Soil Science Society of America Journal 66:178-185 (2002)
© 2002 Soil Science Society of America
DIVISION S-4 - SOIL FERTILITY & PLANT NUTRITION
Subsurface Drip Irrigation and Fertigation of Broccoli
II. Agronomic, Economic, and Environmental Outcomes
Thomas L. Thompson*,a,
Thomas A. Doergeb and
Ronald E. Godinc
a Dep. of Soil, Water, and Environmental Science, University of Arizona, 429 Shantz Bldg. #38, Tucson, AZ 85721
b Pioneer Hi-Bred International, Inc., P.O. Box 1150, Johnston, IA 50131
c Colorado State University, Rogers Mesa Research Center, 3060 Highway 92, Hotchkiss, CO 81419
* Corresponding author (thompson{at}ag.arizona.edu)
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ABSTRACT
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Subsurface drip irrigation offers potential for increased water and N fertilizer use efficiency, and decreased groundwater NO3 pollution. Replicated factorial experiments consisting of four rates of N fertilizer application (60–500 kg ha-1) and three target soil water tensions (SWT) (low, medium, and high) were conducted on subsurface drip-irrigated broccoli (Brassica olearacea L. Italica) during three winter growing seasons in southern Arizona. Objectives were to (i) determine effects and interactions of irrigation water and N inputs on net economic return, residual soil NO3-N, and unaccounted fertilizer N, and (ii) use abstract spatial analysis techniques to simultaneously evaluate agronomic, economic, and environmental production functions during three growing seasons. Spatial analysis was used to identify overlap of acceptable zones of marketable yield, net return, and unaccounted fertilizer N. Acceptable yields and net return were defined as 
95% of maximum predicted response within the range of the treatments, and acceptable unaccounted fertilizer N was defined as 
40 kg ha-1. During this study, >95% of maximum net return encompassed N rates of 300 to 500 kg ha-1, and SWTs of 7 to 25 kPa. There was little accumulation of NO3 in the top 0.9 m of soil when 350 kg N ha-1 were applied. Unaccounted N increased with excessive N and water inputs, and accounted for as much as 46% of N applied. Overlap of acceptable zones of agronomic, economic, and environmental production criteria was achieved in each year. Areas of overlap were bounded by 300 to 325 kg N ha-1 and 8.5 to 12 kPa in 1993–1994, 350 to 500 kg N ha-1 and 11 to 14 kPa in 1994–1995, and 340 to 410 kg N ha-1 and 11 to 24 kPa in 1995–1996.
Abbreviations: SWT, soil water tension
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INTRODUCTION
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CONCERN ABOUT the impacts of agricultural practices on the environment is increasing. These concerns include the leaching of nitrate from crop production areas into aquifers. Nitrate contamination of aquifers is especially pronounced in the irrigated Southwest. The percentage of wells testing above the federal drinking water standard of 10 mg NO3-N L-1 in Arizona, California, and Texas ranges from 9.4 to 13.9%. In contrast, an average of 6.4% of all wells sampled in the USA were above 10 mg L-1 (Fedkiw, 1991).
The use of subsurface drip irrigation is a practice that offers the potential for increased water and N fertilizer use efficiency, and decreased groundwater NO3 pollution (Phene, 1999). The use of subsurface drip irrigation is increasing in the desert Southwest and California. Currently, 3600 ha in Arizona and 22300 ha in California are irrigated in this manner (Anonymous, 1994; 1998). Several recent studies have illustrated the efficient nature of subsurface drip irrigation for delivery of water and nutrients (Pier and Doerge, 1995b; Thompson and Doerge, 1996b).
Water and N are the two inputs to irrigated cropping systems having the most impact on agronomic, economic, and environmental outcomes (Letey et al., 1977). These three criteria have only recently been evaluated simultaneously for drip-irrigated crops. The interactive effects of water and N management on yields have been reported for several drip-irrigated vegetable crops (Phene and Beale, 1976; Bar-Yosef and Sagiv, 1982a, 1982b; Feigin et al., 1982; Yanuka et al., 1982; Pier and Doerge, 1995b; Thompson and Doerge, 1996a). Recently, Pier and Doerge (1995a), Thompson and Doerge (1996b), and Thompson et al. (2000) have evaluated agronomic, economic, and environmental outcomes for several subsurface drip-irrigated crops. Similar methods were used in this study to simultaneously evaluate marketable yield, net economic return, and unaccounted fertilizer N for subsurface drip-irrigated broccoli.
The objectives of this study were to (i) determine effects and interactions of irrigation water and N inputs on net economic return, residual soil NO3-N, and unaccounted fertilizer N, and (ii) use abstract spatial analysis techniques to evaluate agronomic, economic, and environmental production functions during three growing seasons.
