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Department of Plant Science, U-67, Univ. of Connecticut, 1376 Storrs Rd., Storrs, CT 06269-4067
* Corresponding author (Mark.Brand{at}uconn.edu)
Received for publication August 31, 2000.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: CRF, controlled-release fertilizer • FWC, flow-weighted concentration • LF, leaching fraction • Ov, overhead sprinkler application of water • OvF, overhead sprinkler plus controlled-release fertilizer • Tr, trickle irrigation • TrF, trickle irrigation plus controlled-release fertilizer
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Increasingly, nursery stock is produced in containers, and this trend away from field production is likely to continue (Brand et al., 1993; Colangelo and Brand, 1997). Transition to container production is driven by market demands and numerous production advantages including greater production per acre, faster plant growth, higher plant quality, and lack of dependence on arable land (Davidson et al., 1988). Nursery container production involves growing plants in pots filled with a porous artificial medium, usually composed of bark, peat moss, compost, and sand (Whitcomb, 1988). These mixes require frequent irrigation and high fertilization levels for optimum crop production (Davidson et al., 1988; Whitcomb, 1988).
Although acreage in nursery container production is less than acreage in field crops, the potential environmental effect of the container nursery industry may not be reflected in the acreage involved, due to the intensive, year-round nature of container culture. A six-state survey of runoff and well water from container nursery operations revealed that runoff water alone can exceed the USEPA 10 mg L-1 limit for nitrate N in drinking water at certain points in the production cycle (Yeager et al., 1993).
Because the production of nursery crops in containers involves the use of artificial potting media, native soil is not an important factor for container nursery siting (Davidson et al., 1988). Nurseries often sell the topsoil on a site and then grow containerized plants on graded B and C horizon soils, or on fill soil, to provide a level growing surface. A fill or subsoil base has a disturbed structure and little organic matter that may help to hold nitrogen lost from the container crop. Furthermore, areas where plants are container-grown are kept free of noncrop vegetation for a number of reasons: preventing weed seed introduction, limiting competition with crop plants, and the inability of cover crops to grow well beneath the crop canopy. Cover crops have been documented to ameliorate some nitrogen runoff and waste situations in field nurseries (Bir and Hoyt, 1993). The absence of these crops in a container nursery precludes their use as a "filter" for excess nitrogen from the crops.
Cultural practices that are widely believed to reduce nitrogen leaching from container crops include the substitution of controlled-release fertilizers (CRFs) for water-soluble fertilizers (WSF) and cyclic trickle irrigation systems in place of inefficient, overhead irrigation systems. Controlled-release fertilizers are designed to release nutrients over an extended period of time with the objective of meeting the nutrient requirement of the crop (Sharma, 1979). Nutrients supplied with WSFs are often in excess of crop needs and have been shown to be more subject to leaching than CRFs (Rathier and Frink, 1989; Broschat, 1995).
Trickle irrigation can significantly increase water application efficiency when compared with overhead irrigation (Weatherspoon and Harrell, 1980; Harbaugh and Wilfret, 1980; Bonaminio and Bir, 1983), because emitters are in the container and deliver water directly to the medium (Furuta, 1973). Typically, 74% of the water applied through overhead irrigation systems falls outside the containers, missing the pots completely (Weatherspoon and Harrell, 1980). The use of cyclic irrigation has been shown to increase nutrient retention and water use efficiency in container production systems when compared with noncyclic systems (Tyler et al., 1996a). In cyclic irrigation, daily water allotments are made in a series of cycles, comprised of alternating small volume irrigations and resting intervals (Karmeli and Peri, 1974; Mostaghimi and Mitchell, 1983).
Various studies have provided evidence suggesting that that the use of trickle and cyclic irrigation in combination with CRFs can reduce nitrate leaching from container crops when compared with conventional overhead irrigation systems (Rathier and Frink, 1989; McAvoy et al., 1992; McAvoy, 1994; Tyler et al., 1996b). However, most studies did not include the overwinter period or the soil beneath containers, or were performed in greenhouses, where precipitation is not a factor.
