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How Much Is Enough?

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A New Look at Optimum Nitrogen Levels

Is the conventional wisdom regarding nitrogen (N) critical values for almonds valid? Are annual applications of N necessary in mature almond orchards growing in coarse-textured soils in nitrate-sensitive areas?

These are two of the questions that Steve Weinbaum of the Department of Pomology at UC Davis hopes to answer by the end of 1995. His project, funded by the Almond Board of California, also seeks to develop management guidelines for maintaining almond productivity, while reducing the leaching of fertilizer N below the root zone in orchards growing in coarse-textured soils. Working with Steve on various phases of the project are Dave Goldhamer, Wes Asai and F. Niederholzer.

Although final data is still being developed, the project team offers some insights that may be useful to growers planning their fertilization regimes for the 1995-96 growing seasons. The two test plots chosen in 1990 were ten-year-old orchards on Hanford sandy loam in nitrate-sensitive areas of Stanislaus County. Four different rates of N fertilization have been tested: 0, 125, 250 and 500 pounds of actual N per acre, one-third applied in April and two-thirds in October.

Residual N

A considerable amount of residual N was present in the soil and irrigation water at the research site. These conditions now only affected this research project, but similar conditions are faced by almond growers in many areas of the state. Consequently, Steve developed the conversion table below for growers to use in computing the amount of residual N in their orchard soils and irrigation water. Knowing this will help a grower decide how much fertilizer N to apply (or not apply).

Weinbaum notes, "The lack of significant yield reduction in unfertilized trees in 3 of 4 years of our tests indicates that annual fertilization is not necessarily required to maintain productivity under these conditions...sufficient N is available from other sources to meet tree demand."

Weinbaum also noted a decline in yields throughout the Salida orchard regardless of the amount of N applied. He concludes that "annual yield fluctuations are not necessarily linked to N availability. Reduced flower formation and poor pollination are among the possible factors that can limit yield."

Other fluctuations complicate the issue as well. The team noted that a certain amount of annual variation in leaf N concentration may occur that is not directly linked to the amount of N applied. In the Ceres orchard for example, the leaf N concentration of unfertilized trees varied from 2.69 percent down to 2.29 percent and back up to 2.51 percent between 1990 and 1994.

Enough's Enough!

Another point made by Weinbaum is that there is virtually no difference in leaf N concentration between trees receiving a N application of 250 pounds per year and those receiving 500 pounds. He concludes, "This means that at higher levels of available soil N, tree capacity for N uptake is probably saturated." Moreover, applying excess N not only wastes fertilizer and money, it can get you in trouble, he adds: "N applied in excess of the trees' capacity to use it accumulates in the soil and becomes vulnerable to loss (leaching) in coarse-textured soils."

The Magic Number

The ideal range for leaf N concentration, to both maintain yield and minimize leaching, appears to be between 2.3 percent and 2.5 percent says Weinbaum. The fertilizer rate needed to achieve these values will vary from orchard to orchard.

This test suggests that any variation below 2.3 percent leaf N will affect yields. Tree yields were reduced when leaf N dipped below 2.2 percent and preliminary analyses suggest that yields may even be reduced at leaf N between 2.2 percent and 2.3 percent.

Think Ahead

It pays to think ahead to the next crop when planning a fertilizer program, says Weinbaum. "Flower number per acre determines the yield potential in almonds, and flowers begin development 6 months prior to bloom and a year before fruit maturity and harvest," he notes. "We believe, therefore, that leaf N concentrations determined in July may be linked more directly to yield in the subsequent year than in the current year." His tests showed a significant yield reduction the following year in unfertilized trees.

Nitrogen Conversion Table

Pounds of N applied per acre for selected levels of nitrate concentration in irrigation water

N Concentration In Crop Water*

Equivalent pounds of N/acre for selected irrigation rates 

ppm NO3-N ppm NO3  2.5ac/ft  3.0ac/ft  3.5ac/ft  4.0ac/ft 
2.26 10 15.4 18.4 21.5 24.6
4.52 20 30.8 36.8 43.0 49.2
6.78 30 46.1 55.2 64.5 78.8
9.04 40 61.5 73.6 86.0 98.4
11.30 50 76.9 92.0 107.5 123.0
13.56 60 92.2 110.4 129.0 147.6
15.82 70 107.6 128.8 150.5 172.2

*Agricultural laboratories may report their results of water analyses as either N03-Nitrogen (ppm N) or ppm N03.

Go Figure

In case your N-concentration or irrigation-rate data doesn't match what's in the above table, Weinbaum offers the following conversion factors to use in calculating the amount of N applied annually in your irrigation water:

1 ppm N03-N in the water = 2.72 lbs N/acre foot of water applied

1 ppm of nitrate (N03) = 0.614 lb N/acre foot of water applied

To convert ppm N03 to ppm N, multiply ppm N03 by 0.2259 (the atomic weight of the N atom is 22.59 percent of the atomic weight of N03). Therefore, ppm N03 x 0.2259 = ppm N.