Program calculates processing tomato fertilizer needs
Processing tomato growers can use online software to estimate how much nitrogen to apply, and when, to produce a crop of maximum yield and quality and while minimizing leaching to groundwater.
The University of California Nitrogen Calculator for Processing Tomatoes generates fertilizer recommendations based on planting and harvest dates, expected yield and site-specific information about soil and irrigation water nitrogen.
“The calculator is readily available on my lab’s website,” said Daniel Geisseler, the UC Cooperative Extension assistant specialist who led the research reflected in the nutrient management software program.
The program is available online at geisseler.ucdavis.edu/ Tomato_N_Calculator.html.
Geisseler made his remarks, and took the software on a trial run, as UC and Cooperative Extension researchers presented their latest information during an online Processing Tomato Conference in mid-December. The conference was sponsored by Seed Central, a consortium of academic researchers and private companies in the Davis area, formed to “bring science to market faster.”
If a farmer has a field where processing tomatoes will be planted on April 1 and harvested the following Sept. 15, at an expected yield of 55 tons an acre, the software calculates the plants will take up 247 pounds of nitrogen an acre during the course of the season.
Geisseler said the figure is based on UC studies showing that processing tomatoes from a commercial field in the Central Valley have on average 3 pounds of nitrogen per ton of fruit, and an additional pound-and-a-half of nitrogen in the vines.
Though the total amount of nitrogen taken up by the plants in this example would be 247 pounds an acre, much of that would come from sources other than fertilizer.
Agricultural soils commonly have fairly high levels of residual nitrate-nitrogen from previous crop residues and fertilizer applications, and Geisseler said tomato plants can scavenge for this nitrogen to a depth of at least 2 feet.
If the field has 10 parts per million nitrate-nitrogen in the top 2 feet of soil, for example, the software calculates the available nitrates will contribute 46 pounds per acre — basing that figure on the estimate that 80% of the nitrates in the second foot will be available to the crop, while only 50% of the nitrates in the drier top foot will be accessed by the roots.
But agricultural soils also have significant amounts of organic nitrogen, and the software calculates an additional 53 pounds per acre would mineralize and become available to the crop during the course of the season. This figure is based on UC studies showing that 80 pounds of nitrogen typically mineralize during the season — a figure the software adjusts based on CIMIS weather information — and that most but not all of it will be taken up by the plants.