The turfgrass was just starting to become active and the turf plots had only been irrigated a couple of times, primarily to check out the irrigation system. Therefore, the amount of soil moisture in the upper 10 cm was directly related to the level of irrigation, but below 30 cm, soil moisture was mostly a factor of previous rainfall and there were no difference among the 3 irrigation levels.
August 8 was the end of the driest period of summer when less than 1 inch of rain was received during the preceding 54 days. There were clear differences among soil moisture contents in the profiles of the three irrigation levels on August 8. Volumetric moisture content in the soil profile receiving no irrigation was less than 10% in the upper 30 cm. The moisture content increased with depth, but even at 100 cm, it was lower than in the 50% Ep and 120% Ep irrigation levels. Volumetric moisture levels in the soil profile that received irrigation to replace 50% Ep were generally 10% higher than in the 0% Ep profile, but otherwise showed a similar trend with depth. Below 80 cm, soil moisture content for the 50% Ep irrigation level was nearly identical to that in the 120% Ep irrigation level. Moisture levels in the 120% Ep irrigation profile were nearly identical to the levels on May 8 before irrigation was started. Obviously, irrigating at 120% Ep was enough to maintain soil moisture near field capacity at all depths in the soil profile throughout even the hottest, driest weather.
October 8 represents the post-irrigation season after >10 inches of rain were received in the preceding 35 days. At that time, soil moisture had been replenished in all the soil profiles and there was no obvious difference among the three profiles due to irrigation level.
At this point in the study, we must collect, analyze, and interpret additional data before specific conclusions can be made regarding proper irrigation practices for maintaining healthy turf without excessive water use. However, it is possible to make some initial observations about potential water conservation. Turf quality data collected from the LGIS study area (not shown in this report) showed that turf irrigated at the 40 to 70% Ep level maintained an acceptable level of quality. In fact, previously published research on the LGIS reported that Tifway 419 Bermudagrass maintained acceptable turfgrass quality with irrigation inputs of 35% Ep (Qian and Engelke, 1999). The transition from healthy (dark-shaded) to stressed (light-shaded) turf seen in Fig. 1 corresponds approximately to the 50% Ep irrigation level. Since the grass at that level of irrigation still maintained an adequate level of quality and vigor, the difference in the soil water content between the 120% and 50% Ep irrigation levels represents potential water savings. Based on the data from August 8, the difference in water storage in the upper 100 cm of soil was more than 7 inches (see shaded area in Fig. 1). This is the difference in water actually stored in the soil profile and not the amount of irrigation water applied. In the 54 days prior to August 8, approximately 14 inches of irrigation were applied for the 120% Ep irrigation level versus approximately 6 inches for the 50% Ep rate. It is obvious that applying irrigation water at more than the actual evaporation rate is excessive when the goal is to maintain healthy turf.
Nitrogen Fertilizer: A second component of our study was the establishment of 15N microplots adjacent to each of the soil moisture monitoring sites. Consequently, each microplot received irrigation equivalent to 0, 50, and 120% of evaporation losses. Prilled 15N-labeled ammonium sulfate fertilizer [(15NH4)2SO4, 5 at%] was applied to each microplot at a rate equivalent to 2 lbs N 1000 ft-2 (9.8 g m-2) on June 24, 2003. The microplots received no additional fertilizer except for a slow-release 28-3-10 fertilizer applied on June 10, 2003 to the entire LGIS area at a rate equivalent to 0.5 lbs N 1000ft2 (2.4 g m-2). In order to determine the fate of 15N fertilizer, we extracted soil samples from the 0-7.5 and 7.5-15 cm depth of each microplot in fall 2003 along with thatch and grass samples. At the time of this report, we have not completed the chemical analysis of these samples.
Additional needs
related to project activities
One year after starting the project, we have set up the core apparatus needed to monitor soil moisture trends in turfgrass plots due to irrigation and rainfall. We plan to continue monitoring soil moisture for two to three growing seasons so that we can evaluate irrigation practices for different weather cycles. During the summer 2004, we will measure water infiltration rates when the soil moisture contents in the 0%, 50% and 120% Ep irrigation profiles are similar to those observed for August 8, 2003 (Fig. 4). This information will help determine the rate at which water should be added to turfgrass in order to prevent runoff.
Starting with the second growing season, we plan to collect more thorough data on turfgrass health and vigor so that we can more clearly identify the amount of irrigation and soil moisture levels needed to maintain good quality turf. We will use visual ratings, chlorophyll readings, infrared measurements, and grass clipping weights to assess turfgrass quality.
In order to relate soil moisture levels to heat and temperature issues in an urban setting, we will also collect thermal conductivity data from the study area at different times throughout the growing season. It is possible that efficient irrigation practices that reduce water inputs into the soil may also reduce heat storage and humidity, which in turn may reduce night time temperatures.
References
Qian, Y.L. and M.C. Engelke. 1999. Performance of five turfgrasses under linear gradient irrigation. HortScience. 34:893-896