Effect of Rate, Timing of Application, and Placement of Nitrogen Fertilizer on Broccoli Yield and Nitrogen Uptake (1992)

Introduction

Broccoli growers in the Willamette Valley use high rates of nitrogen fertilizers, often exceeding 250 to 300 pounds actual N/acre per season. While the common experience has been that these rates are necessary to achieve maximum yields and quality, a considerable portion of the applied fertilizer may not be taken up by the crop. This has raised concerns that the remaining N may be contributing to nitrate pollution of groundwater. Improved efficiency of nitrogen management in broccoli may be possible if the fertilizer could be applied at the time of maximum crop need and placed for maximum contact with the root system. Trials conducted in 1989 and 1990 indicated that 250 pounds N/acre are needed for maximum yields, and that N source (potassium nitrate, sodium nitrate, calcium nitrate, urea, ammonium nitrate, ammonium sulfate) has very little effect on crop yield and nitrogen uptake. The purpose of the 1991 and 1992 trials was to study the response of broccoli yield to a wide range of rates of nitrogen as well as different methods of fertilizer placement and timing of the sidedressed applications. In addition, plots fertilized at 250 pounds per acre were used to determine the course of nitrogen uptake over time.

Methods

In 1991, 'Gem' broccoli was direct-seeded in a Willamette silt loam, pH 5.7, at the NWREC on 12 June. Plot preparation included a broadcast and incorporated application of triple superphosphate at 200 pounds/acre, potassium sulfate at 240 pounds/acre, boron at 2.0 pounds/acre, trifluralin at 0.75 pounds/acre, and chlorpyrifos at 1.3 pounds/acre. Fifty pounds N/acre as urea were shanked in at 2 inches beneath and 2 inches to the side of the seed row on the appropriate plots. All other plots except a zero-nitrogen check received a broadcast application of 50 pounds N/acre on 13 June, followed immediately by an irrigation of 0.5 inches. Plot size was 15 feet with four rows on an 80-inch bed. Spacing between rows alternated between 12 and 28 inches. The seedlings were thinned to approximately 10 inches between plants in the row on 26 June. The plots were sprinkler-irrigated as necessary and harvested on 21 and 28 August, and 4 September. Only main shoots were cut.

Urea was the sole nitrogen source. Of the total of 13 treatments that received N fertilizer, the remainder of the nitrogen was applied on 16 July for 12 treatments. For the remaining treatment, half the remaining N was applied on 9 July, the other half on 29 July. Treatments consisted of various combinations of fertilizer rates, and methods and timing of fertilizer placement (Table 1). In addition, two sets of plots were reserved for application of 15N-enriched urea to monitor N uptake and movement of applied N through the soil profile. Treatments were in randomized complete block design with four replications.

Following completion of the last harvest and soil sampling for residual N, the four blocks were split and half of each block seeded to 'Wheeler' cereal rye on 17 Sept. Broccoli residue was left standing.

In 1992, methods were similar except as follows. The seeding date was 9 June. Plot size was 20 feet with four rows on an 80-inch bed. Spacing between rows was 16 inches. The initial application of nitrogen was broadcast immediately after seeding and irrigated in. Depending on treatment, all N was applied at planting; 40 pounds per acre were applied at planting and the remainder on 14 July; or 40 pounds were applied at planting, half the remainder on 7 July, and the final application on 27 July (Table 8). The plots were harvested on 18 and 25 August.

The cereal rye cover crop was seeded on 3 Sept. after mowing the broccoli and lightly disking the soil surface.

Results and Discussion

1991
When all N fertilizer was broadcast, yield increased with increasing rate of N to a maximum at 250 pounds N/acre (Table 2). Not all N rates were included when the second N application was banded between the paired 12-inch rows or when the first N application was shanked-in rather than broadcast. However, the trend in yield response to increasing rate of N was similar to the yield response when all N was broadcast (Tables 3 and 4). The greatest yield was obtained when the sidedressed N was split between two applications at four and seven weeks after planting rather than a single application at five weeks (Trt. 11, Table 2). However, the yield with the split application was not significantly greater than the yield with a single sidedressing of N at the same rate. These results are consistent with those obtained in 1990, when maximum yield occurred at 250 pounds N, regardless of source of the applied N. Rates of N in excess of 250 pounds do not appear to increase potential return to the grower.