The main objectives of this study were to (i) determine if cyclic trickle irrigation results in less nitrogen moving through soil beneath containers when compared with overhead irrigation in a system where natural precipitation events contribute to water inputs; (ii) determine when and in what forms nitrogen moves through the soil beneath containers over single and repeated growing seasons, including the winter period; and (iii) determine a nitrogen balance for a nursery production system that includes the plant, potting medium, fertilizer, soil beneath the container, and leachate.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Soil Column and Collection Vessels
Fourteen soil columns were prepared using plastic 208-L (56-cm diameter, 85-cm depth), food-grade, industrial barrels that were modified for use as soil-monolith lysimeters (Fig. 1). The tops of all barrels were removed and one 1.9- x 1.3-cm elbow-shaped fitting was attached to the threaded outlet at the bottom of each barrel. The drain hole of each soil column was fitted with polyester quilt batting, approximately 3.0 cm thick, and the entire bottom of the soil column was covered with woven fiberglass fabric. This was done to prevent suspended soil particles from moving with leachate and draining into the collection vessel.
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A sloped site was chosen so that soil columns could be recessed at the top of the slope, and collection vessels could be easily recessed at the bottom of the slope (Fig. 1). Recessing the columns and collection vessels into the ground was necessary to keep the soil inside the columns under approximately the same conditions as the surrounding soil. It was important to prevent the soil within the columns from freezing prematurely and to keep leachate within the collection vessels from freezing. All soil columns were recessed at the top of the slope, leaving the column 20.0 cm above the soil grade to prevent contamination by outside runoff and splash. Collection vessels were recessed at the bottom of the slope leaving approximately 20.0 cm above the soil grade to allow for removal of leachate. Soil columns were set at a 1.5- to 2.0-cm pitch (front to back) to force percolating water to move to the exit hole. To simulate soil conditions at a typical container nursery, columns were filled with homogeneous, screened (1-cm2 mesh), B and C horizon soil consisting of 77% sand, 20% silt, and 3% clay (determined by hydrometer method; Gee and Bauder, 1986) that contained 2.4% organic matter (dry combustion determination; Ball, 1964). Soil in the columns was packed by hand to a bulk density of approximately 1.5 g cm-3 following the methods of Blake and Hartage (1986). The soil surface of each column was kept level.
All columns were constructed, installed, and filled between May 1995 and August 1995. Columns were allowed to settle for 9 mo before experimental treatments were applied. This settling period enabled us to test the reliability of each column after a freeze–thaw cycle had occurred, and permitted soil organisms to recolonize the soil in the columns following disturbance.
Throughout the study the soil surface of all columns was kept free of weeds by hand removal. Precipitation, air temperature, soil temperature, and percent soil moisture were continuously monitored at 30-min intervals using a tipping bucket rain gauge, a Model 107 air temperature thermocouple, Model 108 soil temperature thermocouples, and Model CS615 soil moisture reflectometers controlled by a Campbell Scientific (Logan, UT) CR10 data logger, respectively. Soil temperature and moisture were taken at a depth of 15 cm.
Nursery Crop
Micropropagated rhododendron plants were grown on top of the soil columns in 2.6-L (300S) plastic containers (Nursery Supplies, Fairless Hills, PA) for both years of the study. The 300S containers had a top diameter of 17.0 cm, a bottom diameter of 13 cm, and a height of 16.5 cm. Rhododendron was chosen because it is a commonly grown crop in the northeastern and northwestern U.S. nursery production regions. The potting medium used was a standard nursery mix composed of three parts aged pine bark, two parts peat moss, and one part sand (3:2:1 by volume) supplemented with dolomitic lime (3.0 kg m-3). Four containerized plants were arranged in a 2 x 2 pattern on top of each soil column, 5 cm apart, during each growing season. Plants were 12 cm tall and 12 cm wide, and had three shoots and an average dry weight of 2.2 g based on measurement of 25 plants not used in treatments. The containers shielded 36.9% of the total surface (2463 cm2) of each column. Plants were grown from 13 June through 31 October for both years.
Fertilization Treatments
A controlled-release fertilizer (CRF) (Sierrablen 17–6–10 plus minors; The Scotts Co., Marysville, OH) was applied as a topdressing to containers in fertilized treatments. Fertilizer was applied at a rate of 10.0 g (1.7 g N) per container (278.1 kg N ha-1) at the beginning of each growing season (13 June). The fertilizer analysis was 17N–2.6P–8.3K and provided 9.1% of N as ammonium N and 7.9% as nitrate N, according to the manufacturer. This fertilizer has an 8- to 9-mo release period. The nitrogen, phosphorus, potassium, and micronutrient sources in this fertilizer are resin coated to provide slow release of 15% N, 5% P2O5, and 9% K2O. Two percent, 1%, and 1% of N, P2O5, and K2O, respectively, are uncoated and immediately soluble.
