Vegetable Research at the North Willamette Research and Extension Center, 1995-1996

CONTENTS

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AUTHOR:

Dr. Delbert D. Hemphill, Jr., Professor of Horticulture, has conducted research on vegetable crops culture and management since 1976 at Oregon State University's North Willamette Research and Extension Center, 15210 NE Miley Rd., Aurora, OR 97002-9543.

COOPERATORS:

Dr. N.S. Mansour is Extension Vegetable Specialist and Professor, Department of Horticulture, Oregon State University, Corvallis, OR 97331

Dr. John Hart is Extension Soil Scientist and Professor, Department of Crop and Soil Science, Oregon State University, Corvallis, OR 97331

Dr. Richard Dick is Professor of Soil Science, Department of Crop and Soil Science, Oregon State University, Corvallis, OR 97331

Dr. John Luna is Assistant Professor and On-Farm Research Coordinator, Department of Horticulture, Oregon State University, Corvallis, OR 97331

Dr. John Selker is Associate Professor, Department of Bioresources Engineering, Oregon State University, Corvallis, OR 97331

Dr. Carol Miles is a Washington State University Extension Agent, 360 NW North St., MS:AESOL, Chehalis, WA 98532

Mr. Ernie Marx is Faculty Research Assistant, Department of Crop and Soil Science, Oregon State University, Corvallis, OR, 97331

Many of the research projects reported here involved cooperation with Experiment Station and Extension Service colleagues at Oregon State University. Their contributions are gratefully acknowledged. The financial support of the Oregon Processed Vegetable Commission, UNOCAL, Biagro Western Sales, and the Agricultural Research Foundation was essential to completing these projects and is greatly appreciated.

This report is the tenth in a series of biennial reports initiated in 1979.

DISCLAIMER: The use of trade names does not constitute an endorsement by the Oregon State University Agricultural Experiment Station. Always check pesticide labels for currently registered uses.

Vegetable growers in the Willamette Valley use high rates of nitrogen fertilizers, often exceeding 250 to 300 pounds actual N/acre per season. While growers believe that these rates are necessary to achieve maximum yields and quality, a considerable portion of the applied fertilizer is not 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 may be possible if the fertilizer could be placed for maximum contact with the root system.

Trials conducted in 1989 through 1994 indicated that broccoli is more efficient than is sweet corn in recovering applied N, even though higher rates of N are typically applied to broccoli. This may be due to differences in root architecture but between-row spacing may also play a role. Sweet corn is typically grown at row spacings of 30 or 36 inches. Broccoli is most commonly grown at spacings that average 20 inches. Therefore, we may deduce that broccoli roots might explore a greater proportion of the space between rows. Broccoli is also typically planted at higher populations than is sweet corn. The purpose of this trial was to investigate whether decreasing row spacing, but maintaining a constant population by increasing in-row spacing, would increase N uptake efficiency in sweet corn, as measured by the amount of mineral N remaining in the soil at harvest.

Methods

'Jubilee' sweet corn was seeded in a Willamette silt loam, pH 5.9, at the NWREC on 24 May using a hand-push garden planter. Plot preparation included a broadcast and incorporated application of 10N-8.3P-16.7K (10-20-20 fertilizer) at 500 pounds per acre, disking, and cultimulching. Fifty pounds N/acre as urea was applied to all plots immediately after planting. Plot size was 15 feet wide by 30 feet long. Spacing between rows was 18, 27, or 36 inches. Immediately after planting, atrazine was applied at 2.0 pounds/acre and alachlor at 3.0 pounds/acre for weed control.

During the third week of June corn stands were thinned to a nominal population of 25,000/acre. The remaining N was broadcast to the appropriate plots on 22 June (split application). The six combinations of the two fertilizer rates and three spacings were in randomized complete block design with four replications. The plots were sprinkler-irrigated as necessary and harvested on 24 August.

After harvest, the stover was mowed and left in place on the plots. The plots were sampled to a depth of 2 feet in 1-foot increments on 27 September. The samples were submitted to the OSU Central Analytical Laboratory for determination of residual nitrate and ammonium concentration.

Results and Discussion

The effect of row spacing was tested both at an optimal and at a suboptimal rate of N. Yield, mean ear weight, ear length, and residual nitrate levels were greater at 200 than at 50 pounds applied N/acre (Tables 1 and 2). Only main effects are shown in the tables as there were no significant interactions of N rate and row spacing affecting any yield parameter or residual N. Seeding with the garden planter was not entirely satisfactory and stands were somewhat less than the desired 25,000/acre, particularly for the 18- and 36-inch spacings. Thus yield tended to be slightly greater at the 27-inch spacing, but the effect of spacing on yield was not signficant (Table 1). The effect of spacing on residual soil nitrate and ammonium concentrations was also not significant (Table 2), giving no support to our hypothesis that closer row spacings might result in more efficient N uptake and reduced residual mineral N. However, residual N at 200 pounds applied N/acre was unusually low in this experiment compared to many of our trials.

Table 1. Main effects of rate of urea-nitrogenz and row     
spacing on the yield of sweet corn, NWREC, 1995          
Treatment           Yield         Ear wt.    Ear length    
                    (T/A)           (g)       (inches)       
N rate (lb/acre)
 50                  4.5            220         10.5
200                  9.0            283         11.6
     Significance     **             **          **
Row spacing (inches)
18                   6.5            266         11.1
27                   7.4            232         10.9
36                   6.4            257         11.1
     Significance     NS             NS          NS      
zFifty pounds N/acre broadcast at planting, remainder 
 broadcast five weeks later.
**, NS:  Significant at p=0.01 and nonsignificant, 
 respectively.


Table 2. Main effects of N rate and row spacing on post-harvestz
soil nitrate and ammonium concentrations, 27 September, 1995        
Treatment         Nitrate-N                    Ammonium-N       
          0-12 inches  12-24 inches   0-12 inches  12-24 inches
N rate (lb/acre)       
          -------------------------ppm-------------------------
 50            0.2           0.2            3.0           2.9 
200           10.9           2.3            4.2           3.5
  Significance  *             *              NS            NS
Row spacing (inches)       
18             4.7           1.6            3.6           3.2
27             7.2           1.1            3.7           3.5
36             4.7           1.1            3.6           2.8
  Significance  NS            NS             NS            NS   
zPre-plant nitrate levels were 0.2 ppm at both 0-12 and 12-24 inch
depths. Pre-plant ammonium levels were 2.8 and 3.2 ppm at 0-12 and 
12-24 inch depths, respectively.
*,NS:  significant at p=0.05 and nonsignificant, respectively.

Introduction

The possibility that the high rates of N needed for maximum yield and quality of vegetable crops are contributing to nitrate contamination of groundwater is a major concern in Oregon's Willamette Valley. We feel that our research has contributed significantly to an understanding of the yield response of vegetables to N fertilizer and the critical stages for N uptake, particulary as related to residual mineral N in the soil after harvest. For the crops of our major efforts (sweet corn, broccoli, cauliflower), we feel that more work on rates of fertilizer application, timing, placement, and N source will not lead to significant changes in the way we grow these crops.

Our focus now is on the most important remaining questions and movement toward possible solutions to the problem of high levels of residual applied N following vegetable crops, particularly sweet corn. We are attemping to develop methods to predict crop response to sidedressed N and to determine how much sidedressed N is needed, particularly in sweet corn. Research on silage corn grown with high inputs of manures or other organic sources indicates that a pre-sidedress soil nitrate test (PSNT) may be useful in determining the amount of additional N needed. Some of this work with silage corn has been done in Oregon. We are interested in extending this avenue of research to sweet corn in Oregon, basing our initial efforts on results of PSNT research on sweet corn in New Jersey.