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MATERIALS AND METHODS
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A detailed description of the field experiments is given in the companion paper (Thompson et al., 2002). During each year, harvested broccoli heads were trimmed to ‘U.S. Fancy’ specifications (USDA, 1943). Marketable heads and trimmings were weighed fresh and dried separately at 65°C in a forced-air oven, ground, and analyzed for total N by the micro-Kjeldahl method modified to recover NO3 (Bremner and Mulvaney, 1982). Soil samples were taken from each plot immediately after harvest at the end of each growing season using a hydraulic drill rig and a 1.5-m long steel coring device. Groupings of three adjacent soil cores were taken at distances of 0, 0.25, and 0.50 m from the drip tubing at three randomly selected locations within the harvest area in each plot. Soil samples to 0.9-m depth were separated into 0 to 0.30, 0.30 to 0.60, and 0.60 to 0.90-m depth increments. The nine subsamples from each depth increment were composited within each plot, thoroughly mixed, subsampled, and air-dried and ground to <2 mm. Analysis of 1 M KCl extractable NH4-N and NO3-N was performed by steam distillation (Keeney and Nelson, 1982).
Estimates of net return were calculated by:
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where Rnet equals the net return ($ ha-1), Rgross is the commodity price ($ Mg-1), Cinput represents the cost of N plus water ($ Mg-1), Charvest signifies the cost of cutting, loading, and hauling ($ Mg-1), and Ymar is the marketable yield (Mg ha-1). Gross return was calculated by assuming a unit price of $482.2 Mg-1. This is the average price in Arizona during the period 1990–1995 (Sherman and Erwin, 1996). Harvest cost was assumed to be $265 Mg-1 (Wade and Harper, 1991). The cost of N was assumed to be $0.35 kg -1 and the cost of water was assumed to be $260.00 ha-1 m-1. This is the approximate current price for Central Arizona Project water. All other production costs were assumed constant across all N by water treatments.
A partial N mass balance was developed using the difference method (Bock, 1984) for broccoli grown during each season. Postharvest unaccounted fertilizer N was calculated as:
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where UNi represents unaccounted fertilizer N in plot i; FNi is fertilizer N applied to plot i; WNi signifies N applied in irrigation water to plot i; WNo equals N applied in irrigation water to control plot, including water used for stand establishment; SNi corresponds to the residual soil NH4-N plus NO3-N to a depth of 0.9 m in plot i; SNo accounts for the residual soil NH4-N plus NO3-N to a depth of 0.9 m in unfertilized control plot harvest areas; PNi represents the total crop N uptake in plot i and; PNo equals the total crop N uptake in control plot harvest areas receiving no N fertilizer. All equation variables are in units of kg ha-1.
The average PNo was 22, 28, and 20 kg ha-1 for the three growing seasons. These values represent crop N uptake from this field following exhaustive cropping. It was assumed that (i) the fate of indigenous N in control and fertilized plots was the same, and (ii) there was no net change in soil organic matter or microbial biomass N. The entire experimental area was subjected to exhaustive removal of available soil N by multiple harvests of unfertilized sudangrass [Sorghum sudanenses (Piper) Stapf.] as well as leaching by several flood irrigation events. This should have resulted in a low potential for soil N mineralization during the broccoli growing season. Therefore, any differences in N losses observed between fertilized and control plots were assumed to be the result of the N and water treatments or their effects on broccoli growth and N recovery in plant biomass. Average irrigation water NO3-N was 2.0 mg L-1.
Response surface equations for marketable yield, net return, and unaccounted fertilizer N were derived for each season using the SAS RSREG procedure (SAS Institute, 1988), which fits a two-variable quadratic response model. This procedure also allows for estimation of critical values on the response surface, such as maxima and minima, if they exist. The general model for each dependent variable was:
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where N is N fertilizer applied (kg ha-1), and SWT is mean soil water tension (kPa). Nine response surface models (year by variable combinations) were generated in this manner. In each case, the lack-of-fit statistic for the model was not significant (P < 0.1), thus indicating that the fit of the model was adequate and no higher-order terms were needed to improve the fit of the model. Therefore, the full quadratic model was retained and plotted as a response surface.
Abstract spatial analysis (Pier and Doerge, 1995a) was used to concurrently evaluate the response surfaces for a given season. For the analysis, an acceptable zone for each of the three production criteria was defined. An acceptable response for marketable yield and net return was defined as 95% of the maximum predicted response within the range of the treatments. An acceptable range for unaccounted N was defined as 40 kg ha-1 of unaccounted fertilizer N. This is an estimate of the quantity of N that could have been leached and still maintain a NO3-N concentration of 10 mg L-1 in the drainage water. This assumes a consumptive water use of 500 mm (Erie et al., 1981), an irrigation efficiency of 85% (state-mandated), 80 mm of rainfall (average rainfall), 30 mm of water containing 2 mg NO3-N L-1 applied during stand establishment, and the same amounts of water in the soil profile at the beginning and end of the experiment. All excess irrigation water, rainfall, and water applied during stand establishment was assumed to leach below the root zone. Because this does not account for immobilization or denitrification of fertilizer N, this should result in an environmentally conservative interpretation, i.e., a worst-case scenario.
