Soil sampling is important in producing organic tomatoes

The first University of California study of the nutrient needs of organic processing tomatoes points to the importance of sampling the soil for nitrate nitrogen after the crop is established.

Unless there is a substantial reservoir of nitrogen in this form that is already available to be taken up by plants, the crop could suffer deficiency during its period of peak demand.

"In the middle of the season, the average requirement is three to four pounds of nitrogen per acre per day," said Tim Hartz, UC Cooperative Extension vegetable crops specialist.

Hartz discussed his research on the challenges of organic processing tomato nutrient management with the growers and others who came to the Russell Ranch Sustainable Agriculture Field Day 2015 in Winters.

This period of extraordinary nitrogen demand lasts all the way from four to five weeks after transplanting until 11 to 12 weeks after, and it can be a tough test of the soil's ability to make enough nutrients available to the crop.

"Knowing your post-establishment soil nitrate is a good piece of data, because none of these soils are going to be able to keep up with a crop taking up three or four pounds a day for weeks at a time," Hartz said. "If you are at 12 to 15 parts per million nitrate nitrogen to two feet, chances are you will be fine. Below that and you run a risk of deficiency."

Hartz said he looked closely at nitrogen in 37 organic processing tomato fields, taking soil nitrate tests three weeks after planting, and plant tissue nitrogen tests 11 weeks after planting, which is before the late-season decline in tissue nitrogen.

"There was a huge variation in soil nitrate nitrogen from five or six parts per million to two feet deep, all the way past 30 parts per million," Hartz said.

The fall application of manure or compost was, by far, the most widely used source of nitrogen for these organic fields, as farmers used this practice on 29 of the 37 fields Hartz studied.

The farmers also made pre-transplant fertilizer applications to 14 of these fields, and post-transplant applications to nine of them.

A cover crop was planted in the winter and incorporated before transplanting on only one of the 37 organic fields.

This research was needed because organic producers have far fewer tools than conventional growers to play catch-up if their tomato crop falls behind in nitrogen during the season, Hartz said.

"Organic processing tomatoes is not a big part of our industry, but it is growing and potentially profitable," he said.

Four years ago, California farmers grew nearly 8,000 acres of organic tomatoes, both processing and fresh market, according to U.S. Department of Agriculture statistics, which was well over 80 percent of the national total.

Although organic growers need relatively large pools of available nitrate nitrogen to meet crop needs in the middle of the season, in the northern growing region, at least, there is relatively little danger of leaching.

"Most of the tomato acreage in the Sacramento Valley is heavy soil, and nitrates leach slowly," Hartz said.

(Bob Johnson is a reporter in Santa Cruz. He may be contacted at bjohn11135@aol.com.)