Irrigation Treatments
Trickle irrigation was accomplished using drip emitters (2 L h-1 flow rate; Netafim Co., Valley Stream, NY) with one emitter per container and four per soil column. Irrigation scheduling was controlled by a Nelson 8425 electronic irrigation controller (L.R. Nelson Corp., Peoria, IL), a solenoid, and a tensiometer equipped with a circuit switch (Irrometer Co., Riverside, CA). A Kent C700 flow meter (Kent Meter Sales, Ocala, FL) was used to measure irrigation volume. The irrigation controller was programmed to irrigate for 4 min (approximately 226 mL per container) three times each day (8:00 am, 12:00 pm, and 4:30 pm). Automated trickle irrigations could not proceed if the moisture tension of fertilized containers was less than 0.01 MPa based on tensiometer measurements. A moisture tension of 0.01 MPa was determined to be the point beyond which rhododendron would exhibit symptoms of moisture stress based on preliminary testing and observation. A full compliment of three daily trickle irrigations provided 11.4 mm (2712 mL) of water per soil column with a leaching fraction (LF = volume leached/volume applied) of approximately 0.14.
Overhead irrigation was provided by hand watering above the foliage of the plants, using a water break and a flow meter to fix irrigation volume. A plastic shield was used during overhead irrigation to ensure that all water was applied to the crop and interplant spaces, with no water falling outside the soil column. During overhead watering, the water break was moved rapidly and randomly to simulate how water would fall on plants under nursery conditions. Overhead irrigation was provided when moisture tension of fertilized containers was observed to reach 0.01 MPa, based on tensiometer measurements. At each irrigation, 16 mm (3800 mL) of water was applied per soil column (approximately 650 mL per container with a LF of 0.20) at a flow rate of approximately 600 L h-1. Approximately 1200 mL of each irrigation fell between the containers, depending on the canopy cover provided by the plants.
Collection and Analysis of Leachate and Irrigation Water
Leachate samples from the soil columns were collected weekly during the growing season, or more frequently during periods of high precipitation to avoid loss of sample due to overflow. During the overwinter period, samples were collected as needed depending on precipitation and thawing of the soil columns. Leachate samples from the individual collection vessels were obtained separately at each collection and total leachate volume was determined. Irrigation water was sampled directly from the irrigation hose at monthly intervals during the growing season.
Nitrate N and ammonia N concentrations in leachate and irrigation water were determined using colorimetric procedures on a Scientific Instruments continuous flow auto analyzer (Westco, Danbury, CT) following USEPA Methods 353.2 and 350.1 (USEPA, 1983). Total nitrogen in samples was quantified using block digestion Kjeldahl and colorimetric procedures following USEPA Method 351.2 (USEPA, 1983) to facilitate organic nitrogen determination. Flow-weighted concentration (FWC) of nitrate N leached from soil columns was calculated as the final cumulative mass nitrate N (mg)/final cumulative volume of leachate collected.
Collection and Analysis of Soil from Columns
Ten soil cores were collected from each soil column to a depth of 80 cm using a 4-cm auger at the beginning and end of each growing season. Auger holes were refilled with unused sample and compacted by hand to approximate the bulk density of the surrounding soil. Cores from each soil column were homogenized and 50-g subsamples were dried. Duplicate two-gram subsamples of dry soil from each column were extracted in 20 mL of 2 M KCl, shaken for 30 min, and filtered through Whatman (Maidstone, UK) No. 40 filter paper. Concentrations of nitrate N and ammonia N were determined as previously described for leachate samples. Samples were also analyzed for total nitrogen using a LECO (St. Joseph, MI) FP-2000 nitrogen determinator following AOAC Method 993.13.
Soil mass per column was determined by calculating soil volume per column and repeated bulk density measurements. Nitrogen mineralization potential under both short-term aerobic conditions and long-term anaerobic conditions was also measured (Stanford and Smith, 1972).
