Nutrient Management Research Database

A series of trials were conducted in drip-irrigated commercial fields near Santa Maria to: - Evaluate vegetable crop productivity at reduced N application rates. - Document soil and plant N dynamics in cool-season vegetable production under typical coastal conditions. - Evaluate the accuracy, labor requirement and cost of on-farm monitoring techniques for soil and plant N status.

Eight drip-irrigated fields in long-term vegetable rotations, representing a range of soil texture, were monitored through typical crop rotations (including broccoli, cauliflower, celery and lettuce) from summer, 1994 through fall, 1995. At the beginning of each crop in each field, 4 replicate plots were fertilized by a banded preplant application of slow-release fertilizer at 100 lb N/acre. These plots, which also received all N applications delivered through the drip irrigation system, served as a high N >reference= treatment against which the rest of the field (designated >field management= treatment) was compared with respect to crop performance and N status of crop and soil. An intensive program of crop and soil monitoring was initiated at crop establishment, including soil NO3-N determination (by an on-farm >quick test= procedure and conventional laboratory analysis), petiole NO3-N (by on-farm sap analysis and conventional laboratory technique) and relative chlorophyll content (by leaf absorbance meter). The results of these on-farm monitoring techniques, together with the visual comparison of field management and reference plots, helped guide fertility decisions. In nearly all fields monitored, seasonal N applications considerably lower than industry norms were used without reducing marketable yield (compared to the high N reference plots); in only one field (of fall broccoli) was yield significantly affected, the effect being a slight delay in maturity. Total plant biomass was equivalent in both N treatments in all fields, as was total N uptake; the only consistent result of the increased N rate of the reference plots was to enlarge the pool of soil NO3-N. A consistent seasonal pattern of soil NO3-N was observed. At the start of the summer/fall 1994, season soil NO3-N concentration was >30 ppm in all fields; these levels were maintained throughout the season, diminishing substantially only during heavy winter rains. Despite conservative fertilization, soil NO3-N increased throughout the spring, 1995 season; by fall, NO3-N concentration was back to levels similar to the previous year. Mineralization of soil organic nitrogen was significant contributor to the development and maintenance of elevated soil NO3-N. Rates of 1.4 to 2.2 lb N/acre day were measured by various anaerobic and aerobic incubation techniques. On-farm monitoring techniques, although not as accurate as conventional laboratory analysis, provided useful information in a timely and cost efficient manner. Petiole NO3-N analysis documented that adequate plant N nutrition was maintained, but could not reliably be used to infer soil N status; soil NO3-N testing was required to assess soil mineral N supply. A routine monitoring program of soil NO3-N testing (2-3 times per season) to assess N sidedress requirement and companion petiole NO3-N analysis to ensure plant N sufficiency would be cost effective. This study suggests that N rate reductions of 50-100 lb N/acre below average industry application rates are possible, with a cost savings of 5-10 times the cost of monitoring.

• This project sought to:

1)      Document the accuracy, labor intensity and cost of on-farm monitoring techniques for soil and crop N status.

2)      Develop baseline data on N uptake rates and tissue N sufficiency levels for the important cool season crops.

3)      Demonstrate the effect of “best management practices”(drip irrigation and fertigation, intensive monitoring) on crop yield, and water - and nitrogen use.

• To achieve these objectives, fields from a range of textures and organic matter contents were monitored using on-farm techniques that do not require laboratory testing. The tests conducted included:

1)      Petiole nitrate analysis to ensure nutritional sufficiency of plants

2)      Soil nitrate testing 2-3 times per season to assess soil N availability.

• The on-farm tests were compared to laboratory analysis
• The fields studied were fertilized at a rates at or below industry averages. The rates at the time of the study (1995) were approximated as follows (range of rates applied in study in parenthesis)

1)      180 lbs N/ac for iceberg lettuce (132-216)

2)      250 lbs N/ac for broccoli (102-205)

3)      300 lbs N for cauliflower and celery (170-300)

• Only one plot showed yield differences between the experimental, reduced N fertilization rate and the “Reference rate” that was 100 lbs N/acre higher.
• Increasing N fertilization did not increase total crop N accumulation, rather it increased soil nitrate levels.
• Plant tissue sampling was not able to accurately provide information about soil N levels.
• The on-farm soil nitrate “quick test” was able to indicate high levels of soil nitrate, but there were issues with accuracy of the results. These errors resulted from:

1)      Variability in soil moisture content between farms.

2)      High levels of soil nitrate made interpretation of the test strip results difficult.

• Mineralization of soil organic matter provided a continued source of soil N, resulting in sustained increases in soil N levels though the course of the season.
• The reduction is N fertilization that on-farm monitoring allowed outweighed the overhead required.