Test strips quickly determine nitrogen level in soil

Ranch managers and pest control advisors on the Central Coast commonly use color-coded test strips to get rapid estimates in the field of the nitrates already in the irrigation water or soil, to help decide whether to fertilize or how much to apply.

Vegetable growers and researchers find these tests more than pay for themselves by reducing fertilizer applications, without hurting the crop.

There are so many different nitrate test strips on the market that the University of California Cooperative Extension decided to help growers choose the right model for the job, by comparing their cost and accuracy at different ranges.

"All of the strips are used in a similar fashion: The strip is briefly dipped into an extractant solution for soil or in water, and allowed to develop color during a standard interval of time, usually ranging between 30 and 60 seconds," said Michael Cahn, UCCE irrigation farm advisor in Salinas. "After color develops on the strip, a color chart, calibrated to either parts per million of nitrate or expressed in equivalent parts per million of nitrate-nitrogen, is used to determine the nitrate concentration of the sample."

The standard strip in most UC studies is the Merckoquant, which was already known to be accurate measuring soil nitrate-nitrogen between 10 and 40 parts per million. But the cost of this strip, at 47 cents per strip, is higher than most and there may be more economical alternatives for vegetable growers.

"The Merckoquant test strip was the brand originally tested by UCCE for use with the soil nitrate quick test, and was considered the standard in this evaluation. This strip measures to a maximum of 500 parts per million nitrate, but was only evaluated up to 250 parts per million for this test," Cahn said.

In collaboration with UCCE research associates Thomas Lockhart and Laura Murphy, Cahn tested six brands or models of the strips. Each of them was dipped into a solution of known nitrate concentration, and the accuracy of the color change recorded. This test was repeated four times for each brand at the strategic nitrate levels.

Two other strips, Hach Aquacheck and LaMotte Instatest NO3/NO2, also showed themselves to be accurate enough in these lab tests to be suitable for vegetable field tests of water or soil nitrates.

"We identified three brands of test strips that accurately measured nitrate and can be used to quickly assess the concentration of nitrate in soil or water," said Cahn. "Both the Merckoquant NO3/NO2 and the Hach Aquacheck strips were accurate for measuring concentrations of nitrate as low as 10 parts per million, which would roughly correspond to 5 parts per million nitrate-nitrogen in soil. No brand of test strip measured nitrate accurately below 10 parts per million."

Of the three brands that are accurate in the range needed for cool-season vegetable fertilizer decisions, the LaMotte Instatest NO3/NO2, at just 25 cents a strip, is the most economical.

This particular model of test strip, however, is not accurate at the lower ranges needed to make informed fertilizer decisions for strawberries and other crops that do not use nitrogen as quickly as cool-season vegetables, Cahn said.

"For strawberry production, and other crops that have a slower nitrogen uptake rate than vegetables, growers need test strips that are accurate over a narrower range of 5 to 15 parts per million nitrate-nitrogen in soil," he said.

Strawberry growers are advised to use either the Hach Aquacheck or the Merckoquant test strip to help with fertilizer decisions.

On the Central Coast, the strips have proven to be particularly useful in testing the soil before the summer vegetable crop, because there is usually a significant amount of residual nitrogen left from the spring crop.

The top foot of soil is usually tested before planting cole crops or lettuce, and the top six inches is recommended for spinach or baby lettuce. In either case, the top two inches of soil are best left out of the sample, because there may be nitrates in ground too dry for the crop to take up.

The threshold for deciding that additional fertilizer is not needed for lettuce is 20 parts per million, which pencils out in most soils to around 80 pounds of nitrogen an acre.

Most well water in agricultural areas also has enough nitrate-nitrogen to warrant testing it and including it in a fertilizer budget.

The charm of the test strips is that, while they are less accurate than a lab report, they let ranch managers know roughly how much nitrogen is in the soil or water in the field after about a minute, Cahn said.

"While laboratory analysis of nitrate is generally more accurate than using colorimetric test strips, the strips tested in this study appear to be sufficiently accurate to estimate the level of residual mineral nitrogen in soil samples and for determining the nitrate contribution from irrigation water, and should be useful for quickly assessing soil nitrogen status before making a fertilizer decision," he said.

In addition to the test strips, usually sold by the hundred, all that is needed to perform quick tests in the field are distilled water, a small centrifuge tube and calcium chloride of a grade available at aquarium or canning supplies stores.

Other strips UCCE tested proved to not be accurate within the ranges needed by either cool-season vegetable or strawberry growers.

"The remaining brands of test strips, LaMotte Instatest 5-way, API 5 in 1, Tetra 6 and 1 Easystrips, all measured less nitrate than the standard solutions over the range of 20 to 200 parts per million nitrate. These strip brands should probably not be used for the soil nitrate quick test and for assessing nitrate concentration in irrigation water," Cahn said.

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