Collection and Analysis of Container Media, Root Balls, and Fertilizer
The container medium was sampled at the beginning of both growing seasons prior to potting. Ten 100-cm3 samples of medium were ground in a ball mill (Spex Industries, Scotch Plains, NJ) and two 1-g subsamples of the ground medium were analyzed for total nitrogen content as described for soil. The mass of medium per container was estimated by potting nine plants, then unpotting them and determining the oven-dried mass of the medium used.
At the end of each growing seasion, each root ball (medium, roots, and unreleased CRF fertilizer) was oven-dried and weighed. Each root ball was subsampled (100 cm3) and processed as described for medium samples. Prior to potting at the beginning of each season, the root balls of 25 control plants not included in treatments were harvested and analyzed to obtain data on initial nitrogen contributions.
At the end of the growing season, 15 fertilizer prills were removed from selected plants in each irrigation treatment and compared with new fertilizer prills. Fertilizer prills were weighed and analyzed for total N using a LECO FP-2000 nitrogen determinator as previously described.
Collection and Analysis of Shoots
At the end of each growing season, individual plants were cut at the media surface and fresh and dry mass was determined. One-gram subsamples of the ground tissue (to pass a 2-mm screen) from each plant were analyzed for total nitrogen content using a LECO FP-2000 nitrogen determinator. Shoots from 25 control plants not used in treatments were harvested and analyzed at the beginning of each season to obtain data on nitrogen contributions for the initial shoots.
Nitrogen Balance
Information collected from the various samples and analyses was used to calculate a nitrogen balance using the following equation based on Stewart et al. (1981):
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NM = change in potting media N; NS = change in soil column N; NP = change in plant N; NL = N recovered in leachate; and NG = gaseous loss of N.
Experimental Design and Statistics
Fourteen soil columns were arranged in a completely randomized design with four treatments. Experimental treatments included overhead irrigation plus CRF (OvF), overhead irrigation without CRF (Ov), cyclic trickle irrigation plus CRF (TrF), and cyclic trickle irrigation without CRF (Tr). Fertilized treatments were replicated four times and unfertilized treatments three times. Leachate data from two of the soil columns in the OvF treatment were not included in statistical analyses or calculations of nitrogen concentration and mass after 31 Oct. 1997 due to malfunction. All statistical analyses were calculated using SAS (SAS Institute, 1994). The general linear models procedure and orthogonal contrasts were used to make comparisons between fertilized and unfertilized, OvF and TrF, and Ov and Tr treatments. For analysis of cumulative leachate collected, orthogonal contrasts were made between overhead and trickle, OvF and Ov, and TrF and Tr treatments.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Both TrF and OvF treatments exceeded the USEPA nitrate N limit of 10 mg L-1 at several times during the 2-yr study (Fig. 3). The TrF treatment maximum nitrate N leachate concentrations were greater than those observed for the OvF treatment. The TrF treatment nitrate N leachate concentration reached 26 mg L-1, while OvF treatment concentration only reached a maximum of 21 mg L-1. For the majority of the study, no differences existed between the nitrate N concentrations of leachate from Ov and Tr treatments.
Year 2 nitrate N leachate concentrations were higher than Year 1 concentrations and the peak concentrations occurred later in Year 2 than in Year 1 (Fig. 3). These annual differences are probably due, in part, to weather conditions. In Year 1, a period of high precipitation occurred from late August through mid-October. This rainfall moved nitrate N rapidly through the soil columns and diluted the nitrate N concentrations as well. In Year 2, the period from mid-August to mid-October received little precipitation, delaying nitrate N movement through the soil columns. Nitrate N concentrations also became concentrated during this period because irrigation leached nitrogen from the containers and into the soil column, but was insufficient to leach the nitrate N from the soil columns.
Nitrate N concentrations in TrF leachate exceeded the USEPA limit of 10 mg L-1 from late September through early November in Year 1 (Fig. 3). In Year 2, TrF leachate exceeded 10 mg L-1 briefly in late August (mineralization) and then from late October to late January. Nitrate N concentrations on OvF leachate approached the 10 mg L-1 limit in Year 1, but never exceeded it. In Year 2, the limit was exceeded by OvF leachate intermittently from late August to early October, then continuously from late October to January.
