When it comes to fertilizing young walnut orchards, nitrogen tops the list of critical nutrients.
In fact, for the first five or six years of age, it’s the only fertilizer material walnut trees typically require, according to Katherine Pope, University of California Cooperative Extension farm advisor for Yolo, Solano and Sacramento Counties.
Until then, most soils in California’s walnut-growing areas can meet the minimal needs of young walnut trees for phosphorus and potassium. And, except in unusual conditions, that’s usually the case with micronutrients, too.
At first, most of the nitrogen taken up by a young walnut tree is used to grow roots, branches and leaves. But, that gradually changes once the tree starts setting nuts. Then, nut production begins to account for an increasing share of the tree’s N demand.
“The nuts require much more nitrogen than any other nutrient,” Pope says.
In the first year, a walnut orchard planted at the rate of 65 trees per acre requires only about 10 to 20 pounds of N per acre. That’s about one-quarter pound of N per tree.
The amount of N already in the soil from previous crops and the level of nitrate N in groundwater used to irrigate the orchard may be enough to meet that need without added fertilizer N, Pope notes.
For example, applying three acre-feet of groundwater containing 10 ppm nitrate, a fairly common reading, provides 13 pounds of N/acre.
How to determine N needs
To determine N fertilizer needs in young walnut orchards, Pope offers this rough rule of thumb:
After the first year, double the rate for each of the next two seasons. (That works out to one-quarter pound N per tree the first year, one-half pound N per tree the second year and one pound N per tree the third year.)
For each of the fourth, fifth and sixth years, fertilize the orchard with 1 pound N per tree to meet the tree’s growth needs plus 40 pounds N per acre for each ton of nuts harvested from each acre.
Once walnut trees reach full production, Pope advises fertilizing orchards to replace the 30 to 40 pounds N per acre for every ton of nut production per acre.
However, not all the N you apply to an orchard will actually be used by the tree. That’s why she recommends adjusting the fertilizer rate to account for the loss in efficiency of N use, depending on how it was applied.
For example, estimating an orchard will produce 1.5 tons (3,000 pounds) of walnuts per acre in the coming year and using an application rate of 40 pounds per acre, the trees will need 1.5 tons per acre x 40 pounds of N = 60 pounds of N per acre.
With a fertigation system applying multiple times over the growing season, it’s reasonable to assume 60 percent of applied N will be taken up by a walnut tree. Divide 60 pounds by the efficiency factor of .6 to get the required fertilizer rate of 100 pounds of N per acre.
The lower efficiency that you’d get with broadcasting a single application, around 30 percent to 40 percent, works out to a fertilizer rate of 150 to 200 pounds per acre.
Slower N uptake
But, there’s more to fertilizing young walnut orchards effectively with N than using the correct application rate.
“These young trees take up nitrogen slowly over the course of the season as roots continue growing from leaf-out until late summer,” Pope says.
“If you put on a big slug of nitrogen in May and irrigate as the season goes on, much of that nitrogen can leach below and out of reach of the roots, which are in the top two or three feet of the soil profile,’ she says. “So, you’ll make the best use of nitrogen by applying it frequently at lower rates until late August.”
She cautions against continuing to add nitrogen after the start of September and encouraging trees to keep growing into fall. The idea is to get the trees to begin shutting down to avoid any damage from a late fall or early winter freeze.
Also, if broadcasting N in young orchards, Pope recommends adjusting your coverage area to match the smaller root systems of these trees.
“If you broadcast fertilizer over the middle of the rows as you would in a mature orchard, you can waste nitrogen that the roots can’t reach,” Pope says.
More information on fertilizing young and mature walnuts as well as other crops is available at: http://apps.cdfa.ca.gov/frep/docs/Guidelines.html.