Method of N placement did not significantly affect yield in this trial. Banding the sidedressed N between the closely-spaced paired rows was no more efficent than spinning the N over the entire area at this plant population (31,360/acre) and row spacing (Table 3). Apparently, fertilizer landing in the 28-inch space between paired rows was effectively utilized. Alternatively, concentrating the N between the paired rows may have resulted in excessive N in the band or exposure of too small a fraction of the root system to the available N. Likewise, shanking the initial fertilizer application in a band 2 inches to the side and 2 inches beneath the seed line did not increase efficiency of utilization as reflected in mean head weight (Table 4). Also, there was a single comparison of shanked at-planting fertilizer and banded sidedressed fertilizer at a total N rate of 100 pounds/acre (Treatment 4). This treatment produced an average head weight for the season of 133 g, less than that obtained for all-broadcast N at 100 pounds (141 g) or the shanked-broadcast combination at 100 pounds N/acre (136 g).

Thus, it appears that at high plant populations and small between-row spacings, broadcast applications are as effective as any other means of application. Irrigation in this trial was controlled to prevent movement of nitrogen out of the root zone. A total of 10 inches of water was applied and there was no significant rainfall. Wells (vacuum lysimeters) were installed in the 15N plots. There was no movement of water below the two-foot depth. A comparison of pre-plant and post-harvest soil samples taken down to the 40-inch depth indicated no movement of nitrate or ammonium below a depth of 20 inches during the growing season. Only at the highest rate of applied N was there any evidence of residual nitrate or N movement to a depth greater than 10 inches (Table 5). With greater irrigation amounts or significant precipitation, band placement might still keep more of the applied N in the root zone.

Results from the 15N uptake studies indicate that about two-thirds of N taken up by broccoli fertilized at the optimal 250 pounds/A N rate comes from applied fertilizer. Total N uptake was 310 pounds/A and essentially all the 15N was taken up. This indicates that an optimally-fertilized and irrigated broccoli crop leaves almost no unutilized N in the soil. However, non-uniform plant stand and growth in these plots caution against extrapolating the results to commercial fields.

The cereal rye cover crop planted in 1991 was very effective in removing residual nitrate and ammonium from the soil profile on plots fertilized with 250 pounds N/acre (Table 6). The plots without cover show an increase in nitrate, but not ammonium, concentration with depth. The plots with a cover crop show a much lower level of nitrate and ammonium at all depths and less tendency for higher levels of nitrate in the 20 to 40-inch depth. The cover crop did not accumulate a great amount of dry matter. However, up to 33 pounds N/acre were trapped in the cover crop biomass at the highest rate of N application to the broccoli crop (Table 7). When there was no broccoli left standing, more than 40 pounds N/acre were taken up by the rye following broccoli fertilized with 250 pounds N/acre. The amount of nitrogen in the standing broccoli crop, which was not killed by the mild winter, was not determined.

	
Table 1. List of N application treatments, 1991 broccoli N utilization trial, 
NWREC                                                                        
No.   Total N applied   N applied at planting  N applied at five weeks       
      ----------------------lb/A--------------------------------------    
 1           0           0                        0
 2         100          50 broadcast             50 broadcast
 3         100          50 shanked               50 broadcast
 4         100          50 shanked               50 banded
 5         100          50 broadcast             50 banded
 6         175          50 broadcast            125 broadcast
 7         175          50 broadcast            125 banded
 8         250          50 broadcast            200 broadcast
 9         250          50 broadcast            200 banded
10         250          50 shanked              200 broadcast
11         250          50 broadcast              0z
12         250          50 broadcast            200 broadcast 15N
13         250          50 broadcast 15N       200 broadcast
14         325          50 broadcast            275 broadcast                 
z100 broadcast at 4 weeks; 100 broadcast at 7 weeks.