After definition of acceptable zones for production criteria, the three response surfaces for each season were superimposed. Zones of overlap were then identified and delineated visually.
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RESULTS AND DISCUSSION
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Net Return
Maximum net return each year was obtained at N rates of 300 to 500 kg ha-1 (Table 1). During 1993–1994 (Fig. 1b)
and 1995–1996 (Fig. 3b)
, maximum predicted net return occurred very close to the SWT and N values for predicted maximum yield; in 1994–1995 (Fig. 2b)
no predicted maximum net return occurred within the range of the treatments. Analysis of variance (Table 2) showed that N rate significantly affected net return in each season (P < 0.01), and SWT significantly affected net return in two of three seasons. In none of the three seasons was there a significant SWT x N interaction (Table 2) at P < 0.05. During 1994–1995 net return was more adversely affected by low SWT (wet treatment) than by high SWT (dry treatment). Rainfall during this season (115 mm) was the highest of the three seasons during this study, therefore the risk of yield loss in the high irrigation treatment was likely higher than during the other two seasons. The shaded areas in Fig. 1b, 2b, and 3b illustrate zones of 95% of maximum net return within the range of the treatments.
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Table 2. Analysis of variance summary for net return, residual soil NO3-N, and unaccounted fertilizer N for broccoli, 1993–1996, as affected by N rate (N) and average soil water tension (SWT).
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With respect to marketable yield, the optimum SWT was about 10 kPa during each season (Thompson et al., 2002). The optimum SWT for maximum net return was similar to that for marketable yield. The close correspondence between the response surfaces for marketable yield (Fig. 1a, 2a, 3a) and net return (Fig. 1b, 2b, 3b) illustrate the overriding importance of yield on economic return. Excessive water and N applications had little effect on net returns other than their adverse effect on marketable yields. Similarly, Sanchez et al. (1996) reported that excessive irrigation reduced marketable yields and net returns for sprinkler-irrigated broccoli grown in western Arizona. They found that profit maximizing N and water rates depended mostly on yield and changed little regardless of input or crop prices.
Residual Nitrate
Postharvest soil NO3-N depth profiles (Fig. 4)
show the effects of water and N rates. Trends were similar among years, but the amounts of residual soil NO3 were highest in 1994–1995 and lowest in 1993–1994. At N rates 350 kg ha-1, postharvest soil NO3-N concentrations never exceeded 10 mg kg-1 in any depth increment examined. In contrast, NO3 accumulated in the soil profile when 500 kg N ha-1 were applied, except in the high irrigation treatment. For example, residual NO3-N in the 0- to 0.9-m depth at rates of 300 to 350 kg N ha-1 averaged across all three seasons was 137, 110, and 90 kg ha-1 for the low, medium, and high irrigation treatments, respectively. In comparison, average amounts of residual soil NO3-N (0–0.9 m) for plots receiving 500 kg N ha-1 during the three seasons were 300, 280, and 130 kg ha-1 for the low, medium, and high irrigation treatments. In the low irrigation treatment, NO3 accumulated mostly in the top 0.3 m of soil, except during 1994–1995 when it accumulated mostly at 0.3 to 0.6 m, probably because of the higher rainfall during that season, compared with the other two seasons. Availability of residual NO3 to subsequent crops will be highly dependent on factors such as rooting depth, rainfall, and irrigation management. In the high irrigation treatment, NO3 was lost from the profile, compared with the low and medium treatments.
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Fig. 4. Postharvest soil NO3 concentrations for broccoli during the 1993 to 1996 growing seasons. Bars represent Fisher's least significant difference (P = 0.05).
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The lower amounts of residual NO3 under conditions of low SWT (wettest soils) probably reflect increased N losses caused by leaching and denitrification, which are favored under these wet conditions (Ryden and Lund, 1980). Pier and Doerge (1995a), and Thompson and Doerge (1996b) reported similar results for residual soil NO3 after subsurface drip-irrigated watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai var. lanatus) and leaf lettuce (Lactuca sativa L. var. crispa L.). It is not known whether this N was lost by leaching or denitrification; however, companion studies suggested that leaching was most likely because of low rates of denitrification in these desert soils (Figueroa, 1999).
