Rice Farming

Shades Of Green

Leaf color chart estimates tissue nitrogen levels
  

By Brenda Carol

Nutrient analysis is often time consuming, expensive and just downright inconvenient, but it’s critical for mid-season nitrogen (N) management in rice production – particularly during the most demanding stages of plant growth. In-season nitrogen needs can be assessed in a number of different ways, but tools such the chlorophyll meter, tissue analysis and soil sampling are cumbersome.

In recent years, California growers have increasingly turned to the leaf color chart for nitrogen analysis. The leaf color chart was developed by Randall “Cass” Mutters, UCCE Butte County farm advisor. It is a quick, convenient way for growers to estimate tissue nitrogen levels at critical points during the rice plant’s life cycle. It consists of a series of panels with colors based on the wavelength characteristics of rice leaves. Mutters field-tested it extensively before releasing it.

“I evaluated it by estimating the nitrogen based on the leaf color chart and then took samples of those leaves and sent them to the laboratory to have them chemically analyzed,” he says. “From there, I established an ‘R-square’ value.”

The “R-square value” is a statistical value of how closely N measurements based on a leaf color chart correlate with conventional tissue analysis. An “R-square” value of 1.0 is 100 percent, which indicates perfect correlation. The leaf color chart doesn’t quite obtain that level of accuracy, but it gets impressively close.

“I can get an R-square value across nine different California rice varieties of 0.9 or better compared to laboratory analysis of nitrogen,” Mutters says. “On some varieties I can get an R-square value of .94 or better.”

That’s how accurate Mutters and his field crew, who have had training, can get with the leaf color chart. However, even growers and PCAs who haven’t had much training can get pretty close. A couple of years ago, a network of farmers who tried it – with minimal training – came up with an overall R-square value of .86 first time out of the chute.

Chart works best during mid-season
The leaf color chart works best between maximum tiller and a few days past panicle initiation. The convenience, relative accuracy and instant feedback have been a hit with growers and PCAs who are always pressed for time during mid-season.

“By the time growers get to that stage of the game, testing for nitrogen can take up to two weeks to get results,” says Arnie Cramer, PCA with United Agri Products in Yuba City, Calif. “The problem with that is they’re wanting to put out that last nitrogen application before panicle initiation. By the time they get sampling results, it’s too late. With the leaf color chart, you go out, start checking your field and get immediate feedback.”

However, it’s not a one-time deal, Cramer cautions. “You have to be diligent to determine if your N levels are going up or down, and it’s best to sample frequently. However, it’s a very simple system to learn and use, and it’s amazingly accurate. The only trick is that it’s somewhat subjective. But after a while, I could hold it out the window and take a reading that was almost as accurate as if I walked out in the field. There’s a field chart and a single leaf chart. Overall, it’s pretty easy to learn how to use.”

The more meticulous the grower, the more likely he is to take advantage of the leaf color chart.

“The growers who are planning on topdressing will use it,” Cramer says. “In the past, a lot of growers put all their N on up front and walked away. They kept an eye on it, but didn’t really bother with mid-season nitrogen applications. That could start changing with the increasing price of fertilizer. No one is too interested in wasting anything these days. It pays to stay up on what’s going on with nitrogen levels in the plant, and this is an easy, convenient, cost-effective tool to accomplish that.”

Brenda Carol is a freelance writer based in California. Contact her at (805) 226-9896 or brenda@brendacarol.net.