Methods

NWREC plots, 1995

'Jubilee' sweet corn was planted on 30-inch row spacing on 16 May. Forty pounds N/acre as 10-20-20 was broadcast and disked into the entire area before planting. Nitrogen as urea was applied at four rates (0, 40, 80, 120 pounds/acre) at planting. Before application of mid-season sidedress N on 21 June, a pre-sidedress soil nitrate test (PSNT) sample was collected from the surface foot of soil and analyzed for nitrate- and ammonium-N (Table 3). Sidedressed N rates of 0, 40, or 80 pounds/acre were superimposed on each of the initial N rate treatments, resulting in total N rates ranging from 40 to 240 pounds/acre. On 7 July, SPAD chlorophyll meter readings were taken and leaf samples collected and analyzed for total N content. At harvest, stalk samples were collected from six treatments (Table 4, first three and last three treatments) and analyzed for nitrate-N concentration. Corn was harvested from 40 row feet near the center of the plot on 18 August. Following harvest, soil was sampled from the 0-1 and 1-2 foot depths and analyzed for nitrate- and ammonium-N content.

NWREC plots, 1996

'Jubilee' sweet corn was planted to 15 x 30-foot plots on 30-inch row spacing on 26 May. At planting, N (as urea) was applied at rates of 0, 40, 80, and 120 lb/acre to establish four levels of soil mineral N. Before applying mid-season sidedressed N, a PSNT sample was collected from the surface foot of soil and analyzed for nitrate- and ammonium-N (Table 5). At the same time, 10 leaves from each plot were collected for analysis of leaf N content and chlorophyll meter (SPAD) readings were taken on 10 additional intact leaves/plot. Additional N applications of 0, 40, and 80 lb/acre were then superimposed on each of the initial treatments, resulting in total N rates of 0 to 200 lb/acre. Three weeks after the second N application another set of SPAD readings were made. Corn was harvested from 40 row feet on 29 August. Following harvest, soil was sampled for determination of residual nitrate and ammonium content.

On-farm PSNT Trial, 1995

Sixteen experiments were conducted on six farms. Growers planted and managed corn according to their normal practice. The surface foot of soil was sampled and analyzed for nitrate-and ammonium-N content before the grower applied mid-season sidedress N. Unfertilized check plots were left in each field. The checks were divided into three subplots, each four rows wide and 20 feet long. Three subplots were also picked randomly in the fertilized area adjacent to the checks. At harvest, corn from the center two rows of each subplot was harvested. The bottom eight-inch section of stalk was also cut from each of 10 plants in each subplot and analyzed for nitrate content.

We defined percent relative yield as the average yield from the unfertilized plots divided by the average yield from the fertilized plots. If yield from the unfertilized plots was less than 94 percent of yield from the fertilized plots, the site was identified as N responsive. The 94 percent cutoff allows for variability and considers the diminishing rate of return as increasing amounts of fertilizer are needed to attain ever smaller yield increases as maximum yield is approached. A 6 percent reduction represents a yield loss of about 0.5 tons/acre. Relative yield data were then compared to PSNT soil test data to determine if there is a soil nitrate concentration above which response to additional fertilizer N is unlikely. Relative yields were also compared to nitrate concentrations in corn stalks at harvest to determine if the test can be used to evaluate N sufficiency and N management efficiency.

On-farm PSNT Trial, 1996

Twelve growers participated in the on-farm PSNT trials. Plots were approximately one acre, or large enough to fill a truck at harvest. Growers limited pre-plant and at-planting N applications to a total of 50 pounds N/acre. A PSNT sample was collected and analyzed for nitrate-N before mid-season N applications,. Two top/sidedress N rates were then applied; 100 and 150 pounds N/acre. The goal of the project was to determine a PSNT soil test value above which a mid-season N application of 100 pounds N/acre would be sufficient for optimum yield. At three sites, treatments were replicated four times. Plots were harvested into separate trucks by growers. Yield information (weight and grade) was determined by the processor receiving the corn. Following harvest, soil and corn stalk samples from each N rate were analyzed for nitrate-N content. Nitrogen-responsive sites were defined as those where yield from the 100 pounds N/acre topdress was less than 98 percent of yield from the 150 pounds N/acre topdress.

Results

NWREC, 1995

Maximum sweet corn yield was attained at total N rate of 240 pounds N/acre. This yield, however, was not signficantly larger than with several combinations of N at planting and sidedressed N totalling as little as 120 pounds/acre (Table 6). At 120 pounds applied N/acre, yields tended to be reduced when all N was applied at planting. When 160 pounds N/acre was applied at planting, the PSNT value was 36 parts/million nitrate-N (Table 1) and additional sidedressed N did not improve yields or ear weight significantly (Table 6). All treatments including sidedress N at 80 lb/acre attained yields and ear weights not significantly different from the largest yield, regardless of the amount of N applied at planting. These results suggest that split N application may result in more efficient fertilizer use and is in agreement with past results obtained at NWREC.

SPAD meter readings were correlated with leaf N concentration and sweet corn yield (Tables 4, 6, and 7), suggesting that this meter may be useful for evaluating crop N status. The advantage of the SPAD meter is ease of use and instantaneous analysis. The meter has been used successfully in field corn production in many parts of the United States. Because of variability between sites, however, it may be necessary to establish a high N comparison plot in each field where the meter is to be used. The high N plot is used as a reference to determine if the rest of the field is N deficient.

Residual soil nitrate concentrations increased with both N at planting and sidedress N rates. Residual soil nitrate tended to be low at N rates at or below that needed for maximum yield (Table 8). As N rates exceeded crop demand, yields remained nearly constant while residual soil nitrate increased (Figure 1). Residual nitrate was about 35 and 80 pounds nitrate-N/acre for the 160 and 240 pounds N/acre rates, respectively, with no signficant increase in yield over this range of fertilizer rates. Timing of N application had no apparent effect on residual soil nitrate in the surface foot of soil.

NWREC, 1996

Highest yields of 10.7 tons/acre were obtained with a total N application of 120 pounds/acre (80 at planting, 40 mid-season; Table 10). Three other treatments gave yields not significantly different than the maximum. These were 120 pounds N (40 at planting and 80 mid-season), 160 pounds total N, and 200 pounds total N/acre. These results are consistent with those of 1995. Mean ear weight was highest at 160 pounds N/acre but several rates and combinations of at-planting and mid-season applications gave essentially equal ear sizes. When all 120 pounds N/acre were applied at planting, yield was significantly lower than when the same rate of N was split. Tipfill and ear length also increased with increasing rates of N. In these plots, the PSNT value was 26.4 parts/million when 120 pounds N/A was applied at planting. This rate of N at planting was not quite adequate to give maximum yields. However, when the same total rate of N was applied, but split into two applications, maximum yield was obtained. This is in agreement with past research indicating that splitting N applications may result in more efficient utilization of N by the corn plant.

SPAD readings and leaf N correlated with amount of N applied to the soil. However, these tests indicate only that the plant has sufficient N at the time of the test. These tests say nothing about whether the soil will provide sufficient N to maintain the plant through harvest. They may be of more use when the total N rate is divided into several applications such as with feeding liquid N through the irrigation system. However, the late SPAD readings (Tables 11, 12), taken three weeks after PSNT, correlated strongly with eventual yield. A SPAD reading of less than 45 at this point may indicate that additional N is needed.

Residual soil nitrate concentrations were lower than we usually experience in corn plantings (Table 11). Nevertheless, nitrate levels did increase with increasing rate of mid-season sidedressed N. About 68 pounds/acre residual nitrate- plus ammonium-N were present on plots fertilized at 200 pounds N/acre. This rate clearly exceeded that needed for maximum yield.

