Organic nitrogen provides benefits to vegetable crops

Salinas Valley vegetable crops benefit from organic nitrogen in the soil that mineralizes gradually over the course of the season, but farm advisors say it can be difficult to calculate how much of the nutrient will become available, and when.

Unlike nitrate nitrogen in the irrigation water and topsoil, which can be measured before planting, the availability of this organic nitrogen is a moving target that can vary greatly from field to field.

"The amount of nitrogen mineralized in our field evaluations ranged from 0.6 to 3.3 pounds a day; more nitrogen mineralization generally occurred in soils with greater quantities of total nitrogen in the soil," said Richard Smith, University of California Cooperative Extension farm advisor in Salinas.

Smith, UC Cooperative Extension vegetable crops specialist Tim Hartz, assistant Cooperative Extension specialist Dan Geisseler and Cooperative Extension staff research associate Patricia Love studied mineralization rates in 10 Salinas Valley vegetable fields during the 2016 season.

Nitrogen that mineralized in the soil and became available to the crop over the course of the season ranged from a low of just 16 pounds in one of the lettuce fields, all the way up to 82 pounds in a chard field.

"All the crops surveyed in these evaluations have rapid nitrogen uptake patterns in the last half of the crop cycle," Smith said. "It is at this time when robust quantities of nitrogen are needed to supply the crop with the amounts of nitrogen needed to maximize yields."

While the 10 fields in that study were all organic, where the challenge is getting enough nitrogen to the crop, the same team of researchers also looked at using broccoli as a trap crop in 10 conventional vegetable fields, where the challenge is preventing excess nitrogen from leaching into the underground water.

"Many of the vegetable crops, such as leafy greens, are shallow rooted, fast maturing with a high nitrogen demand, and produced in multiple rotations per year," Smith said. "High land rents restrict the ability of producers to effectively rotate to cover crops that could take up excess residual soil nitrate from the soil."

One answer to the leaching challenge could be the use of deeper-rooted cash crops, like broccoli or cauliflower, that can scavenge for nitrogen that has escaped the root zone for lettuce.

"Broccoli roots extend down to 40 inches by the end of the crop cycle," Smith said. "Sufficient root density occurs in the second foot of soil by 50 days after planting, such that the crop can effectively utilize nitrate at that depth in the soil profile."

The study showed broccoli to be particularly effective in ground where high nitrate levels had built up in the top three feet of soil.

"Deep-rooted cover crops and vegetable crops grown in rotation with shallow-rooted vegetables can sequester nitrate," Smith said. "Careful management of nitrogen inputs and use of rotational crops can provide a means of maintaining nitrogen in the rooting zone of the crops, reducing losses to groundwater and improving nitrogen use efficiency."

For organic growers, the challenge is calculating how much nitrogen will become available from mineralization of the soil and of their fertilizer, which can both be difficult to predict.

"These evaluations show a rapid release of nitrogen from the organic fertilizer over the first two to three weeks after application," Smith said. "After that time, the rate of release of nitrogen is very slow."

Development of organic nutrient programs for the later weeks of the vegetable season remains a work in progress, as many of the factors are difficult to predict.

"Organic fertilization of fast-maturing cool-season vegetables is complicated by a number of factors: the difficulty of utilizing residual soil nitrate monitoring, given the potential for nitrate leaching with these heavily irrigated, shallowly rooted crops; uncertain rates of nitrogen release from organic fertilizers due to the variable effects of temperature, fertilizer placement and fertilizer nitrogen concentration; difficulty synchronizing nitrogen release from soil and organic fertilizers with the high crop nitrogen demand over the last half of the crop cycle," Smith said.

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