All of our tree crops require annual application of nitrogen fertilizer. All the other required elements usually are adequate in most of the soils in the county, especially the deep alluvial soils and nutrients other than nitrogen should only be applied if deficiencies have been seen or if leaf analysis shows a problem.
Nitrogen can only be absorbed into the tree through active roots. Few roots are active or growing during the winter, therefore winter is the poorest time to fertilize. Nitrogen fertilizer needs to be either incorporated into the soil or watered in soon after application to avoid losses from volatilization.
I personally don't like to see high rates of nitrogen applied in a single application on either almonds or walnuts. I believe the tree can better utilize split applications and there is less chance of burn on replants.
If you plan to apply a total of 200 pounds of nitrogen, a split application of maybe 125 pounds in the spring followed by the other 75 pounds during the summer is preferable. With micro sprinkler and drip irrigation systems several small applications per season is even more beneficial.
On young trees or on light textured soils split or multiple applications of nitrogen appear to be more efficient. Divide the total amount of nitrogen you want to apply into two, three or several equal applications. Irrigation injection equipment make this operation easy.
Young trees don't require as much N as older trees. The rule of applying one ounce of actual N per year of age of tree, I believe is very good. Some growers say it is conservative but I have seen burned trees from higher rates. That rate can be applied several times per season. On first year trees I also like to see a foot of growth before the first N applications.
On older walnut trees that are being heavily fertilized, I also prefer splitting applications. I have seen tree collapse from high nitrogen rates applied just before a hot spell and heavy water application. Splitting the nitrogen into 2 or 3 applications usually prevents this collapse.
How much fertilizer should I apply?
Ask yourself the following two questions: Last year was the growth adequate (remember, lack of water can also limit growth)? Was an adequate amount of N applied for the crop and tree maintenance? If it was, then start with this amount of N. Adjust the rate up or down depending on expected yield, desired growth and cover crop, then fine-tune the applications using leaf analysis.
Nitrogen is needed by trees and vines to provide growth and leaf development for proper tree health.
Adequate new growth is needed each year to provide the tree structure and fruiting shoots or spurs for next year's crop. It is prudent, though, not to over fertilize. Over-fertilization can cause excess growth, vigor and shade which then can actually decrease production by limb and leaf competition, shading of developing buds and creating non productive wood. Over-fertilization is also costly and leads to ground water contamination.
A grower can take several steps to maximize efficiency of N uptake and minimize N losses.
Apply N only when leaves are present and the tree roots are active.
Apply a uniform irrigation that is adequate to carry the N into but not past the root zone.
Because young fruit trees have a fairly constant N uptake, apply multiple applications of N throughout the growing season.
Mature trees need most of the N in early spring, therefore, a late summer application of part of the N before an irrigation will provide the tree with N for early spring growth. The rest of the N needed should be applied during the spring.
Fertigation has generally been very efficient in N applications.
Analyze leaves in July each year to fine tune N level to the orchard. Maintain the level in the adequate range.
If fertilizer is surface applied, disk or irrigate N into the root zone shortly after application.
Fertilize the tree not the cover crop. Evaluate how best to bypass the cover crop. This may be by applying the fertilizer to the herbicide sprayed strip, mowing, or cultivating the cover crop. Don't over irrigate. Nitrogen is soluble and moves with water. Excessive runoff of tail water or leaching will remove N.