On-farm PSNT Trials, 1995

PSNT and yields are shown in Table 13 and plotted in Figure 2. The graph is divided into four quadrants. Quadrant III contains sites that had low PSNT values and were N responsive. Quadrant II contains sites that had high PSNT values and were not N responsive. Both of these quadrants contain "correct predictions," sites where the PSNT successfully predicted yield response to N fertilizer. Quadrant I contains sites that had low PSNT values but did not show yield response to added N. Quadrant IV contains sites that had high PSNT values and an unexpected yield response to additional N. These quadrants represent "incorrect predictions," sites where using PSNT would have resulted in the wrong decision.

The vertical line in Figure 2 represents the expected PSNT critical value of 25 parts/million nitrate-N, based on sweet corn research in New Jersey and field corn research in Oregon and several other states. The lack of high PSNT sites in this study prevented us from confirming the critical value for sweet corn in western Oregon.

PSNT values on 13 of the 16 sites were below 25 parts/million nitrate-N, suggesting that sidedress N applications are needed on most Willamette Valley sweet corn fields. Only site 16 had a PSNT value above 30 parts/million. This site had a history of manure applications and, therefore, the potential for large amounts of N mineralization.

Stalk nitrate data suggest a critical value of about 2,750 parts/million nitrate-N. If nitrate-N concentrations in the stalk at harvest are below 2,750 parts, lack of N may have limited yield. This is in agreement with the results from the NWREC plots.

Five sites (2, 5, 8, 10, 12) with low PSNT values did not show a difference in yield between fertilized and unfertilized plots (Figure 2, Quadrant I). At first, these appear to be incorrect predictions and raise questions concerning the effectiveness of PSNT. Closer examination of the data, however, shows that four of these sites had low yields in both the fertilized and unfertilized plots (Table 13). Corn stalk nitrate concentrations were also low in the fertilized plots at these sites, suggesting that the fertilized plots were still N deficient. Nitrogen deficiency on the fertilized plots may have prevented identification of a potential N response.

The small number of high PSNT sites suggests that determination of a critical value above which no sidedress N is needed may be of limited value. Nine of 16 sites, however, had PSNT values in the range of 17-25 ppm NO₃-N. Sites in this range are close to the level is expected to be sufficient for maximum economic yield. Future research should focus on establishing a critical PSNT value above which sidedress N applications can be reduced, but not eliminated entirely.

On-farm PSNT Trial, 1996

PSNT values ranged from 8 to 31 ppm NO₃-N (Table 14). The distribution of values in 1996 was similar to 1995 (Figure 3). Sites with PSNT values of 15 ppm or greater did not benefit from the higher N rate, with two exceptions (Figure 4). This indicates that if PSNT values are 15 ppm or greater, a mid-season N application of 100 pounds N/acre is sufficient for optimum yield. Because data is from a single year, a more conservative PSNT critical value of 18 ppm may be appropriate. The distribution of PSNT values indicates more than half of the sweet corn fields sampled during 1995-96 could benefit from this approach to N management.

The two N-responsive sites with high PSNT values were on coarse-textured soils in the Stayton and Coburg areas. They were also the only two sites where the low N plots were placed at the edge of the field. Whether the unexpected yield response was due to soil type, plot layout, or something else, is unknown.

The corn stalk nitrate test is designed to identify N deficiency or excess in the crop just harvested. This information would be used to adjust N management in future years. NWREC data in 1995 (above) suggested a stalk nitrate critical value of 2,750 ppm NO₃-N. This critical value was accurate for 8 of 12 sites (Figure 5) in 1996. The two sites that were outliers for PSNT data were also outliers for corn stalk nitrate data (Tables 14 and 15).

The coefficient of variation (CV) is a measurement of variability in data. On the replicated sites, the CV was extremely low (1.5 percent or less). The low CVs indicate that three replications are sufficient for future field-scale sweet corn research.

There were no statistically significant differences in corn grade (percent no value, grade, cobs per ton) on the replicated sites, as determined by the processors.

Summary

• Split N applications are more efficient than applying all N before or at planting for sweet corn. Maximum yield was obtained at N rates as low as 120 pounds N/acre with split applications at NWREC.
• The PSNT appears promising as an N management tool for sweet corn. Tentatively, PSNT values above 18 ppm nitrate-N indicate a mid-season N application of 100 pounds N/acre is sufficient for optimum yield. Confidence in the PSNT is greater on silty clay loams (Woodburn, Willamette, etc.) than on coarse textured and gravelly soils, such as those found in the Stayton and Coburg areas.
• The corn stalk nitrate at harvest test remains promising as an indicator of N management. Data from more years and sites are needed to establish a critical value.
• Residual soil nitrate can be reduced while maintaining optimum yield by using N management tools such as the PSNT. In most cases, reducing residual soil nitrate to 50 pounds nitrate-N/acre in the surface foot is a reasonable goal.
• Field-scale replicated trials resulted in good data that was believable to participating growers. For future trials, three replications are sufficient.

Table 3. Effect of N at planting on soil nitrate
and ammonium concentration and sweet corn leaf N
concentration five weeks after planting, NWREC, 1995 
N at planting   Soil nitrate-N  Soil ammonium-N     
(lb/acre)             (ppm)          (ppm)          
 40                   12.3            4.4
 80                   21.8            7.9
120                   30.0           13.4
160                   36.0           17.4
Significance           **              *            



Table 4. Effect of N at planting and sidedressed N on 
dry weight, total N concentration, nitrate content, and 
SPAD readings of sweet corn leaves, NWREC, 7 July, 1995                
                                              Leaf              
N at planting  Sidedress    Dry wt.  Total N   N content   SPAD   
(lb/A)          N (lb/A)      (g)      (%)       (g)     reading
 40                 0        11.0      3.04      0.334     40.5   
 40                40        12.2      3.53      0.428     43.2   
 40                80        13.3      3.58      0.475     48.0   
 80                 0        13.4      3.30      0.442     46.7   
 80                40        13.0      3.53      0.459     48.2   
 80                80        12.9      3.71      0.480     46.6   
120                 0        15.1      3.38      0.512     47.0   
120                40        13.3      3.51      0.466     47.8   
120                80        14.0      3.75      0.526     48.4   
160                 0        14.4      3.54      0.509     50.1   
160                40        13.1      3.76      0.492     49.1   
160                80        14.1      3.74      0.526     50.7   
   Significance
       N at planting          NS         *          *       **   
       Sidedress N            NS        **         NS       **   
       Planting x sidedress   NS        NS         NS       **   
       Interaction LSD                                      2.6



Table 5. Effect of N at planting on soil nitrate and ammonium 
levels and leaf chlorophyll (SPAD) readings and N content at 
time of PSNT, NWREC, 1996                                            
N at planting    Soil nitrate-N    Soil ammonium-N      SPAD   Leaf N 
(lb/acre)             (ppm)              (ppm)                   (%)   
  0                   7.1                 2.4           36.8     2.9
 40                  14.3                 4.7           39.9     3.2   
 80                  23.1                 8.2           41.0     3.5   
120                  26.4                 7.4           42.2     3.5   
  Significance        **                   **            **       *  



Table 6. Effect of N at planting and sidedressed N on yield 
and quality parameters of sweet corn, NWREC, 1995                      
N at       Side-     Ear    Ear     No.     Ear    Tipfillx   
plantingz  dress Ny  yield   wt.  ears per  length   
(lb/A)    (lb/A)   (tons/A) (g)    plot   (inches)         
 40          0       4.3    216     42      8.3      2.6    
 40         40       6.3    235     56      8.8      3.0   
 40         80       9.3    306     63      9.1      3.8   
 80          0       7.5    266     55      8.6      3.4   
 80         40       9.1    287     66      8.9      3.5   
 80         80       9.2    301     64      9.2      3.7   
120          0       8.6    276     65      8.7      3.6   
120         40       8.7    293     62      8.8      3.6   
120         80       9.7    297     68      9.0      3.4   
160          0       9.3    311     63      9.0      3.8   
160         40       9.3    304     64      9.3      3.5   
160         80      10.6    309     71      8.9      3.9   
Significancew
  N at planting      **      **      *        *
  N sidedress        **      **     **       **
  Plant x sidedress   *       *     NS       **
  Interaction LSD    1.6     35             0.3            
z40 lb/acre broadcast as 10-20-20 one day before 
 planting. Remainder broadcast as urea one day after planting.
yBroadcast as urea five weeks after planting.
xFive-point scale with 5=perfect fill, 1=2 inches 
 unfilled kernels.
wNS,*,**: not significant and significant at 5% and 
 1% levels, respectively.



