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News

California: enhanced water and nutrient efficiency

20/04/2018 - François-Xavier Branthôme - Lire en français
Processing tomato growers and researchers continue to develop more efficient and sustainable ways to manage the water, energy and nutrients needed to produce the crop. One 2017 study on the UC Davis campus of applying different rates of fertilizer indicates that if it is available, the plants may take up more nitrogen than they need to produce a good crop. 
 
The plants may take up more nitrogen than they actually need,” said Daniel Geisseler, UC Cooperative Extension nutrient management specialist. “If more is available they may take it up, even though less nitrogen would not affect the yield. When we applied more nitrogen, there was more in the fruit.
Geisseler, who is conducting a series of studies on how much nitrogen becomes available to crops from the soil over the course of the season, presented his latest findings to the growers and pest control advisors at the South Sacramento Valley Processing Tomato Production Meeting in Woodland. “The ultimate goal is to develop an online tool to help growers manage their irrigation and nutrient management,” Geisseler said.

When researchers in the Davis study applied varying amounts of fertilizer, from 50 pounds over to 50 pounds under what they thought the crop needed, there was no effect on tomato yields. “We tried 175, 225 and 275 pounds of nitrogen at Davis in our 2017 study,” Geisseler said. “We didn’t see any statistical difference among the three treatments, which was surprising. I would have thought the 50 pounds below optimum would show a yield effect.” The difference was in the concentration of nitrogen in the tomatoes at harvest, which ranged from 2.59 pounds per ton in the low fertilizer plots up to 3.09 per ton in the most fertilized plots.

Learning to be as precise as possible in using inputs like fertilizer and water is now an important part of the relationship between growers and their largest customers. 
After talking to marketing people, environmentalists and growers, we chose five areas to concentrate on — water, fertilizer, greenhouse gases, soil quality and pesticides,” said Dan Sonke, director of sustainable agriculture for Campbell Soup. “Over five years of collecting data, we’ve seen reduced water use from the adoption of drip irrigation. We set a goal of a 20 percent reduction in water per ton of tomatoes by 2020, and reached it by 2016. The California industry can be very proud.”

Research like Geisseler’s is part of the process of learning how to be environmentally and economically sound in managing the nitrogen it takes to produce a processing tomato crop. Previous studies in six commercial fields in Woodland, Stockton and Huron already showed that a ton of processing tomatoes has 2.99 pounds of nitrogen, and that a third of all the nitrogen taken up by the plants ends up in the vines. 
 
That discovery led to a calculation that a field producing a 55-ton crop would take up 246 pounds of nitrogen and, at an efficiency of 90 percent, would require a total of 274 pounds to be available. Some of that comes from the nitrate in the irrigation water, which is fairly easy to measure and calculate, but some comes from the soil, which can be a bit trickier. “We estimated half the nitrate in the top foot and 90 percent in second foot would be available,” Geisseler said. “We estimated about 47 pounds an acre of nitrate-nitrogen would be taken up. We need more research on how much nitrogen the plant can actually take up from above the drip line.”

Less nitrate-nitrogen is taken up from the top foot under buried drip irrigation, because that ground remains dry, but how much less is a subject that calls for further research. “It really makes a difference how much nitrate is in the soil and irrigation water,” Geisseler said. “If the irrigation water had 10 parts per million nitrates and you applied 22 inches, [it means] that would be another 50 pounds of nitrates.”

Additional nitrogen is supplied as the organic material in the soil mineralizes and becomes available over the course of the growing season.
From 75 to 125 pounds a year mineralizes in the San Joaquin Valley, based on common cropping and fertilizer practices,” Geisseler said. “From 30 to 50 pounds of that becomes available during the cropping season. Even in low organic soils, nitrogen still mineralizes during the growing season.”

Fortunately, the rate of mineralization tends to be greatest when the nitrogen will do the crop the most good.
After the harvest of the crop, mineralization decreases because there is no irrigation or rain,” Geisseler said. “It stays low in the winter because the temperatures are low. In the spring it increases with the higher temperatures and the soil is still wet from the rains.”

When someone adds it all up, the nitrate already in the soil, the portion that mineralizes over the course of the season and the portion that is in typical irrigation water can supply as much as half of what the crop needs, he said.
 

Source: dailydemocrat.com, Ag Alert
 
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