Fertilizer applications require careful planning

The complex challenge of matching walnut nitrogen availability with changing crop needs can be simplified a little by holding off on the first fertilizer application at the beginning of the season.

University of California researchers marked fertilizer so they could track its movement and found that walnut trees do not take up new sources of nitrogen until May.

"We know that if you put on nitrogen in April, it's going to be hanging out in the soil or, more likely, it's going to leach out with April rains," said Katherine Jarvis-Shean, UC Cooperative Extension farm advisor in Yolo, Solano and Sacramento counties. "We want to match nitrogen availability with demand over the course of the growing season; trees are best at taking up nitrogen when they need it."

Jarvis-Shean made her remarks during the 42nd Annual Nickels Soil Lab Field Day in Arbuckle, as she discussed strategies for delivering enough nitrogen for walnut trees to produce well, without leaching nitrates into groundwater.

While only the nitrate form of nitrogen is highly susceptible to leaching, urea quickly morphs into ammonium, which in turn, with a little warmth and water, becomes nitrate.

"Ammonium is positively charged, sticks with the soil, and can be taken up by the roots," Jarvis-Shean said. "Nitrate is more mobile, and a lot of it is leached. When it is warm and there is moisture, ammonium can turn to nitrate; half the ammonium that is not taken up by the roots turns to nitrate in two weeks. That's why we have to do the reporting."

The most recent UC studies show that walnuts need a little less nitrogen than previously believed.

"The total nitrogen from our most recent studies show that 29 pounds of nitrogen is taken up for every in-shell ton of walnuts at 8% moisture," Jarvis-Shean said. "This includes the nitrogen in irrigation water, organic nitrogen that becomes available, and fertilizer. The belt is going to get tighter. Stick to the 40 pounds when you fill out your report, but know that the lower 29 pound number is coming when these studies come out."

A small but still significant share of this nitrogen comes from the soil or irrigation water, which essentially supply a little free fertilizer.

"The soil organic matter is kind of a savings account of nitrogen," Jarvis-Shean said. "It comes a little bit at a time over the course of the season because of the work of microbes in the soil. If you're irrigating from groundwater, you almost certainly also have nitrates; that's free fertilizer for you."

For conventional growers, the organic nitrogen is a relatively small part of the budget, but organic growers count on building this contribution through a combination of cover crops and compost.

The timing of fertilizer applications may be the easiest part of a management program because walnut trees need a relatively constant amount of nitrogen over a four-month period.

"Studies have shown that walnuts used only stored nitrogen through April," Jarvis-Shean said. "The nuts are adding about the same amount of nitrogen every month from May through August."

The best type of fertilizer to apply is a management decision that depends on many things, including how it will be delivered, what nutrients are needed along with the nitrogen, and susceptibility of the site to leaching, as well as cost.

"The right source is probably the stickiest one," Jarvis-Shean said. "There is not one source that is best in the Central Valley all the way from Red Bluff to Hanford. There are many fertilizers to choose from. If you're on a fertigation system, you want something you can inject. You need to ask if you broadcast, and whether you need calcium along with the nitrogen."

The materials with the lowest risk of leaching are ammonium sulfate, which is a high soil acidifier, or urea, which is not.

The highest potential for leaching comes with applications of calcium nitrate, which will, however, also deliver calcium to the crop.

Intermediate in their leaching risk potential, and in their soil acidification, are ammonium nitrate, calcium ammonium nitrate, or CAN-17, and urea ammonium nitrate, or UN-32.

In varying degrees, all of the sources of nitrogen are susceptible to leaching, which makes it important to keep the water and the nutrients in the root zone.

"When you pull out old walnut trees, most of the roots are in the top three feet," Jarvis-Shean said. "That's what you see when you pull out an orchard, and that's what we found in our studies. We want to keep our nitrogen where the trees can get at it, in the top three feet. This is as much an irrigation issue as a fertilization issue."

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