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Software program calculates processing tomato fertilizer needs

05/02/2021 - François-Xavier Branthôme
The calculator is readily available

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 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.

An additional source of nitrogen comes from the irrigation water, as underground water in most agricultural areas of the state can have elevated nitrates. There is strong and growing regulatory pressure on agriculture to reduce groundwater nitrate levels, and Geisseler said the software tool is intended to help farmers meet that demand.

Irrigation water that has 20 parts per million nitrates will deliver an additional 87 pounds per acre during the season, if the crop is irrigated with a total of 24 inches.

If a modest 20 pounds of starter nitrogen is applied, and 80% efficiency in taking up nitrogen is assumed, that leaves an additional 52 pounds of nitrogen to be fertigated during the season. But Geisseler said fertigation should not begin until the crop uptake is greater than the starter, residual and irrigation-water nitrogen, and should end well before harvest.

The software calculates for this hypothetical 55-ton crop that if a farmer starts fertigating six weeks after planting, terminates the fertilizer after the 14th week and runs the fertilizer through the drip line nine times in the interim, each fertigation should include 6 pounds an acre of nitrogen.

Most of the information that must be entered into the software is easy to either estimate or learn from tests any lab can perform, Geisseler said.

The planting and harvest dates and expected yield can all be estimated in advance, for example, and relatively simple lab tests will let a farmer know how much nitrogen to expect from the soil and water.

Growers and pest control advisors are generally already familiar with most of the concepts behind the fertilizer recommendations, Geisseler said, but the software instantly performs calculations that could take precious time before making fertilizer decisions in the field.

Although a wealth of information is behind the recommendations for how many units of nitrogen to inject with each irrigation during a growing season, he said it is still necessary to monitor.

Unexpected changes in the weather can affect how the crop grows, and the contribution of nitrogen from the soil estimated by the software is just an estimate, albeit a good one. There is no substitute for walking the field to adjust fertilizer applications to account for unexpected conditions if necessary, Geisseler said.

It is important that you still go out in the field and monitor the crop in time to make corrections,” he advised. “Like all models, these calculations are based on average conditions.”

Some complementary data:
More information on the project can be found here.






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