Nitrogen Use within Young Orchards

Newly planted orchards have different nitrogen needs than mature orchards. Young trees, which should be considered as trees that yield less than 2000 lbs/acre (2.2 metric ton/ha), require a growth factor and a crop use factor. Inefficiencies of application should also be incorporated, which will consider the area outside of the tree’s rootzone  that is receiving water and fertilizer . Taking these variables into account, an equation for young tree nitrogen use would be as follows:

Rate = (GF + YF) + ((GF+YF)*(1-RF)),in which the growth factor (GF) is a fixed variable, and the yield factor (YF) and root system factor (RF) are known variables.Growth factor. The growth factor is relatively constant, on a year-to-year basis. Multiple research projects have suggested that trees will? utilize about the same amount of nitrogen in the first and second growing season. These studies have shown that young trees will utilize around 3-5 ozs of nitrogen per tree (80-140g/tree) in the first growing season, and 4-6 ozs/tree (100-175 g/tree) in the second growing season. This amount is provided as value per tree because increasing densities will utilize more nitrogen. An example of changing nitrogen amounts is shown in table 1.Table 1: The effect of tree density on nitrogen use rates for the first growing season of a newly planted orchard.

Spacing Example (tree x row)

Tree Density/acre

Rate of Nitrogen (lbs/acre) per acre at 4 ozs/tree

Spacing Example – meters (tree x row)

Tree Density/Ha

Rate of Nitrogen (kg/ha) per acre at 110 g/tree

15’x22’

132

33

4.5 x 6.6

337

38

12.5’x22’

158

40

3.75 x 6.6

404

45

10’x22’

198

50

3.3 x 6.6

459

51

12’x21’

173

44

3.6 x 6.3

441

49

Yield factor. The yield factor is more straight forward. This is a similar value that is used within mature trees. Within young trees, yield should be estimated and then multiplied by 0.085 to determine the nitrogen needed for the crop. For example, an orchard that will yield 500 lbs/acre will need 42.5 lbs of nitrogen for the crop.Root System factor. Since young trees have smaller rootzones, they aren’t able to reach all of the nitrogen that is applied through the irrigation system. To account for this, additional nitrogen will be needed.  The variable is the ratio between tree root area and the area of fertilizer application. To estimate this value, estimate the diameter of the tree, and count the drip emitters within the diameter and divide by total drip emitters per tree. For microsprinklers, the area of the tree within the wetting pattern divided by the total area would be used. Since the trees will actively grow throughout the year, this value will increase as the tree’s root system increases.Example from our orchards. Within our orchards, we target 3.5 ozs of nitrogen/tree (100 grams/tree) during the first growing season, and 5.25 ozs of nitrogen/tree (150 g/tree) during the second growing season. At a density of 144 trees/acre (355 trees/ha), the trees will require an estimated 32 lbs of nitrogen/acre (36 kg/ha) of in year one, and 48 lbs of nitrogen/acre (53 kg/ha) in year two. Based on our growth conditions and density, I assume our efficiency to be 50% for the first year, and 80% for the second year. Since there is no crop in the first two growing seasons, we can estimate the total nitrogen need as 48 lbs/acre and 58 lbs/acre (54 kg/ha and 64 kg/ha) for years one and two, respectively. The difference between these two years is not that great due to the improved efficiency of application.For year three, it is a little more complicated. We assume that the tree will utilize between 5.25 ozs/tree in addition to crop needs. If we are forecasting 500 lbs/acre, we will apply 42.5 lbs/acre for the crop (500 lbs of kernel* 0.085 lb nitrogen per lb of kernel) and 48 lbs for the growth factor, totaling 90 lbs/acre. Due to increased tree size and the higher density of planting, the efficiency factor is assumed to be 100%.We have had success with these rates. Tree growth was within expectations (figure 1). This success may be due to our method of application as we are applying small doses of nitrogen two times a week. Our trees have shown the appropriate vigor and leaf tissues have been within expected range of 3.0-3.5% for nonbearing, and ~2.5% for bearing. Residual soil nitrate ranged between 700-1000 ppm at year end/winter sampling across the three yearsBased on these successes, we will continue to utilize the program described, modifying it based on soil texture, residual nitrate, and any source of nitrogen applied to the field.