Table 2. Effect of N rate on yield and mean head weight of broccoli when all 
fertilizer is broadcast, NWREC, 1991                                        
                              Mean head weight (g)      
Trt. No.  N rate (lb/A)  First harvest   All harvests   Total yield (T/A)   
 1              0            95             91                  2.9   
 2            100           137            141                  5.5
 6            175           145            153                  5.4
 8            250           154            158                  5.9
11            250           159            162                  6.0
14            325           123            140                  4.1
             LSD (0.05)      48             40                  1.8         


Table 3. Comparison of N rate on mean head weight of broccoli when all fertilizer 
is broadcast versus band placement of the second N application, NWREC, 1991      
         Broadcast-broadcast                       Broadcast-band            
Trt. No.  N rate (lb/A)  Mean wt. (g)   Trt. no.  N rate (lb/A)  Mean wt. (g) 
 2             100           141           5           100           129
 6             175           153           7           175           145
 8             250           158           9           250           161
                      Meanz  151                                     145         
                                                                                  
zNo significant difference between means (P=0.05).


Table 4. Comparison of N rate on mean head weight of broccoli when all fertilizer is
broadcast versus shanked-in placement of the at-planting N application, NWREC, 1991 
         Broadcast-broadcast                        Shank-broadcast                 
Trt. No.  N rate (lb/A)  Mean wt. (g)   Trt. no.  N rate (lb/A)  Mean wt. (g)
 2             100           141          3            100           136
 8             250           158         10            250           151 
                      Meanz  150                                     144
                                                                                    
zNo significant difference between means (P=0.05).

Table 5. Soil nitrate and ammonium concentrations (ppm) before planting 
and after broccoli harvest, NWREC, 1991                                
                                                 N rate, lb/A          
                                         0    100    175    250     325
Depth of sample (inches)                                                
Nitrate                   Pre-plant    ---------Post-harvest-----------
 0-10                       6.3        0.1    0.4    3.4    9.0    23.3 
10-20                       3.3        0.8    0.1    0.7    2.4     4.2
20-30                       2.0        0.6    0.1    0.3    1.2     2.0
30-40                       2.4        0.8    0.3    0.4    1.0     1.7
Ammonium
 0-10                       3.2        3.7    4.0    7.6   12.9    30.8
10-20                       3.7        2.4    2.7    2.3    4.2    12.1
20-30                       6.7        3.7    4.9    2.8    4.8     5.2
30-40                       8.2        3.0    3.1    2.3    2.7     4.0  


Table 6. Effect of a cereal rye cover crop on soil nitrate and ammonium 
concentrations (ppm) in broccoli plots fertilized with 250 pounds/acre 
nitrogen, NWREC, 15 April, 1992                                        
Depth of sample (inches)    With cover    Without cover    Significancez 
Nitrate
 0-10                         0.07             0.41            *
10-20                         0.09             0.57            **
20-30                         0.12             3.17            **
30-40                         0.18             5.38            ** 
Ammonium
 0-10                         1.05             2.36            *
10-20                         1.23             2.04            *
20-30                         1.27             2.20            *
30-40                         1.25             1.96            NS      
z*,**,NS:  Differences significant at the 5% and 1% levels and no significant 
difference, respectively.

Table 7. Nitrogen content, yield, and nitrogen uptake by a cereal rye cover crop in 
April, 1992, following various rates of N applied to a broccoli crop in 1991, NWREC
N rate, lb/acre  N content (%)   Dry matter yield, lb/acre   N uptake, lb/acre     
  0                  1.55                   430                     6.2
100                  1.48                   580                     7.6
175                  1.29                  1840                    23.0
250                  1.63                  1510                    24.4
250z                 1.27                  3530                    42.5
325                  1.45                  2330                    33.5
     Significance     NS                    **                      **             
zBroccoli plants removed from this plot but were left standing after harvest 
in all other plots.

1992

Averaged over timing of N application, yield increased with increasing rate of N to 180 pounds N/acre (Table 9). Rates of N application beyond this level had little effect on yield. In the previous experiments, yield peaked at 250 pounds N/acre, with a definite decrease at 325 pounds/acre.

Splitting the N application to provide most of the N nearer the time that the plants experience greatest uptake did not increase yield (Table 10). Even at less than optimal rates of N, yield was greatest when all N was applied at planting. This was a late-spring planting in a very dry year and with carefully controlled irrigation. Thus, N loss to leaching was probably not a factor. Results may have differed for a planting early in the season when leaching of the early N application would be more likely.

