A ‘low-tech,
high-tech’ tool

Economical leaf color chart helps you check the crop for nitrogen

By Vicky Boyd
Editor

In this high-tech era where cell phones receive e-mail and tractors run on satellite-guided autopilot, imagine a plastic ruler-like device that allows you to measure your rice crop’s nitrogen level simply by matching colors.

No buttons, no cords, not even a moving part—the leaf color chart (LCC) works on the basic principle that rice leaf color is a good indicator of leaf nitrogen content.

And the easy-to-use, economical tool already has a following, even though it was only unveiled at the Rice Field Day in Biggs, Calif., in August 2000.

"It’s a very low-tech, high-tech tool," says Chris McKenzie, who grows rice near Pleasant Grove, Calif., and who plans to use the LCC this season. "It gives you results instantly compared to when you have to send it off to a lab.

"If you take a leaf sample, you have to apply the fertilizer long before you know what the sample results are. If you do two leaf samples, you find out they’re not very representative and they change. This allows you to keep checking on the nitrogen."

The chart also provides farmers with a frame of reference rather than having to rely on their memory from day to day, he says.

Farmers walk a fine line with nitrogen applications, McKenzie says. Too much, and you promote disease and lodging, lower grain quality and waste money. Too little, and you rob your crop of its yield potential.

With funding from the California Rice Research Board, a team from the University of California Cooperative Extension in Butte County spent two years developing and then validating the leaf color chart.

Involved were Cass Mutters, a UC farm advisor; Jim Eckert, coordinator of the Biologically Integrated Farming Systems program; and David Parks, a California State University, Chico, graduate student and Extension intern.

The 15-inch-long leaf color chart resembles a sheet of green paint samples from a home improvement center that’s been immortalized in plastic. Instead of "olive grove" or "rhubarb leaf," as you’d see on the paint cards, the color chart has eight shades of unnamed green.

The lightest, most yellowish shade is labeled 1, with the deepest, darkest green sample labeled 8.

On the back of the chart, a table explains how the number values correspond to percent leaf nitrogen. For California medium- and long-grain varieties, 3.3 percent to 3.8 percent leaf N at panicle initiation is adequate.

Based on the leaf color chart, you should have enough nitrogen to forego nitrogen topdressing if you had a color value reading of 4 or above at panicle initiation.

The LCC can be used in one of two ways. You can either sample Y-leaves throughout the field and match the individual leaf to the color or you can use the chart to gauge the overall color of the entire field.

Begin sampling at the panicle initiation growth stage of rice. Because the sun angle and light intensity affect leaf color, Eckert says sampling should be done between 10 a.m. and 2 p.m. Always position yourself with the sun to your back.

Regardless of the method, Eckert recommends repeating the process several times within each field to obtain representative results.

Borrowing from an old concept

The concept behind the leaf color chart is nothing new, Mutters says. Researchers at Cal Tech in the 1920s first identified plant leaf color as a good indication of nitrogen content. Subsequent University of California, Davis, research during the 1980s reinforced the earlier work.

The Japanese also saw merit with the concept and in the 1970s produced a series of color panels that estimated leaf nitrogen.

Mutters began the work two years ago, looking at how leaf color equates to leaf nitrogen content of two Japanese varieties, akitakomachi and koshihikari. He soon found that the same principles could be applied to the more conventional California-developed varieties, such as M-202.

What’s unique about the Butte County project is it involved a spectrophotometer to measure reflected light from the leaf surface.

Mutters and his team used the spectrophotometer to define reflective signatures of the rice leaves at various nitrogen levels. He then reproduced the unique spectral characteristics of the rice leaves in acrylic panels.

They also pitted the leaf color chart against a chlorophyl meter (SPAD-502 from Minolta Inc.), a $1,500 hand-held device that measures reflected light wavelengths. In addition,leaf nitrogen was determined by laboratory analysis.

The results from the color chart showed a very high correlation to those from the SPAD meter and the lab tests, Eckert says.

When used alone, the leaf color chart may be better suited to California-bred varieties than those developed for the South. But the low-tech application may still have a place in the South.

Rick Norman, a University of Arkansas agronomy professor specializing in rice fertility, says measuring leaf color is only about 30 percent accurate in predicting whether Southern varieties need additional nitrogen.

"The color method is equivalent in its accuracy to measuring leaf nitrogen concentrations," says Norman, who’s based in Fayetteville, Ark. "That’s worked well in California. However, in the Southern U.S. rice belt, N concentration isn’t as accurate as the Rice Gauge. In the South, the Rice Gauge is more accurate at mid-season."

The Rice Gauge measures plant height and area, which are good indications of whether the crop needs additional nitrogen at specific growth stages.

Based on years of testing, Norman says the Rice Gauge is accurate 70 percent of the time when used following instructions.

But when leaf color tests are combined with the Rice Gauge, they produce results that are accurate 80 percent of the time, Norman says.

E-mail Vicky Boyd at vlboyd@worldnet.att.net or call (800) 252-1924, ext. 8.

For more information, contact UC farm advisor Cass Mutters at rgmutters.ucdavis.edu or by calling (530) 538-7201.