Efficient use and availability of quality water sources has been a major concern in the nursery industry for many years (Yeager, 1992; Irmak, et al., 2003). Without scientific guidelines for proper application of water and nutrients, future choices of nursery-crop-production sites and species will be limited (Beeson, et al., 2004). Due to the current lack of scientific methodologies to guide irrigation practices, nursery growers often apply water to crops by simply turning on valves without knowing how much water is lost through runoff or drainage.
Overhead sprinkler systems are widely used to irrigate container-grown nursery crops, but water applied by this method is usually either excessive or insufficient, resulting in uneven application. During the growing season, more than 80% of the water from sprinkler systems may be lost through runoff, drainage, and evaporation (Weatherspoon and Harrell, 1980).
Nursery growers are using pot-in-pot systems to produce higher-quality tree crops at reduced labor cost. This production system has expanded rapidly during the past decade. The system can moderate root temperature and improve root quality, prevent blowing over of container-grown trees, and reduce harvesting labor costs (Ruter, 1997). However, with this technique, it is essential to apply sufficient water two or more times throughout the day along with supplemental nutrients to sustain rapid tree growth (Ruter, 1998; Beeson and Keller, 2003). Irrigation and fertilization practices have raised concerns over water-use efficiency because of water loss from containers and the extent of nutrient and chemical leaching to soil and ground water from drainage water. This is due to the fact that containers are buried in soil, and it is not easy to observe water and nutrient loss with such production circumstances.
With pot-in-pot production systems, knowledge is lacking on interactions between water and nutrients for optimal growth of plants. Techniques are needed to ecologically monitor nursery production practices for proper use of water resource and nutrient management. To fully explore potential impacts of pot-in-pot production systems on nursery production, knowledge of water quality and quantity to produce healthy trees is needed to improve application efficiency and avoid soil and groundwater contamination.
An experimental system to examine water quality, irrigation efficiency, and drainage from pot-in-pot nursery container production was established in a commercial nursery field. The system consisted of a plot containing 50 trees planted in 50 pot-in-pot containers and irrigated with micro-spray stakes, 10 drainage water measurement devices, 10 container substrate moisture probes, 10 thermocouples, a weather station, and data loggers.
After the system was established in July 2003, data were collected on the amount of irrigation, drainage water loss, substrate moisture content and temperature, weather conditions, and tree caliper 18 cm above the soil surface. The levels of nitrate nitrogen (NO3-N), phosphate (P), and potassium (K) in water drainage were analyzed weekly from water samples. A detailed description of system development is given by Zhu et al. (2004).
Red Sunset maple (Acer rubrum ‘Franksred’) trees were selected for the test because of their popularity in nursery marketing. The system will be expanded in the future to look at three species at one time. Caliper of each tree at 7” above the ground was measured during the growing season. The average tree caliper of bare root trees was 0.55” when they were transplanted to the pot-in-pot system.
The container substrate on a volumetric basis was composed of 55% aged pine bark, 3% sharp silica sand, 5% expanded shale Haydite soil conditioner, 20% steamed composted nursery trimmings and potting mix waste, 12% fibrous light Sphagnum peat, and 5% composted municipal sewage sludge. The container substrate provided for natural suppression of Pythium and Phytophthora root rots (Hoitink and Boehm, 1999).
A 5- to 6-month controlled-release Scotts granular fertilizer 20-5-8 (N-P-K) was applied on the top of substrate at a rate of 119 grams per tree when the bare-root trees were transplanted in the containers. Then, water soluble urea with 28% nitrogen was injected into irrigation water at a constant rate of 200 ppm at every 19-day watering cycle, although the application rate of this liquid feed program was supposed to vary with the condition of plant growth during the growing season.
The system was placed in use on August 6, 2003, with irrigation applied twice a day, once in the morning and once in the afternoon, until November 16 (total of 14 weeks). Irrigation application rate during the rest of the growing season was managed with the 3 GPH spray stakes following the production practice in a 45-acre commercial pot-in-pot production area adjacent to the experimental system. This allowed the researchers to set a base line for future comparison. Between August 6 and November 16, 2003, a total of 7.6” irrigation was applied to the trees, and total precipitation received was 23.5”.
Data in Figure 1 show the comparison of weekly total amounts of irrigation, rainfall, and drainage water collected from 10 rows of the 50 pot-in-pot system between August 6 and November 16 in 2003. During the 14-week period, total volume of drainage water from 50 containers was 490 gallons, while total irrigation water and rainfall to the 50 tree containers was 1,790 gallons. About 38% of irrigation water and rainfall was lost through drainage during September and the first week of October 2003, because of large amounts of irrigation applied to maintain tree caliper growth during this dry period. Many times, when large rainfall periods occurred, there was more runoff.
Figure 1. Weekly total rainfall and irrigation applied to, and drainage from, 50 pot-in-pot production containers between August 6 and November 16, 2003.
The average drainage start time from the 10 rows was 22.3 minutes after irrigation started with 3 GPH flow rate applied for three minutes, and was 7.6 minutes with 7 GPH flow rate applied for three minutes. Higher flow rate caused earlier drainage because of limited substrate capability of holding water in containers.
Figure 2 illustrates the average weekly amount of NO3-N, P, and K leachate in drainage water from 10 rows between August 6 and November 16 in 2003. The system detected that the total amount of NO3-N, P, and K lost through drainage from 50 containers during 14 weeks was 142.8, 7.2, and 97.8 grams, respectively. Most loss of nutrition occurred between week 4 and week 8 because of a large amount of drainage. After week 9, the amount of NO3-N, P, and K leachate decreased considerably because it was close to the end of the growing season, and the residual level of NO3-N, P, and K in the container substrate might be very low.