Flow-Weighted Nitrate Nitrogen Concentrations
Annual FWCs of nitrate N in leachate from the fertilized treatments were higher than the unfertilized treatments in both years (Table 2). The TrF treatment exhibited almost double the annual FWCs of nitrate N in leachate that the OvF treatment exhibited in both years. Annual FWC of nitrate N in leachate from the Ov treatment was similar to the Tr in both years of the study. In Year 1, the annual FWC of nitrate N in leachate did not exceed the 10 mg L-1 USEPA drinking water standard in any of the irrigation and fertilizer treatments. However, in Year 2 annual FWCs of nitrate N in leachate exceeded 10 mg L-1 from the OvF and TrF treatments (10.5 and 18.9 mg L-1, respectively).
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Mass of Nitrate in Leachate
There were no differences in the mass of nitrate N in leachate from fertilized and unfertilized treatments for the first 3 mo of the study (Fig. 4). After this period of time, higher nitrate N mass was found in the leachate from fertilized treatments regardless of irrigation method. Most importantly, nitrate N mass in leachate was as high in trickle-irrigated treatments as in overhead-irrigated treatments for the last 21 mo of the study.
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Trickle irrigation did not reduce the final cumulative amount of nitrate N leached from the CRF-fertilized containers and through the soil columns when examined over one or two annual growth cycles. Although the TrF treatment (leaching fraction [LF] = 0.1) received only 57% of the irrigation water applied to the OvF treatment (LF = 0.2), similar amounts of cumulative nitrate N leached in both treatments. Cumulative nitrate N values were more strongly influenced by the amount of precipitation received than the amount of irrigation received.
Form of Nitrogen in Leachate
Fifty-four percent of the total amount of nitrogen applied per soil column to containerized rhododendron plants in the fertilized treatments was in the form of ammonia N (300 kg ha-1 of the 556 kg ha-1 applied per column). However, at the end of the study, cumulative ammonia N recovered in leachate per column from the OvF and TrF treatments was only 0.54% (1.6 kg ha-1) and 0.35% (1.0 kg ha-1) of the total ammonia N applied, respectively (data not shown). Ammonia N concentrations in leachate from all treatments were below detection for most of the study. There were no differences in ammonia N concentration between any of the treatments on any leachate collection dates.
Forty-six percent of the total nitrogen applied per soil column to containerized rhododendron plants in the fertilized treatments was in the form of nitrate N (256 of the 556 kg ha-1 applied per column). Cumulative nitrate N recovered in leachate per soil column from the OvF and TrF treatments was 40 and 47% (103.7 and 121.0 kg ha-1) of the total nitrate N applied, respectively (Fig. 4). More than 99% of the nitrogen recovered in leachate from the fertilized treatments was in the form of nitrate N, despite the fact that more than half of the applied nitrogen was ammonium N. These data suggest that most of the applied ammonium underwent nitrification either in the container or in the soil beneath containers. Numerous studies have shown that nitrogen applied as ammonium to a container crop can be nitrified before leaching from the container (Warren et al., 1995). Niemiera and Wright (1987) found that ammonium N concentrations in a pine bark media solution decreased from 30 to 0 mg L-1 in 40 d. Rapid increases in nitrate N concentration in the media solution coincided with ammonium N decreases. Fare et al. (1994) reported that 80 to 90% of the nitrogen leached from CRF-fertilized containers receiving cyclic irrigation was in the form of nitrate N. The greater percentage of nitrogen leached as nitrate N in our study may be due to additional nitrification that occurred within the soil columns of the system. In urea-fertilized silage corn, Gold et al. (1990) reported that 83 to 93% of dissolved inorganic nitrogen in soil water percolate was in the form of nitrate N, further supporting the assertion that nitrification occurred within the soil profile.
Other Nitrogen Sources and Sinks
The nitrogen mineralization potential of the soil in the columns was 3339.6 mg N per column under long-term aerobic conditions and 2776.2 mg N per column under short-term anaerobic conditions. Values for total soil nitrogen (g kg-1 soil) within the columns showed no significant differences between any treatments at the end of each growing season (13 June 1996 and 1997) (Table 3). Nitrate N, ammonia N, and organic nitrogen in irrigation water never exceeded the detection level (0.2 mg L-1) and were not considered a significant source of nitrogen in this study.