Table 7. Correlation coefficients (Pearson's pairwise) for leaf 
dry weight, SPAD readings, leaf N concentration, and leaf N 
content, NWREC corn, 1995                                          
Variable      By variable     Correlation coefficient   Probability   
Leaf dry wt.  N concentration         0.1927               0.367
SPAD          N concentration         0.5981               0.002
SPAD          Leaf dry wt.            0.3942               0.057
N content     N concentration         0.6217               0.001
N content     Leaf dry wt.            0.8866               0.000
N content     SPAD                    0.5836               0.003
Yield         SPAD                    0.8528               0.000
Mean ear wt.  SPAD                    0.7839               0.000       



Table 8. Main effects of N at planting and sidedressed N on  
residual soil nitrate- and ammonium-N concentrations following  
sweet corn, NWREC, 1995                                            
N at planting       Nitrate     Nitrate     Ammonium     Ammonium
(lb/acre)           0-1 foot    1-2 foot    0-1 foot     1-2 foot 
                    -------------------ppm-N---------------------  
 40                   3.0          1.6         4.8          4.5
 80                   5.1          2.1         5.0          4.0
120                   9.6          3.6         7.9          4.3
160                  14.8          5.0         5.6          4.2
      Significancez   **            *           NS           NS
Sidedressed N
(lb/acre)
  0                   3.9          1.6         4.4          3.9
 40                   7.3          3.2         6.4          4.3
 80                  13.2          4.4         6.8          4.1
      Significance    **            *           NS           NS  
zNS,*,**:  not significant and significant at 5% and 1% 
 levels, respectively. 



Table 9. Effect of total N applied on corn yield and stalk nitrate-N 
content, NWREC, 1995                                                   
Total N Applied, lb/A      Yield, tons/A     Stalk nitrate-N, ppm     
 40                             4.3                      30
 80                             6.9                     271
120                             9.0                    1107
160                             9.1                    4310
200                             9.5                    7146
240                            10.6                    9203
                LSD (0.05)      1.6                    1319         



Table 10. Effect of N at planting and mid-season sidedressed N on 
yield and quality parameters of sweet corn, NWREC, 1996                
N at     Mid-season    Yield     No.     Ear   Ear    Tipfillz
planting     N                   ears/   wt.  length
(lb/A)    (lb/acre)   (tons/A)   plot    (g) (inches)            
  0          0          2.7       55     102   7.1      1.5
  0         40          7.0       81     181   7.7      2.1
  0         80          8.7       83     218   8.1      2.5
 40          0          6.3       78     169   8.2      2.2
 40         40          8.1       86     196   8.2      2.2
 40         80         10.1       90     236   8.3      2.5
 80          0          8.5       87     203   7.9      1.9
 80         40         10.7       94     238   8.1      2.3
 80         80          9.1       86     225   8.3      2.4
120          0          9.0       87     217   8.3      2.7
120         40         10.2       89     240   8.4      2.6
120         80          9.4       90     218   8.4      2.7
           LSD (0.05)   1.6       10      28   0.6               
zBased on 5-point scale with 5 = perfect fill, 1= 2 inches 
 unfilled kernels.
  


Table 11. Effect of N at planting and mid-season N on leaf chlorophyll 
(SPAD) readings three weeks after second N application and residual 
soil nitrate and ammonium content, NWREC, 1996                         
N at planting   Mid-season N   Late SPAD   Soil nitrate  Soil ammonium
(lb/acre)         (lb/acre)                    (ppm)         (ppm)     
  0                   0           31.3          1.8           4.9
  0                  40           40.5          1.6           4.2
  0                  80           46.2          5.0           5.1
 40                   0           41.3          1.4           4.3
 40                  40           45.6          2.7           4.2
 40                  80           48.2          5.4           4.7
 80                   0           46.6          2.5           5.7
 80                  40           47.6          5.0           5.6
 80                  80           46.2          4.0           5.2
120                   0           46.4          3.3           7.1
120                  40           49.0          2.6           7.2
120                  80           50.3         10.3           6.7
                     LSD (0.05)    3.9          3.1           0.7     



Table 12. Correlation coefficients (Pearson's pairwise) for SPAD 
readings, leaf N concentration, and sweet corn yield, NWREC, 1996      
Variable    By variable   Correlation coefficient     Probability
PSNT-SPAD      Leaf N              0.7649                0.0007
PSNT-SPAD      Yield               0.4592                0.0006
PSNT-SPAD      Tipfill             0.1534                0.2777
PSNT-SPAD      Ear wt.             0.3980                0.0035
Late-SPAD      Yield               0.8818                0.0000
Late-SPAD      Tipfill             0.6477                0.0000
Late-SPAD      Ear wt.             0.8863                0.0000  



Table 13. Sweet corn yield and stalk nitrate concentrations at 
harvest, grower cooperator sites, 1995                              
Site   PSNTz       Stalk nitrate           Yield         Relative  
       (ppm)           (ppm)               (T/A)           yield
    Nitrate-N  No added N  Added N   No added N  Added N    (%)     
 1      11          42       4156        7.7       9.9     78.2 
 2      11         498        756        7.8       7.4    104.8
 3      12        2262       7261        9.1       9.7     93.4
 4      12        1925       2806       10.1      11.6     87.2
 5      17        2297        814        8.4       8.0    105.1
 6      17         657       3943       12.2      13.1     93.0
 7      17         120       3219        8.1       9.9     82.3
 8      19         892       1871        9.5       9.3    101.8
 9      20        1248       3376        9.4      10.3     91.2
10      22        2379       4826       11.7      10.9    107.8
11      22        1473       3242        9.5      13.7     69.3
12      23         893       2312        9.3       9.4     98.6
13      24         447       5793        9.9      11.1     89.0
14      29        6224      10573        8.7       9.5     91.2
15      29        4489       6290        9.1       9.2     98.8
16      47        9044      10309       10.4      11.2     93.5    
zSoil nitrate-N concentration at time of sidedressing.             

    


Table 14. Sweet corn yield at two N ratesz, 
grower-cooperator sites, 1996                                          
Site   PSNT, ppm    Yield (T/A)   Relative yield  
       Nitrate-N   Low N  High N      (%)       
 1        8         9.4    10.1       93.5        
 2       11         9.7    10.1       95.7    
 3       12        12.5    12.5      100.0    
 4       15        10.2    10.4       98.5    
 5       18         9.7    10.6       91.3   
 6       21         9.7     9.8       99.0   
 7       21         8.7     8.9       98.0   
 8       22        10.7    10.9       98.7   
 9       24         9.6     8.7      110.6   
10       26         393     385y     102.2   
11       26        12.4    12.1      102.4   
12       31         8.2     9.0       92.2         
zLow N rate = 100 lb/A after PSNT.  
 High N = 150 lb/A after PSNT.
yFresh market grower.  Yield in cases/A.



