UC advisor reviews handheld nutrient meters

Two portable and relatively inexpensive tools can help grain growers read the midseason nutrient status of their crops, by measuring the red and infrared light reflected or absorbed by the plants.

A University of California Cooperative Extension farm advisor says the GreenSeeker and the atLEAF CHL provide measurements of plant chlorophyll levels that can be combined with other information about the soil, weather and crop growth stage to make informed fertilizer decisions.

"The tools help determine plant vigor by measuring light reflected from the whole plant canopy or absorbed by a single leaf," said Nicholas Clark, a UCCE farm advisor based in Hanford.

Clark is part of a team of Cooperative Extension researchers putting together a relatively simple and economical management system to help grain growers estimate how much nitrogen to apply.

"The plant nitrogen levels indicated by the handheld devices complement other important information, which is closely related to the crop nitrogen status, such as recent soil nitrate concentration, current crop growth stage, crop yield and protein goals, and local weather records to inform a site-specific nitrogen fertilizer recommendation," Clark said during a webinar on small-grain production.

One of the tools, the GreenSeeker, sells for around $500 for a barebones model, he said, and uses light reflected from the canopy to measure vigor.

"It emits red and infrared light, and detects how much of each is reflected from the crop canopy," Clark said. "Chlorophyll in green plant tissue absorbs red light but not infrared light. As more red light is reflected off plants, the (normalized difference vegetation index) value decreases. Higher NDVI levels indicate very-green plants, and lower NDVI levels indicate less-green plants."

The GreenSeeker calculates the average vegetation index as a user walks a 20-to-40-foot stretch of the field. Clark said a number of these samples taken from representative areas of the field combine to provide a reading of the midseason vigor of the crop.

The atLEAF CHL, available for as little as $250 for the lean version, he said, uses a different approach to estimate plant vigor by measuring chlorophyll levels.

"Like the GreenSeeker, it also emits a red and infrared light," Clark said. "However, the atLEAF CHL measures how much of each light passes through a single leaf, instead of light reflected off the canopy."

To come up with a reliable vigor rating of the crop with the atLEAF CHL, he said, a user should test 15 to 20 leaves taken from representative areas of the field.

A significant difference between the two devices is size of the sample they measure for vigor, and the time in the growing season they are most useful, Clark said.

"Since each device measures a different scale of the crop, the whole canopy versus a single leaf, they have different uses," he said. "The atLEAF CHL indicates the concentration of chlorophyll in the leaf. This information becomes increasingly valuable as an indicator of whether the crop has sufficient nitrogen as it begins heading out and filling grain."

The information from the two handheld devices can be combined with measurements of nitrogen already available in the soil and an understanding of how much the crop needs at different times in the season to develop an efficient fertilizer program.

"Once it is decided whether to fertilize with nitrogen, the next step is to figure how much nitrogen is necessary," Clark said. "This can be determined using a crop growth and nitrogen uptake model in conjunction with yield and protein goals."

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