Post-Harvest Mineral Nitrogen Status in Grower Fields (1996)


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.


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.


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).


This is in agreement with our results indicating that sweet corn is relatively inefficient in taking up N and with results obtained in 1993 and 1994 grower surveys. Levels of nitrate following beans were nearly as high as with corn, a trend noticed in 1994. Nitrate levels following broccoli were considerably higher than in previous years, but not as high as for corn or beans. For all crops, nitrate levels were generally elevated, not only in the surface foot of soil, but also at greater depths in the soil profile. This contrasts with our experience at NWREC and may indicate that improvements could be made in grower irrigation practices. Post-harvest ammonium concentrations varied less between pre-season and post-harvest sampling than did nitrate, but ammonium levels tended to be higher than the last two years, particularly for broccoli fields and in the surface foot of soil (Figure 9). However, the high average for ammonium following broccoli harvest was due to one site. Grower-cooperators were provided with a copy of the data from their fields along with the average for all fields.



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