Figure 2. Average weekly amount of NO3-N, P, and K in drainage water from 10 rows of total 50 pot-in-pot containers between August 6 and November 16, 2003.
The mean pH of drainage water samples stayed within the range from 6 and 8 most of the time for all 10-row samples except for weeks 4 and 12 (Figure 3). Unexpectedly, the average pH in week 4 was 5.3, and the average pH in week 12 was 8.6. High water pH can occur when water levels are low in dry periods and result in negative impact on tree uptake, substrate quality, and drainage water quality.
Figure 3. Average weekly drainage water pH from 10 rows of total 50 pot-in-pot containers between August 6 and November 16, 2003.
Figure 4 shows the response of substrate moisture content in four rows to 7 GPH of irrigation applied for three minutes, twice a day, on September 9 and 10. The moisture content of the substrate near the upper root zones reached the saturated point at about 55% in a very short time and then decreased to about 40% within two hours after irrigation stopped. Figure 5 shows the response of substrate moisture content in four rows to 0.78 in., and 1.14 in. of rainfall reached the area within 30 hours. The moisture content varied with the amount of rainfall, duration, and row location. Longer intensive rainfall caused the substrate to remain in a saturated condition longer. Moisture contents for other rows responded similarly to those shown in Figures 4 and 5.
Figure 4. Example of substrate moisture content near the upper root zones for four rows when 7 GPH of irrigation was applied for three minutes, twice a day, on September 9 and 10, 2003.
Figure 5. Example of substrate moisture content near upper root zones for four rows when 0.78 in. and 1.14 in. of rainfall reached the test plot within a 30-hour period.
Daily mean substrate moisture content near upper root zones fluctuated widely during four seasons, with the largest variation in January and February (Figure 6). The substrate moisture content from the end of November through December was higher than in September and October. In late November through December and early January, due to rainfall and snowfall, the top substrate was covered with ice which could hold moisture near the probe-sensing area in the root zone.
Figure 6. Mean container substrate moisture content measured with 10 probes between August 6, 2003, and July 31, 2004.
The moisture content in January and February generally declined below 20% because the probe-sensing area was frozen. However, in later February, due to the high ambient temperature, ice at the top of the substrate melted, and the moisture content increased above 40%. Moisture content of the container substrate varied with rows although the amount of irrigation water and rainfall to all rows were the same. Such differences might be caused by the variations in substrate uniformity, tree sizes in different containers, and other unknown factors.
Figures 7 and 8 show the mean substrate temperature, and the daily maximum and minimum ambient air temperatures in September 2003 and February 2004, respectively. In September, the substrate temperature in 10 rows ranged from 53 to 78°F while the ambient air temperature ranged from 41 to 84°F (Figure 7). Comparatively, in February, the substrate temperature in 10 rows ranged from 24 to 33°F while the ambient air temperature ranged from -3.5 to 60°F (Figure 8).
Figure 7. Mean container substrate temperatures measured with 10 thermocouples and daily maximum and minimum ambient air temperatures during September of 2003.
Figure 8. Mean container substrate temperature measured with 10 thermocouples and daily maximum and minimum ambient air temperatures during February of 2004.
Figure 9 shows the average daily substrate temperature of 10 rows and maximum and minimum daily ambient air temperatures between August 2003 and July 2004. The substrate temperature in the pot-in-pot system had much lower variation than the ambient temperature within a day and was independent of moisture levels before the substrate was frozen. In contrast to the substrate moisture content, the substrate temperature did not have much variation between different rows. Since the pot was not exposed to sunlight, root growth was uniform throughout the pot.
Figure 9. Average daily substrate temperatures in 10 rows and daily minimum and maximum ambient air temperatures between August 6, 2003, and July 31, 2004.
Figure 10 shows the caliper of trees at 7 in. above the ground between July 3 and November 5, 2003. Growth rate of trees was considerably higher in September than other months. Though the fact that growth rate among the 50 trees was not consistent, average tree caliper was 1 in. at the end of growing season, or a 178% increase during the growing season.
Figure 10. Average trunk caliper of 50 trees at 7 in. above the substrate between July 3 and November 5, 2003.
Results from this preliminary study indicated that the amount of drainage water loss and nutrition leachate varied with the amount of water received by pot-in-pot containers. Many growers are applying far more water and nutrient than plants can use. Single long irrigations can cause more leachate than several divided watering schedules during a day.
The moisture content varied with the amount of rainfall, duration, and row location. Longer intensive rainfall caused the substrate to remain in a saturated condition longer. The substrate temperature in the pot-in-pot system had much lower variation than the ambient air temperature within a day and was independent of moisture levels before the substrate was frozen.
Future studies will help growers determine how to apply both water and nutrient as trees actually need them, since growing is always an art due to variations in plant size and substrate uniformity.
The authors acknowledge the following individuals — K. A. Williams, A. Clark, D. Hammersmith, A. A. Doklovic, D. T. Troyer, B. E. Nudd, and L. A. Morris — for technical assistance.
Mention of proprietary products or companies is included for the reader’s convenience and does not imply any endorsement or preferential treatment by either USDA-ARS or The Ohio State University.
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Heping Zhu, Application Technology Research Unit, USDA-ARS, Wooster, Ohio; Randall H. Zondag, Extension Educator, Ohio State University Extension, Lake County, Ohio; Charles R. Krause, Application Technology Research Unit, USDA-ARS, Wooster, Ohio; Richard C. Derksen, Application Technology Research Unit, USDA-ARS, Wooster, Ohio; Tom Demaline, Willoway Nursery Inc., Avon, Ohio.