Table 15. Residual soil nitrate and cornstalk nitrate at two Nz
rates, grower-cooperator sites, 1996                            
Site  Residual soil Nitrate-N (lb/A)   Cornstalk Nitrate-N (ppm)
          Low N      High N                Low N    High N 
 1           12        20                   2219      5761          
 2           16        26                   5358      6124
 3           91       113                   8131      7318
 4           43        37                   9219      8860
 5          110       296                   3584      2918 
 6           75       123                   9385      9089
 7           17        21                   9586     10575
 8           --        --                   8364     11288
 9           96       149                   6975      7950
10          142       242                   6286      5461
11           92       107                   2467      3630
12           64        46                   9492      7013      

Introduction

Although we have established that cauliflower is more efficient that sweet corn or snapbeans in taking up fertilizer N, the large amounts of N applied to this crop (up to 300 pounds/acre) make it a candidate for improvements in N use efficiency. As with sweet corn, a predictive test of the need for late-season N applications could enable growers to cut back on wasteful and environmentally sensitive overapplication of N fertilizer.

Methods

In plots at NWREC, 'Snowball Y Improved' cauliflower was seeded on 8 June in a greenhouse. Plugs were transplanted on 12 July to a Willamette silt loam, pH 6.0. Four levels of soil N were established on 17 July by applying either 40, 80, 120, or 160 pounds N/acre, as urea, just after transplanting. Transplants were set in three-row plots with 30 inches between rows and 18 inches between plants in the row. On 21 August, the plots were sidedressed with either 0, 60, or 120 pounds N/acre.

Results

Although repeated applications of Lorsban and diazinon were made, root maggot damage and loss of stand were severe. Plots were harvested only once, on 4 October. Head size was below normal, attributed to maggot-damaged root systems. Head weight and yield did not vary significantly with the amount of N applied at planting (when averaged over sidedressed N), but both tended to decline with increasing rate of N, indicating that excessive N at planting may have stunted the plants or delayed maturity (Table 16). Both yield and head weight tended to increase with the greater rates of sidedressed N. Approximately equal head sizes were obtained with many different combinations of N at planting and sidedressed N, indicating that a predictive test might be useful in determining the amount of sidedress N to apply in cauliflower fields.

Replicated PSNT trials were also carried out with in two commercial cauliflower trials. In the first trial, the PSNT soil test was 25 ppm nitrate-N and cauliflower midribs contained 12,400 ppm nitrate-N at time of intended sidedress. Addition of the full amount of sidedress N planned by the grower resulted in significantly increased yields at both the first and second harvests. In the second trial, the PSNT value was 35 ppm and the cauliflower midribs contained 16,800 ppm nitrate. Addition of sidedressed N produced an insignificant increase in yield at the first harvest but a decrease in yields at both the second and third harvests, indicating that the soil and plant nitrate levels were sufficient at time of the intended sidedressing (Figure 7). These results again point to the possible value of a predictive soil or tissue nitrate test in cauliflower. However, the multiple harvests necessary with cauliflower and the lack of uniformity of maturity of most varieties makes it unlikely that a useful PSNT can be developed for cauliflower. The rate of applied N also affects maturation in cauliflower, further complicating the picture.

Table 16. Main effects of rate of N at planting and sidedressed   
N on yield and head size of cauliflower, NWREC, 1995           
N rate (lb/A)       Mean head wt. (g)     Total yield (T/A)    
At planting
 40                     673                    9.2  
 80                     777                    9.0  
120                     705                    7.3  
160                     702                    7.0
                         NS                     NS
Sidedressed
  0                     650                    6.5
 60                     788                    8.3
120                     763                    9.5
                         NS                     NS                    

Cooperator: Dr. John Hart, Dept. of Crop and Soil Science

Introduction

Vegetable growers in the Willamette Valley use high rates of N fertilizers, often exceeding 250 pounds actual N/ acre per season. While growers believe that these rates are necessary to achieve maximum yields and quality, a considerable portion of the applied fertilizer is not taken up by the crop. This has raised concerns that the remaining N may be contributing to nitrate pollution of groundwater. Our research has shown that broccoli needs 250 pounds N/acre for maximum yield. At this N rate very little nitrate-N remained in the root zone. The same was true of cauliflower. However, when sweet corn was fertilized for maximum yield, significant quantities of applied N were not taken up by the crop.

A survey of grower fields was instituted in 1993, where 30 fields were sampled for nitrate and ammonium-N concentrations before spring fertilization and were then cropped to beans, beets, broccoli, carrots, cauliflower, and sweet corn. At the end of the growing season, the same fields were tested for residual nitrate and ammonium concentration. Residual levels were correlated with amounts of N applied and other grower cultural practices. This survey was repeated in 1994 and 1995 on 34 fields. To provide a basis of comparison with the grower fields, crop yield and residual mineral N were measured as a function of applied N for beans, beets, carrots, and sweet corn at NWREC. The sweet corn trials involved other factors such as timing of N application and results are presented elsewhere. The bean, beet, and carrot results are presented here.

Methods

All crops were seeded to a Willamette silt loam, pH 6.1, at NWREC. Plot preparation for all crops included a broadcast and incorporated application of potassium sulfate at 250 pounds/acre and triple superphosphate at 150 pounds/acre, disking and cultimulching. Pre-plant soil samples were obtained to 4-foot depth, in 1-foot increments, on 27 April. The samples were submitted to the OSU Central Analytical Lab for analysis of nitrate and ammonium content. The N source for all trials was urea. Post-harvest soil samples to 2-foot depth were obtained on 21 September. Irrigation of all crops ceased on the harvest date to minimize nitrate leaching and further N uptake by the crop.

Beans

The plot area received a broadcast, incorporated application of 0.75 pounds trifluralin, 2.0 pounds EPTC, and 1.2 pounds chlorpyrifos/acre. 'Oregon 91G' snapbeans were seeded at 65 pounds/acre on 15 May with three rows per plot on 20-inch centers. The first 40 pounds N/acre were broadcast on the planting date; the remaining N was broadcast on 16 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 25 July.

Beets and Carrots

The plot area received a broadcast, incorporated application of EPTC at 2.0 pounds/acre (beets). Carrot plots were treated with linuron at 1.25 pound/acre one day after seeding. 'Detroit Dark Red' table beet and 'Six Pac' carrot were seeded on 15 May with three rows on a 5-foot bed. The first 40 pounds N/acre were broadcast on the planting date; the remaining N was broadcast on 10 June. Plots were sprinkler-irrigated and cultivated as necessary and harvested on 8 August (beets) and 13 September (carrots).

Results and Discussion

Bean yield was highest at 80 pounds N/acre (Table 17). As in 1994, carrot yield was highest at 120 pounds N/acre (Table 17). Carrot yields were lower in 1995 than in 1994, reflecting, in part, siting the experiment on Willamette rather than Latourell soil. Also as in 1994, beet yield increased markedly with the first increment of applied N and continued to increase in quadratic fashion with the maximum at 240 pounds N/acre (Table 18). This is also consistent with previous research at NWREC and with grower experience. Table beet yield response to N rate was nearly identical in 1994 and 1995.

Nitrate concentration in the surface foot of soil after snapbean harvest increased dramatically between the rates of 80 and 120 pounds applied N/acre (Table 19). Soil ammonium content also increased with increasing applied N for the surface foot of soil. The largest increase came between the rates of 120 and 160 pounds applied N/acre (Table 19). Rate of applied N did not significantly affect soil nitrate or ammonium content at the 1-2 foot depth.

Soil nitrate concentration in the surface foot of soil increased with increasing rate of applied N for both beet and carrot (Tables 20 and 21). In the second foot of soil, the increase was significant only for carrots. Residual mineral N following beet was low compared to most crops. For carrots, there was not much increase in soil nitrate content in the surface foot of soil until the applied N reached the optimal level for yield (Table 21). This is consistent with 1994 results. Soil ammonium content appeared to increase at both depths following the higher rates of applied N for table beets. However, the increases were not statistically significant. Soil ammonium content in the surface foot of soil increased slightly, but significantly, with the highest rate of N applied to carrots.