Unaccounted Fertilizer Nitrogen
Accounting for all known inputs and outputs of N within a cropping season allows calculation of unaccounted fertilizer N. This includes N lost by gaseous emissions from soils or plants, leached below the root zone, or immobilized in soil organic matter. Unaccounted fertilizer N was significantly affected by both N rate and irrigation treatment (Fig. 1c, 2c, and 3c; Table 2), and increased most dramatically when optimum N rates were exceeded, and under conditions of low SWT. There was a significant N x SWT interaction during 1994–1995, but not during the other two seasons (Table 2). In a few cases, unaccounted fertilizer N was 0 kg ha-1. This apparent over-accounting of fertilizer N is most likely because of errors in soil and plant sampling caused by the natural spatial variability of the system. Over-accounting of N averaged only 7 kg ha-1 and was <66 kg ha-1 in any plot.
Amounts of unaccounted N increased with increasing N rate and lower SWTs (Table 1; Fig. 1c, 2c, 3c). Pier and Doerge (1995a) found similar results for subsurface drip-irrigated watermelon, and Thompson and Doerge (1996b) and Thompson et al. (2000) found similar results for leaf lettuce and cauliflower (Brassica oleracea L. var. botrytis L.), respectively. Feigin et al. (1982) also observed increased N losses, presumably by leaching, because of excessive irrigation applied to drip-irrigated celery (Apium graveolens L. var. dulce (mill.) Pers). Sexton et al. (1996) estimated NO3 leaching in sprinkler-irrigated corn (zea mays L.) by the difference method. Leaching losses of N increased when optimum N rates were exceeded. They recommended fertilizing for 95% of maximum yield to minimize NO3 leaching losses. Nitrate leaching losses of as much as 40% of applied N were reported in California cauliflower fields by Lund (1979). In our study, unaccounted N was equivalent to as much as 46, 46, and 42% of fertilizer N in the first, second, and third seasons. The highest amounts of unaccounted N, as high as 230 kg ha-1, were always in the plots receiving the highest N treatment and the lowest SWT. Our results show that while excessive irrigation had only moderate effects on crop yield, quality, biomass N (see Thompson et al., 2002), and net returns (Table 1), it resulted in much higher N losses from the top 0.9 m of the soil profile.
Response Surface Analysis
Regression equations for response surfaces are shown in Table 3. The F values were significant at P < 0.001 for all models. Spatial analysis of response surfaces for marketable yield, net return, and unaccounted fertilizer N (Fig. 1d, 2d, 3d) showed that during each season these three criteria were optimized simultaneously. The areas of overlap were bounded by 300 to 325 kg N ha-1 and 8.5 to 12 kPa during 1993–1994, 350 to 500 kg N ha-1 and 11 to 14 kPa during 1994–1995, and 340 to 410 kg N ha-1 and 11 to 24 kPa during 1995–1996.
Similar production conditions resulted in overlap of acceptable zones of the three production criteria during each season. Pier and Doerge (1995a) found that overlap of these three production criteria occurred at N rates of 60 to 315 kg N ha-1 and SWT of 7 to 17 kPa for subsurface drip-irrigated watermelon grown in southern Arizona. Thompson and Doerge (1996b) reported that all three criteria were optimized simultaneously for subsurface drip-irrigated leaf lettuce at N rates of 240 to 250 kg N ha-1 and SWT of 6.6 to 7.3 kPa. Thompson et al. (2000) reported that overlap of all three criteria was achieved for subsurface drip-irrigated cauliflower in one of three years. Our results indicate that with proper management of water and N inputs, including maintaining an appropriate SWT, subsurface drip-irrigated broccoli production can result in outcomes that are acceptable to growers, and result in minimal environmental impact.
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CONCLUSIONS
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Agronomic, economic, and environmental production criteria were evaluated for subsurface drip-irrigated broccoli grown in southern Arizona. During this study, 95% of maximum net return encompassed N rates of 300 to 500 kg ha-1, and SWTs of 7 to 25 kPa. Concentrations of postharvest soil NO3 were <10 mg kg-1 in treatments receiving <350 kg N ha-1. Treatments receiving >350 kg N ha-1 often had high postharvest soil NO3, except in the wettest irrigation treatment. Therefore, even though excessive amounts of irrigation water and N had only moderate effects on marketable yield and net returns, they had dramatic effects on residual soil N. The maximum amounts of residual NO3-N occurred under conditions of high N rates and high SWT. The maximum amounts of unaccounted fertilizer N occurred under conditions of high N rates and low SWT. Overlap of acceptable zones of agronomic, economic, and environmental production criteria was achieved during each season.
Received for publication November 2, 1999.
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Fertigation Frequency for Subsurface Drip-Irrigated Broccoli
Soil Sci. Soc. Am. J.,
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910 - 918.
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TOP
ABSTRACT
INTRODUCTION