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Total nitrogen in fertilized plants was higher than in unfertilized plants in both years (Table 3). No differences in total nitrogen were detected between overhead-irrigated and trickle-irrigated plants. Plants grown under both fertilized treatments were high quality, marketable specimens, while plants in the control treatments produced only one growth flush and were obviously nutrient deficient. Plants from Year 1 and Year 2 were statistically similar in dry weight and averaged 21.5, 22.4, 4.3, and 5.5 g for OvF, TrF, Ov, and Tr treatments, respectively. Percent total N values for leaves and stems of fertilized plants were in agreement with those reported by Ticknor and Long (1974), while values for control plants were well below reported values for healthy rhododendron plants.
Nitrogen Mass Balance
Total percent recovery of the initial nitrogen in the system (fertilizer N + root ball N + soil column N + plant tissue N + leachate N) ranged from 94.3 to 101.5 in Year 1 and from 91.2 to 102.9 in Year 2 (Table 4). Percentages greater than 100 may have been due to nitrogen input from precipitation, or measurement error. Combined results for the entire experimental period showed statistically higher nitrogen recovery for the TrF treatment than for the OvF treatment, although the differences were not large. Denitrification occurs readily under low-oxygen conditions, therefore, the higher irrigation volume of the OvF treatment may have resulted in conditions more favorable to denitrification than the lower irrigation volume of the TrF treatment. The high nitrogen recoveries in this study indicate that gaseous loss (denitrification and/or ammonia volatilization) of nitrogen from the plant–artificial potting medium–soil profile system was small. These results are in agreement with the findings of Fare et al. (1994) and Stewart et al. (1981) who, in similar experiments examining nutrient dynamics in container production (excluding the soil beneath containers), reported 83 to 104% nitrogen recovery.
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SUMMARY AND CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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The trickle irrigation system was more water efficient than the overhead irrigation system, delivering 57% less water while maintaining crop quality. However, the TrF treatment did not reduce the cumulative amount of nitrate N leached from the containers and through the soil columns, when compared with the OvF treatment. The large amount of precipitation received during the experimental period, relative to the irrigation volume, rendered the trickle irrigation treatment ineffective at reducing nitrate N leaching when examined over a 1- or 2-yr outdoor crop cycle. Although trickle irrigation is considered to cause less nitrate N leaching than overhead irrigation in general, this study clearly shows that under outdoor container production practices, leaching losses from trickle irrigation can be as high as, or higher than, losses from overhead irrigation.
Nitrate N was the primary form of nitrogen found in leachate, regardless of irrigation method. Because 54% of the applied nitrogen was in the form of ammonium N, it is likely that a considerable amount of nitrification occurred. Percent nitrogen recovery from all treatments was high, indicating that denitrification was not a major factor in this system.
The magnitude of nitrate N export in leachate from the plant–artificial potting medium–soil profile system examined in this study was comparable with that from other plant agricultural systems, which are considered potential sources of nonpoint-source nutrient pollution. Nursery container production strategies that limit nitrogen leaching need to be further developed in order to make container production more sustainable.
A large container nursery in Connecticut of 40 ha may have approximately 20 ha of actual container crops in production during the growing season. Although the acreage involved in container production may not be as vast as that of other plant agricultural systems, the amount of nitrate N leaching from container production should not be ignored. According to the findings of our study, nitrogen export from a 40-ha nursery growing rhododendron could be as high as 1210 kg of nitrate N per year. Nitrate N export of this magnitude from a container nursery, could pose a significant threat to a surrounding watershed. Furthermore, the rhododendron used in this study requires nitrogen fertility levels well below those required by many other ornamental nursery crops. Deciduous, flowering shrubs and shade trees typically receive twice as much CRF as rhododendrons. It is likely that leaching losses from these nursery crops are significantly higher than what we have found for rhododendrons.
It is important to point out that most northern nurseries move containers into plastic-covered hoop houses from approximately November to April. Although containers within the hoop houses receive occasional irrigation during the winter, the surface area of the container production area exposed to natural precipitation is less during the winter than during the growing season. Leaching from the covered areas may not occur during the winter months, but nitrate N that has accumulated in the soil beneath these areas will likely become vulnerable to leaching when the hoop houses are uncovered in the spring.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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0.05; NS = not significant.