Table 17. Effect of rate of urea-nitrogen on the  
yield of green beans and carrots, NWREC, 1995   
N rate           Bean        Carrot         
(lb/A)           (T/A)       (T/A)                    
  0               2.9         10.9 
 40               3.9         14.7
 80               7.3         15.9
120               6.1         19.6
160               6.8         18.0     
   Significance   L*Q*        L*Q*               
L=linear, Q=quadratic, *significant, p=0.05.



Table 18. Effect of rate of urea-nitrogen  
on the yield of table beets, NWREC, 1995 
N rate                   Yield
(lb/A)                   (T/A)           
  0                       3.6
 60                      14.1
120                      14.8
180                      17.5
240                      19.8
      Significance      L**Q*            



Table 19. Effect of rate of broadcast N on soil nitrate and 
ammonium concentrations (ppm N) at two depths following bean 
harvest, NWREC, 1995                                               
Sample depth                       N rate, lb/A           
(inches)                    0     40    80   120    160  LSD(0.05)
               Pre-plant   ---------Post-harvest--------
Nitrate
 0-12             0.2      5.5   7.3   7.5   17.1   13.8    6.1
12-24             0.2      1.4   1.9   2.0    2.0    3.4    NSD
Ammonium
 0-12             3.6      3.1   3.1   3.8    5.6    8.4    1.7
12-24             3.1      3.2   3.1   3.3    3.6    4.6    NSD   



Table 20. Effect of rate of broadcast N on soil nitrate and 
ammonium concentrations (ppm N) at two depths following beet 
harvest, NWREC, 1994                                               
Sample depth                       N rate, lb/A        
(inches)                    0     40    80   120    160  LSD(0.05)
               Pre-plant   ---------Post-harvest--------
Nitrate
 0-12             0.2      0.9   1.3   4.6    6.6   7.7     4.2   
12-24             0.2      0.5   1.1   0.8    1.0   2.6     NSD   
Ammonium
 0-12             3.6      2.9   3.3   5.8   10.2  18.2     NSD 
12-24             3.1      3.2   2.8   3.5    5.0   8.2     NSD  
  


Table 21. Effect of rate of broadcast N on soil nitrate and 
ammonium concentrations (ppm N) at two depths following 
carrot harvest, NWREC, 1995                                       
Sample depth                       N rate, lb/A         
(inches)                     0     40    80   120   160  LSD(0.05)  
               Pre-plant   ---------Post-harvest--------
Nitrate
 0-12              0.2     1.0   1.1   4.7  14.5   21.0     4.0   
12-24              0.2     0.2   1.5   1.1   2.7    3.3     1.8   
Ammonium
 0-12              3.6     3.5   3.1   3.6   3.4    5.8     1.3 
12-24              3.1     2.8   3.5   2.8   3.0    3.2     NSD  

Introduction

This grower trial, initiated in 1993, was undertaken to determine whether residual nitrate and ammonium levels in grower fields following vegetable crops were similar to those found in our experiments at NWREC. The data should be useful in indicating which of the major processed vegetable crops leave significant quantities of residual mineral N in the soil at harvest. This trial should also show the extent to which grower cultural practices, particularly fertilizer application, influence the amount of residual N that is available for leaching by heavy winter rainfall. Results from 1993 and 1994 may be found in: Vegetable Research at the North Willamette Research and Extension Center, 1993-1994, Oregon Agricultural Experiment Station Special Report 944, April, 1995. Results of the 1995 survey and a 3-year summary are included below.

Methods

Soil samples were taken to a depth of 5 feet both before and after crops of snapbeans, beets, broccoli, carrots, cauliflower, and sweet corn, for determination of mineral N (ammonium-N and nitrate-N) content. Thirty-four fields were again sampled, representing 12 growers in Marion and Lane counties and 9 soil types. The growers were interviewed to determine field history and cropping and fertilization intentions and were asked to keep records of fertilizer applications. Samples were taken from 4 beet, 4 carrot, 6 broccoli, 5 cauliflower, 8 snapbean, and 7 sweet corn fields.

In order to preserve anonymity, only average soil test values are presented in this report. Pre-plant nitrate concentrations in the surface foot of soil were nearly identical to those found in 1993 and 1994, averaging 5.2, 5.1, 2.8, and 2.9 ppm nitrate-N at the 1-, 2-, 3-, and 4-foot depths, respectively. Ammonium levels were slightly higher than in previous years, averaging 6.5, 4.6, 4.1, and 3.9 ppm, respectively. As in 1993 and 1994, most cases of higher levels of nitrate or ammonium could be explained by a past history of manure application or by the presence of a legume cover crop.

Results

Not surprisingly, much greater differences among fields existed at harvest than at planting. Average nitrate and ammonium levels at harvest vary both with crop and with grower cultural practices (data not shown). For example, in top foot of the soil, nitrate concentrations were greater for sweet corn than for the other crops (Figure 8).

The three-year summary of data is found in Table 22. Notable findings include the following:

• The range of N applications by growers is greatest for snapbeans, when the range is expressed as a percentage of the mean of all grower applications. Growers are in much closer agreement on the proper rate of N application to sweet corn.
• The largest amounts of N are used on broccoli and cauliflower, which are relatively efficient in utilization of applied N.
• When expressed in terms of pounds residual N/acre, sweet corn leaves behind the most N, carrots the least.
• When expressed as a percentage of the N applied by the grower, snapbeans have the highest residual N, carrots the lowest.

Table 22.  Residual nitrate-N in the surface foot of grower 
fields following vegetable crop harvest, 1993-1995                     
Crop     Residual     Residual       Range of N    Mean N   Range/Mean
         nitrate-N    nitrate-N        applied     applied
          (lb/A)    as % of applied    (lb/A)      (lb/A)       %     
Bean        51           48             55-157      107        95
Beet        24           11            155-278      214        57
Broccoli    31           12            199-328      259        50
Carrot      16            9            130-220      171        53
Cauliflower 33           15            184-277      225        41
Sweet corn  70           36            159-228      195        35     

Cooperator: Dr. N.S. Mansour, Dept. of Horticulture

Introduction

Phosphorus pollution of the Willamette River and its tributaries is a problem affecting agriculture in western Oregon. Many of our soils are high in available (soluble) P, as measured by the tests commonly used to determine plant-availability of P. Consequently, many streams have background levels of P that are conducive to algal blooms and poor water quality for fisheries and recreational use. Agriculture may also contribute to P pollution of streams through the use of large amounts of phosphate fertilizers and is under pressure to reduce P applications.

Growers of vegetables and certain other crops tend to use high levels of P fertilizer even on soils that test very high for available P, especially in early-season plantings. This use is supported by past research at OSU which indicates that stand establishment and early growth of seedlings is stimulated by banded applications of phosphate on soils which do not benefit from broadcast applications of P. Planting typically takes place when soils are cold and wet and P uptake is limited. Growers and water quality would both benefit greatly if a product were available which would provide soluble, mobile P but at rates greatly reduced from those commonly used in vegetable production. Biagro Western Sales, Inc. distributes a line of "Nutri-Phite" fertilizers based on phosphite rather than phosphate sources of P. The company has data indicating that these products provide a more plant-available source of P.

The purpose of this research was to evaluate the response of bulb onions and broccoli to a Nutri-Phite P fertilizer on a soil which tests very high in available P but where banded phosphate fertilizers have resulted in crop responses and are routinely applied beneath the seed row of vegetable crops. This was a challenging test of the ability of the Nutri-Phite P Foliar (4-30-8) formulation to enhance yield in a situation where soil P levels were high, banded phosphate was applied in combination with the Nutri-Phite P, and the crops have a waxy cuticle resistant to foliar uptake of nutrients.

