Introduction
This grower trial was undertaken to determine whether residual nitrate and ammonium levels in grower fields 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 and 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.
Methods
Soil samples were taken in one-foot increments to a depth of 5 feet both before and after crops of green beans, beets, broccoli, carrots, cauliflower, and sweet corn, for determination of mineral N (ammonium-N and nitrate-N) content. Thirty fields were sampled in 1993 and 34 in 1994, representing 15 growers and 7 soil types. The growers were interviewed to determine field history and cropping and fertilization intentions and they kept records of fertilizer applications and irrigations.
Results and Discussion
Mean pre-plant nitrate and ammonium concentrations for 1993 are seen in Tables 34 and 35, respectively. Corresponding data for 1994 are in Tables 36 and 37. In order to preserve anonymity, only means are presented. However, there were few surprises in the pre-plant data. The relatively heavy rainfall during the late winter and spring resulted in low levels of nitrate in most fields in both years. Most individual 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.
Mean nitrate and ammonium levels at harvest varied by crop (Tables 34-37) and grower cultural methods (data not shown). For example, in top foot of the soil, nitrate concentrations were greater for sweet corn than for cauliflower or broccoli, although the mean N application to these crops was 203, 226, and 262 pounds/acre, respectively, in 1993 and 193, 223, and 253 pounds/acre, respectively, in 1994 (Tables 34 and 36). Nitrate levels were generally elevated for sweet corn, not only in the surface foot of soil, but also at greater depths. This contrasts with our experience at NWREC and may indicate movement of nitrate as a result of the unusually wet late spring and early summer, or may indicate that improvements could be made in grower irrigation practices. For carrots, the elevated nitrate levels at the 12 to 36-inch depths may indicate soil mixing during digging. Post-harvest ammonium concentrations varied much less between pre-season and post-harvest sampling than did nitrate (Tables 35 and 37). Presumably, this is due to conversion of urea and ammonium-N to nitrate during the growing season.
Table 34. Mean post-harvest nitrate concentration (ppm) in grower fields as a function of crop and depth in the soil profile, 1993 Crop Depth of sample (inches) 0-12 12-24 24-36 36-60 Pre-plant 4.2 2.7 2.5 2.5 Beans 13.9 8.0 6.4 5.7 Beets 15.3 4.4 2.8 3.2 Broccoli 10.4 3.9 2.5 2.7 Carrots 9.2 11.8 7.1 4.3 Cauliflower 15.3 6.8 5.0 5.5 Sweet corn 21.9 10.0 8.8 7.2 Table 35. Mean post-harvest ammonium concentration (ppm) in grower fields as a function of crop and depth in the soil profile, 1993 Crop Depth of sample (inches) 0-12 12-24 24-36 36-60 Pre-plant 3.6 2.9 2.6 2.6 Beans 4.8 3.2 3.1 2.7 Beets 4.6 3.1 3.1 2.7 Broccoli 4.8 5.0 3.1 3.4 Carrots 1.0 0.3 0.3 0.1 Cauliflower 6.7 4.1 3.5 3.8 Sweet corn 4.3 3.2 3.3 3.8 Table 36. Mean post-harvest nitrate concentration (ppm) in grower fields as a function of crop and depth in the soil profile, 1994 Crop Depth of sample (inches) 0-12 12-24 24-36 36-60 Pre-plant 5.0 4.6 5.5 5.2 Beans 20.2 5.8 7.1 6.6 Beets 13.3 6.1 4.1 3.2 Broccoli 7.2 3.0 2.3 1.9 Carrots 12.8 7.5 7.4 6.3 Cauliflower 9.9 6.9 6.9 6.8 Sweet corn 22.2 9.1 9.6 8.0 Table 38. Mean post-harvest ammonium concentration (ppm) in grower fields as a function of crop and depth in the soil profile, 1994 Crop Depth of sample (inches) 0-12 12-24 24-36 36-60 Pre-plant 4.2 3.9 3.0 4.1 Beans 7.4 3.5 4.0 3.6 Beets 4.5 3.6 3.0 2.2 Broccoli 6.0 4.7 3.5 3.0 Carrots 3.1 2.5 1.9 1.5 Cauliflower 7.2 3.6 3.2 4.0 Sweet corn 4.7 3.1 2.6 2.7