It is critical to have an adequate supply of N at planting. Even after a dry winter and with significant available N in the soil at planting (5.6 ppm nitrate in the surface six inches of soil), yield was increased by applying N at planting. Delaying the first N application to four weeks after planting reduced mean head weight from 163 g to 148 g for plants receiving a total of 180 pounds N/acre. In the comparison of the single application versus a 2-way or 3-way split, perhaps the split applications would have been more favorable if a greater proportion of the nitrogen had been applied at planting.

The broccoli crop effectively depleted soil mineral nitrogen at rates of applied nitrogen up to 180 pounds/acre (Table 11). At higher rates the soil nitrate, but not ammonium, concentration was elevated slightly. The results obtained at 240 pounds N/acre are almost identical to those obtained with the optimal rate of 250 pounds in 1991.

Four years of results indicate that the optimum nitrogen rate for broccoli grown on a Willamette silt loam at a plant density of 30,000 to 40,000 per acre is near 250 pounds/acre. Rates in excess of 300 pounds/acre leave significant amounts of residual nitrate available for leaching, whereas rates of 250 pounds/acre or less are efficiently utilized by the crop. Winter cover crops can catch a large proportion of the residual nitrate. Neither method of placement of N nor the timing of N application appear to have much effect on the efficiency of N utilization by broccoli.

Table 8. List of N application treatments, 1992 broccoli N utilization trial, NWREC
No.   Total N applied   At planting   At five weeks   At seven weeks               
      ---------------------------lb/A-------------------------------    
 1           0              0               0             0
 2          60             40              20             0
 3         120             40              80             0
 4         180             40             140             0
 5         240             40             200             0
 6         300             40             260             0
 7          60             60               0             0
 8         120            120               0             0
 9         180            180               0             0
10         240            240               0             0
11         300            300               0             0
12         120             40              40z           40
13         180             40              70z           70
14         240             40             100z          100
15         300             40             130z          130
16         180              0              90z           90
_____________________________________________________________________________      
zAt 4 weeks rather than 5 weeks.


Table 9. Main effect of N rate on yield and head size of broccoli, NWREC, 1992
N rate       Mean head wt.,     Mean head wt., total     Total yield
(lb/acre)   first harvest (g)   of two harvests (g)      (tons/acre)           
  0               76                    52                   1.6
 60              112                    95                   3.9
120              167                   142                   5.5
180              195                   167                   6.8
240              196                   169                   6.7
300              205                   170                   7.4
      LSD (0.05)  41                    31                   1.0               


Table 10. Effect of splitting nitrogen application on broccoli head size at 
several rates of applied nitrogen, NWREC, 1992                             
N rate (lb/acre)   Single applic.   Two applics.     Three applics.        
                   --------------------g/head---------------------- 
 60                     101             88                --
120                     159            137               129
180                     177            163               162
240                     170            169               168
300                     171            164               176
Mean (120-300 lb)       169            158               159               


Table 11. Effect of rate of broadcast nitrogen on soil nitrate and ammonium 
concentrations (ppm) following final broccoli harvest, NWREC, September, 1992  
                                               N rate, lb/A                   
                                      0   60   120   180   240   300  LSD(.05) 
Depth of sample (inches)                                                      
Nitrate                  Pre-plant  -------------Post-harvest--------------
 0-10                       5.6     0.9  0.4   3.1   3.4   9.8  12.1    4.2  
10-20                       3.8     0.2  0.2   0.9   1.1   4.7   3.6    3.2
20-30                       3.1     0.6  0.4   0.3   0.6   1.4   0.9    NSD
30-40                       3.8     1.2  0.7   1.2   1.0   1.9   1.4    NSD
Ammonium
 0-10                       4.8     2.0  1.1   1.8   2.0   2.0   2.3    NSD
10-20                       2.8     1.8  1.6   2.2   2.7   1.3   1.5    NSD
20-30                       5.1     1.5  2.0   1.1   0.9   0.8   1.3    NSD
30-40                       4.4     1.5  0.8   1.2   1.1   0.9   1.4    NSD   

Tags:

Fresh Market Vegetable Production, Processed Vegetable Production, Willamette Valley, Broccoli, Soil and nutrient management, Vegetable production