Methods

Onions. The test site was a Willamette silt loam typical of soils commonly used for vegetable production in the area and tested high for both available P and K (Table 23). 'New York Early' onion was seeded on 29 May, following plowing, disking, and cultimulching to form a seedbed. The stand was thinned three weeks after emergence. No pre-plant pesticides were used. Each plots size was 10 x 20 feet, with four rows/plot. All rows were treated but yield data were taken from the centermost 10 feet of the two center rows of each plot. Treatments were replicated four times in completely random design. Weeds were controlled by three applications of oxyfluorfen at 0.1 pound/acre and by hand-hoeing. Nitrogen applications included 50 pounds/acre broadcast just after emergence (3 June) and another 50 pounds/acre broadcast on 9 July. All N was in the form of ammonium nitrate. Irrigation was by ovehead sprinkler, as needed.

Treatments are presented in Tables 24 and 25. The check treatment received no P. The banded P treatment received concentrated superphosphate (0-45-0) at a rate of 90 pounds/acre, banded two inches beneath and to the side of the seedline. The Nutri-Phite applications were in addition to the 90 pounds phosphate-P/acre and included DyneAmic spray adjuvant at 1 ml/liter. The initial application of Nutri-Phite P was on 19 July, 50 days post-emergence, with a second and third application on 5 and 19 August, respectively. Applications were made approximately two feet over the plot rows with a two-nozzle boom attached to a backpack sprayer operated at 40 psi pressure. Total amounts of phosphite-P applied in the Nutri-Phite treatments were 1.13 and 2.27pounds/acre for the 2 and 4 pint rates, respectively. Corresponding rates of K applied were 0.58 and 1.16 pounds/acre, respectively.

Onions were lifted on 10 October, removed to a greenhouse bench on 16 October, topped on 23 October, and weighed and graded on 25 October. Bulbs were graded as large (over 2-inch diameter) or boiler size. Leaf samples were collected for P and K analysis on 10 October and submitted to the Central Analytical Laboratory, Oregon State University.

Broccoli. Methods were as above, for onions, except as follows. 'Gem' broccoli was direct-seeded on 16 July following a broadcast, incorporated application of trifluralin herbicide at 0.75 pound/acre and chlorpyrifos insecticide at 1.3 pounds/acre. Nitrogen, as ammonium nitrate, was applied at 50 pounds/acre just after emergence, 100 pounds/acre on 22 August, and 100 pounds/acre on 19 September. Seedlings were thinned starting on 23 July and some hand-hoeing was necessary for good weed control. Two applications of diazinon (1.0 pound/acre) were made for control of cabbage root maggot. Nutri-Phite applications were made on 19 August (30 days after emergence), and 3 and 17 September. Maturity was concentrated and a single harvest was made on 14 October from the center two rows. Leaf samples were collected at the same time.

Results

Given the combination of high soil test for P and K, the application of Nutri-Phite P Foliar in combination with banded phosphate, the low rates of phosphite-P and K applied, and the resistance of the crops to foliar uptake of nutrients, it is not suprising that there was little broccoli or onion response to the Nutri-Phite phosphite-P fertilizer. Visual observations of the plots did not indicate improved plant growth in response to either the banded phosphate treatment or to Nutri-Phite.

The number and weight of large onion bulbs appeared to increase with application of P in any form (Table 24) as did the total weight harvested. This would be in agreement with the long history of response of vegetable crops to banded P applications. However, as can be seen from the probability levels for statistical significance given in Table 24, these differences were not near the normally accepted probability level of 0.05 for statistical significance. Onion leaf P and K concentrations also failed to respond to treatment. Broccoli yields showed no trend in reponse to P application (Table 25). Given the late planting date on warm soil, the lack of response is not surprising. Broccoli leaf P levels appeared to increase with application of Nutri-Phite but the results were not significant at p=0.05.

Table 23. Nutrient levels in Willamette silt loam soil, 
NWREC, on May 29, 1996                                    
pH    P      K       Ca          Mg       NH4-N    NO3-N
    (ppm)  (ppm)  (meq/100g)  (meq/100g)  (ppm)    (ppm)
5.7  228    367      5.5        0.80       23.1     5.3     



Table 24. Yield and nutrient content of onion as affected 
by banded P fertilizer and Nutri-Phite P Foliar, NWREC, 1996           
Treatment        Large bulbs         Total bulbs     Leaf  Leaf
             No./   kg/    bulb  No./   kg/    bulb   P     K
             plot   plot  wt.(g) plot   plot  wt.(g) (%)   (%)  
Check         8.5   0.86   101    133   3.36    25   0.42  2.17  
Banded P     17.0   1.75   103    146   5.13    35   0.40  2.30 
Nutri, 2 pt  15.0   1.30    87    149   5.43    36   0.42  2.25 
Nutri, 4 pt  20.5   2.24   109    132   5.27    40   0.46  2.31  
  P level    0.65   0.64   0.60  0.70   0.50   0.47  0.17  0.56 



Table 25. Yield and nutrient content of broccoli as affected  
by banded P fertilizer and Nutriphite-P Foliar, NWREC, 1996  
Treatment              kg/plot      Leaf P (%)     Leaf K (%)      
Check                    4.5           0.35           2.36  
Banded P, 90 lb/A        3.8           0.36           2.46
Nutri-Phite, 2 pt/A      5.1           0.38           2.45
Nutri-Phite, 4 pt/A      4.0           0.38           2.38
  Probability level      0.93          0.15           0.69   

Introduction

Nitrate pollution of groundwater from the application of high rates of N fertilizers to vegetable crops in a concern in the Willamette Valley. Excess N not taken up by the crop remains in the soil and can be leached to groundwater during the wet winter months. These concerns led us to initiate in 1990 a study of the cycling and availability of N in vegetable cropping systems. These are the sixth and seventh years of a study in which winter cover or "catch" crops have been seeded following vegetable crops and in which the N uptake of the cover crop and its contribution to a succeeding vegetable crop has been measured in comparison to a winter-fallow control. In 1994, sweet corn was grown on these long-term rotation plots at NWREC and fertilized at three rates of N. Following harvest the plots were seeded to cereal rye or a mixture of cereal rye and Austrian winter pea. In 1995, broccoli was grown on these plots at three rates of N to determine the cover crop contribution to broccoli yield and N uptake. Following harvest, the plots were again disked, harrowed, and seeded (drilled) to triticale or a mixture of triticale and Austrian winter pea. In addition, other plots in both years were overseeded (relay intercropped) to cereal rye, triticale, or red clover about one month after sweet corn or broccoli emergence. These cover crops were permitted to grow through the winter. In 1996, sweet corn was again grown on the plots and fertilized with three rates of applied N.

In autumn 1993, passive capillary wick samplers were installed beneath the winter-fallowed plots and fall-planted cereal rye (later triticale) plots. All three N rates were also represented. The samplers have allowed us to collect leachate on a continuous basis and determine both the nitrate concentration of the leachate as well as the total nitrate loss on an area basis.

Methods

The overseeded cover crops of 'Kenland' red clover and 'Wheeler' cereal rye were broadcast on 7 July, 1994, and 11 July, 1995, into four plots each of the standing sweet corn (1994) or broccoli (1995) crops. The direct-seeded cover crops were seeded on 7 October, 1994 and 6 October, 1995, after disking and harrowing to form a seedbed. These plots had been cropped to sweet corn in 1990, 1992, and 1994, and broccoli in 1991 and1993, with N rates of 0, 50, and 200 pounds/acre for sweet corn and 0, 125, and 250 pounds/acre for broccoli. Four plots (30 x 60 feet) were planted to 'Wheeler' cereal rye (1994) or 'Celia' triticale (1995) at 65 pounds/acre. The other four plots were planted to a mixture of rye (1994) or triticale (1995) at 35 pounds/acre and Austrian winter pea at 100 pounds/acre. No fertilizers were applied to the cover crops. Nitrogen rate subplots of 600 square feet each were determined by the N applied to the previous vegetable crop. The identity of the three N rate subplot treatments was maintained from year to year.

