Tim Hartz, under funding by the California Lettuce Research Board, showed that the pre-sidedress soil nitrate testing, or PSNT, method was consistently accurate in maintaining lettuce yield and quality.
At the same time, it reduced seasonal nitrogen applications by an average of 45 percent.
Average commercial yield was nearly identical between the PSNT and growers standard treatments. Postharvest evaluation at UC, Davis, including storage of 10 to 14 days at 41 degrees, showed "virtually identical visual quality, decay and discoloration rankings between nitrogen treatments in all fields," he said.
Hartz’ did his 1999-2000 season trials on 11 fields (10 planted to iceberg and one to romaine) from Castroville to Soledad with soil types from sandy loam to clay and harvest dates from June through September. Fields were irrigated by furrow or sprinkler.
For the trials he set up 36-row-wide strips across cooperating growers’ fields. The majority of each field was under the grower’s normal fertility program, but on the strips he sidedressed nitrogen in amounts according to nitrate-nitrogen content of the top one foot of soil.
After his earlier research for the board, Hartz noted that lettuce showed no short-term response to nitrogen fertilizer where the residual nitrate content of the soil was greater than 20 parts per million (PPM). Factoring 4 million pounds of soil per acre-foot, he calculated the amount to apply based on the PPM.
In the PSNT strips he applied nitrogen as needed, ranging from 20 pounds per acre for those showing 15 to 20 PPM to 80 pounds for those showing 0 to 5 PPM.
‘Quick test’ works
He analyzed nitrate in the soil with the "quick test," a field method using colormetric strips and found it to be reasonably accurate and comparable to conventional laboratory analyses.
The cost of soil nitrate monitoring, by either field or laboratory means, would generally be more than offset by reduced fertilizer costs, he said.
Nitrogen management practices varied widely among cooperating growers, the seasonal applications ranging from 158 pounds to 339 pounds per acre in one to four sidedressings.
The average grower application was 247 pounds per acre, and the PSNT method reduced that to 135 pounds, or 45 percent less, much of it in the first sidedressing.
"As expected," Hartz reported, "the spring fields, planted after winter fallow conditions, had somewhat lower soil nitrate at the first sidedressing than did the summer/fall fields, which were planted after incorporation of spring crop residues."
Hartz observed that fertilizer nitrogen not taken up by the crop presents a potential threat to ground- and surface-water quality. "Sampling at harvest showed that the grower-standard plots averaged approximately 60 pounds per acre more nitrate-nitrogen in the top three feet of soil than did the PSNT plots.
"That additional nitrate-nitrogen presents a leaching hazard, particularly in fields entering the rainy, winter-fallow period."
Hartz also evaluated another method, measuring nitrate content of lettuce midribs, in the trials and concluded it is of "very limited value" in determining field-specific sidedress nitrogen requirements.