Highlights

• Processing tomatoes contain about 3 lbs N/ton at harvest
• About two thirds of the N is in the fruits, one third in the vines
• Little N is taken up during the first month after transplanting
• The results were incorporated into an online N calculator and CropManage

The Team

Kelley Liang, Brenna Aegerter, Mike Cahn, Gene Miyao, Tom Turini, Daniel Geisseler

Background

Processing tomatoes are an important California crop grown on about 260,000 acres in 2016. Over the last 15 years, the wide adoption of drip irrigation by the tomato industry has resulted in a dramatic shift in production practices, with fertigation through the drip system now being the most common practice. To achieve high yields while reducing the risk of N losses, the time and quantity of irrigation water and fertilizer applications need to match crop demand. With stricter regulatory and reporting requirements and technological advances, computer based decision support tools are becoming a central component of field-specific crop management.

Objectives

The objective of the project is to develop a decision support tool for irrigation and N management in processing tomatoes based on the framework of an existing tool, CropManage, which has been successfully developed and introduced for cool season vegetables on the Central Coast.

Our Approach

Eleven commercial processing tomato fields were selected for the study. In-season soil and plant samples were collected in three-week intervals. In addition, canopy coverage was measured by infrared camera.

In addition, a replicated trial has been set up at UC Davis in spring 2017 and again in 2018. The trial includes four treatments, each replicated five times. The treatments differed in their N application and irrigation rate. Each plot is 200 feet long and three beds wide. Based on the results from the commercial fields, the optimal N application and irrigation rates were determined.

Results

The preplant nitrate-N concentrations in the commercial fields ranged from 18 to 325 lbs/acre in the top foot and from 15 to 160 lbs/acre in the second foot of the profile. These values are based on an estimated bulk density of 1.2 g/cm3. The large differences among sites highlight the need for a site-specific tool that takes into account residual soil nitrate.

Tomato plants took up between 257 and 395 lbs N/ac. It took approximately 40 days for the plants to accumulate 40 lbs N/ac, indicating that early season N requirements are low. Between 170 and 264 lbs N/ac of the total aboveground N was allocated to the fruits, which contained close to 3 lbs N/ton. Across all sites, the plants took up an average of 320 lbs N/ac by harvest, with 33% of the N being in the vines and 67% in the fruits.

The canopy cover first started with a slow initial growth phase, followed by rapid growth eventually reaching its full canopy, at that point the coverage stabilized.

For the replicated trial, N uptake was estimated to be 246 lbs/acre. This estimate is based on the results from commercial fields and an expected yield of 55 tons/acre. Subtracting residual soil nitrate and N in the irrigation water, the fertilizer N requirements were estimated to be 225 lbs/acre. This application rate includes starter N and an N use efficiency of 90%.

In 2017, the marketable yield averaged 58 tons/ac. Increasing or lowering the optimal N application rate by 50 lbs/acre had no statistically significant effect on yield. However, N uptake by the plants increased with increasing N application rate. The N content of the fruits ranged from 2.75 to 3.15 lbs/ton. This result suggests that tomato plants can adjust the N content in the fruits in response to the availability. Irrigation water need was calculated based on modeled canopy development and average weather data of the previous four years. Increasing the irrigation water application rate by 30% at the optimal N application rate had no effect on yield either.