On 1 April of both years, samples were taken from all subplots to determine shoot dry weight and N uptake. The shoots were clipped about one inch above ground. All cover crops were mowed down and disked on 15 April, 1995 and 17 April 1996. The plots were plowed, disked, and harrowed in early May.

On 5 June, 1995, 1.3 pounds chlorpyrifos and 2.0 pounds B (as Solubor)/acre were applied to the plots which had been overseeded. The plots which had been in drilled cereal or cereal plus pea or were fallowed were treated with the above plus trifluralin at 0.75 pounds/acre. The pesticides were rototilled into the surface three inches of soil. 'Gem' broccoli was seeded in 20-inch rows on 7 June with three rows/bed.

On 16 June, 1995, N applied as urea was broadcast at rates of one-half the total N rates of 0, 125, and 250 pounds/acre. On 28 June, the broccoli was thinned to a stand of 10 inches between plants in the row. The remainder of the urea was sidedressed on 11 July, when the appropriate treatments were again overseeded with triticale or clover, using a whirly-bird fertilizer spreader. The seed was scratched in with a garden rake.

Plots were tractor-cultivated on 13 July and hoed as needed later in July. Plots were harvested on 31 August from 15 feet of two inner rows of each subplot. Following harvest, the appropriate plots were again planted to cover crops.

On 30 May, 1996, 'Jubilee' sweet corn was seeded in 30-inch rows. Phosphorus was banded at 60 pounds P₂O₅/acre two inches to the side and two inches beneath the seed row. Plots overseeded to rye or clover were treated with EPTC at 3.0 pounds/acre, which was incorporated before planting. All other plots received a broadcast application of 2.0 pounds atrazine and 3.0 pounds alachlor/acre immediately after planting.

On 6 June, 1996, N was broadcast as urea at rates of one-half the total N rates of 0, 50, and 200 pounds/acre. All N rate subplots were in the same location as the corresponding N treatments on the previous vegetable crops. The remainder of the urea was sidedressed on 15 July, when the EPTC-treated plots were again overseeded with rye or clover, using a whirly-bird fertilizer spreader. EPTC-treated plots were hand-hoed as necessary before overseeding. Harvest on 5 September was from 15 feet each of the two innermost rows of each subplot. Following harvest, the appropriate plots were again prepared for planting of cover crops.

Details of the installation, maintenance, and use of the passive capillary wick samplers can be found in Brandi-Dohrn, F. et al. 1997. Nitrate leaching under a cereal rye cover crop. J. of Environ. Qual. 26:181-188.

Results

Cover Crop Yield and N Uptake, 1995 and 1996

Biomass accumulation of cover crops was low in both years compared to our previous experience on these crops, attributable to loss of stand caused by flooding in both years and unusually poor survival of the Austrian winter peas in 1995-1996.

In 1995, the largest biomass accumulation was with the autumn-drilled rye/pea mixture in plots previously fertilized with 200 pounds (224 kg/ha) N/acre (Figure 10). The biomass of both the drilled rye and rye/pea mixture tended to increase with increasing rate of N applied to the 1994 sweet corn crop on these plots. Overseeded (relay) rye did not respond in this fashion and the yield of overseeded clover tended to be reduced at the high rate of N. Nitrogen uptake followed the same trends (Figure 11). However, the N uptake of the rye/pea mixture on plots previously fertilized with 200 pounds N/acre was disproportionately high, presumably due to N fixation by the peas.

Nitrogen Rate and Cover Crop on Broccoli Yield, 1995

Stands were greatly reduced by root maggot damage. Yield and mean head weight did not vary significantly by previous cover crop, but head weight responded normally to increasing rate of applied N, with greatest size at 250 pounds N/acre (Table 26). The largest mean head weight was with the combination of cereal rye cover crop and 250 pounds N/acre.

Nitrogen Rate and Cover Crop on Sweet Corn Yield, 1996

Yield tended to decline following a triticale cover crop, regardless of N rate (Table 27). The overseeded (relay) clover cover crop tended to increase yield but the effect was not significant. Yield with drilled triticale or drilled triticale plus winter peas was essentially equal due to poor growth and survival of the peas. Initial pea stand was adequate but the stand was lost during the winter. Yield increased normally with increasing rate of N. The highest-yielding treatment combination was the high rate of applied N following fallow, with 11.8 tons/acre (Figure 14). However, at the suboptimal N rate of 50 pounds applied N/acre, yield following overseeded clover was 9.4 tons/acre compared to 8.2 tons/acre following fallow (Figure 14). Corn yield was also greater following clover than following fallow when no N was applied. These results indicate that a legume cover crop can contribute significant N to the following vegetable crop. This N contribution is consistent with previous results from these plots.

Cover Crops on Nitrate Leaching

The nitrate-N concentration in leachate from the high-N subplots had been reduced by an average of 50 percent by the cereal rye cover crop in the winters of 1992-93 and 1993-94. The reduced nitrate concentrations in leachate coming from covered plots was very consistent throughout the fall, winter, and spring. In the winter of 1994-95, nitrate leaching was reduced under a cover crop during the first part of the winter season, but the trend was reversed in February through May (Figure 15). The reversal of trend suggests a combined piston-preferential water flow through the soil during this period. However, the previously established trend held through the winter of 1995-96 (data not shown). For the entire 4-year period, the presence of a cover crop reduced cumulative nitrate leaching loss by 45 percent at the highest rate of applied N. The range of the reduction was from 33 to 63 percent, with the greatest reduction in the winter of 1993-94. The mean nitrate concentration of the leachate was reduced by 40 percent under a cereal cover crop over the same period, with a range of 22 to 58 percent.

Summary

Consistent with past results, winter cover crops reduced leaching of nitrate from the root zone. Leguminous cover crops made N available to the following vegetable crop. A cover crop consisting only of a winter grain tended to depress yield of the following crop.

Table 26. Main effects of preceding cover crop and rate   
of applied N on yield of broccoli, NWREC, 1995         
Treatment               Yield         Mean head        
                      (kg/plot)        wt. (g)         
Cover crop
Fallow                   1.1             132     
Cereal rye               1.0             145     
Rye + pea                1.3             130     
Overseeded rye           1.5             143     
Overseeded clover        1.5             148
                          NS              NS  
N rate, lb/acre
  0                      1.5             115
125                      1.1             134
250                      1.2             170
         LSD (0.05)       NS              52           



Table 27. Main effects of preceding cover crop and rate 
of applied N on yield of sweet corn, NWREC, 1996                       
Treatment              Yield  No. ears/  Mean ear  Ear length  Tipfill
                       (T/A)    plot     wt. (g)    (inches)          
Cover crop (avg. over N rates)
Fallow                  7.5      54         199        8.1       2.2
Overseed triticale      6.1      45         186        7.4       2.2
Overseed clover         8.0      56         218        8.1       2.3
Fall-seed triticale     6.7      49         197        7.9       2.2
Fall-seed triticale/pea 6.9      49         205        8.3       2.2
             LSD (0.05) 1.0       6          NS         NS           
N rate, lb/acre (avg. over covers)
  0                     2.4      27         129        6.7       1.5
 50                     7.8      58         209        8.3       2.3
200                    10.9      66         265        8.9       2.9
        LSD (0.05)      0.8                  23        0.6