Easy-to-use Leaf Color Chart helps growers gauge crop’s mid-season nitrogen needs.
By Vicky Boyd
Editor
The University of California, in collaboration with the California Rice Commission and the California Cooperative Rice Research Board, has released the UC Leaf Color Chart, version 2.0.
The ruler-like device contains eight color cells of varying shades of green that growers can use to gauge crop mid-season nitrogen levels and whether they need to topdress.
Originally introduced by UC Cooperative Extension farm adviser Cass Mutters in 2000, the Leaf Color Chart was recently revisited as a tool to be used in tandem with the UC publication, “Rice Nutrient Management in California,” to help rice growers address the state’s nitrogen management plan.
“We were at some water board meetings this summer and held up the publication, ‘Rice Nutrient Management in California,’ along with the Leaf Color Chart,” says Roberta Firoved, manager of industry affairs with the Sacramento-based California Rice Commission. “Oh my gosh—you would have thought that (the Leaf Color Chart) was the coolest thing in the world. We have a great opportunity to do educational outreach by providing growers with these tools, and it’s going to demonstrate that our industry is proactive.”
With funding from the Rice Research Board, Mutters produced 2,300 copies of the updated Leaf Color Charts. This winter, the California Rice Commission plans to mail to all of the state’s growers the Leaf Color Chart and either the nutrient management guide or a discount so producers can buy the guide. Included in the mailing will be a note explanation why growers are receiving the tools, Firoved says.
Mutters says UC researchers also plan to discuss the Leaf Color Chart and accompanying nutrient management guide during their winter rice meetings.
Simple yet effective
The Leaf Color Chart works on the simple principle that rice leaf color is a reliable indicator of mid-season leaf nitrogen content. The 15-inch-long gauge is made of durable UV- and heat-resistant acrylic plastic.
On the back is a calibration table that equates each color cell to leaf nitrogen levels.
For California medium-grain Calrose-types in general, Mutters says, UC has set a critical leaf nitrogen level of 4.6 percent at mid-tillering and 3.2 percent at panicle initiation.
Based on the Leaf Color Chart, a grower is able to determine if leaf nitrogen levels are high enough to forego midseason nitrogen topdressing.
Seth Fiack, an Ord Bend-area rice producer, was chairman of the Rice Research Board in 2015 when it approved funding to update the gauge. And it was the tool’s simplicity that sold the board.
“It’s an effective tool, and it’s easy for guys to use,” says Fiack, who has used the gauge since it was fi rst introduced in the early 2000s. “That’s what makes it so good—it’s not complicated.”
Based on readings from the color chart, he only had to topdressed 300 of his 525 acres of rice this season.
But Fiack was quick to point out that the color chart is just one of the tools he uses to manage nutrients; another is the UC degree-day model.
Like many other new devices, he says growers have to use the color chart regularly to become comfortable with it and learn how it relates to their individual fields.
Two sampling options
You can either sample Y-leaves throughout the field and match the individual leaf to the color cells or you can use the chart to gauge overall canopy color.
Regardless, begin sampling at mid-tillering, Mutters suggests. Because the sun’s angle and light intensity can affect color, he recommends using the leaf color chart between 10 a.m. and 2 p.m. He also prefers keeping the sun to one’s back for consistency.
As you would with soil or tissue sampling, take Leaf Color Chart readings in several areas of the field to gain representative results, Mutters advises. If you have one portion of a field that appears to be different, consider taking separate readings so you can manage it on a site-specific basis.
Behind-the-scenes development
For the updated color chart, researchers worked with a Los Angeles area laboratory to spectroanalyze and digitally render the individually unique leaf colors to ensure that the color cells accurately described the actual color of a rice leaf.
The results were used to formulate high-temperature resistant, UV-stabilized, colorized acrylic plastic. Test plates were created to compare with the original Leaf Color Chart, and the process was repeated until the colored plastic matched the original color chart.
The color cells are linear based on a component of the leaf spectrum, allowing growers to “split the difference” if a leaf color falls between two different color cells.
During the 2016 season, Mutters also conducted a trial that involved nine rice varieties and six different nitrogen levels at the California Rice Experiment Station to develop calibration curves for the Leaf Color Chart.
“It’s a very complex tool that’s deceptively simple,” he says.
The need for nitrogen stewardship
California’s nutrient management requirements fall under the conditional Irrigated Lands Regulatory Program, adopted by the state in 2003 to reduce agricultural pollutant runoff into surface water. It was later expanded to include groundwater. The conditional ILRP is now a permanent regulation implemented as Waste Discharge Requirements Orders. Until 2003, agriculture had received a discharge waiver from the State Water Board.
Since then, growers who irrigate their land and have runoff from irrigation or rainfall have an option. They can either apply to the Regional Water Quality Control Board for individual discharge permits or they can join a regional water quality coalition. Most choose the latter.
The roughly dozen coalitions statewide collect per-acre fees from members to help fund water quality monitoring, develop best management practices and educational programs, and data collection.
The state’s rice industry falls under a commodity-specific program because the industry already had a framework in place for rice pesticide monitoring. In addition, the CRC commissioned a study that examined rice production’s possible nitrogen contribution to groundwater, says Roberta Firoved, manager of industry affairs with the Sacramento-based California Rice Commission.
The results showed rice was an extremely effi cient nitrogen user and when combined with the clay soils of most fields, rice fertilization posed little risk to underground water sources.
But in initial meetings with the Central Valley State Water Board, Firoved says regulators assumed incorrectly that growers intentionally over-applied nitrogen, since the nutrient isn’t regulated and doesn’t carry mandatory reporting like pesticide use.
She and her colleagues have had to educate regulators that rice growers generally are conscientious about not over-applying nitrogen. That’s because rice is extremely sensitive to nitrogen, which can actually reduce yields by causing blanking and increase disease pressure and lodging if over-applied.