Effect of Nitrogen Fertilizer Application Timing on Apple Trees and Fruit

Timothy J. Smith, WSU Extension
Dr. Steve Drake, USDA ARS- Wenatchee
COOPERATORS: Mike Mrachek, Soil Scientist, Cascade Consulting, Tim Meyer, Grower, East Wenatchee

This work was carried out starting in Fall 1993, and continued for four seasons of production. This effort was partially supported by a grant from the Washington Tree Fruit Research Commission. Post-harvest fruit quality evaluation was provided by Dr. Drake of USDA ARS- Wenatchee.

JUSTIFICATION:

Many mature apple trees in Washington are being managed at marginal nitrogen fertility levels in order to improve fruit quality. Standard recommendations for August leaf N vary by variety, but generally fall into the range of 2 to 2.2 percent of dry leaf weight. Growers find that maintaining leaf N level about 0.2 percent lower than the recommended level often improves fruit color and over-all eating quality. Low N status often leads to smaller than industry average fruit and a stronger than usual tendency toward alternate bearing.

Late summer nitrogen application timings have been reported to enhance the movement of the applied nitrogen into the flower buds and early season foliage. These reports did not outline the effect of this alternative N application timing on the physical aspects of the tree and the quality of the fruit.

Late summer tree and fruit growth prevents mechanical application of prilled fertilizers, as the fertilizer striking the fruit will cause surface marking. Application of N by fertigation through sprinkler leads to overgrowth of cover crops, as the fertilizer is applied relatively evenly throughout the orchard. Due to the impracticality of other application methods, it was also necessary to determine the practicality of application of the N product by hand.

None of the results obtained in this test should be extrapolated to orchards which are not being managed at borderline N deficiency for fruit quality enhancement. A similar trial carried out on well fertilized D’Anjou pears growing on deep, fertile soil resulted in no fruit, blossom, or tree growth differences, even on the trees that had no N applied for five years.

Also, these apple results were obtained in an irrigated orchard. Treated trees had naturally halted vegetative growth for the season, and were never under any significant moisture stress. Non-irrigated orchards are likely to respond differently. Late summer N application might lead to late season re-growth and subseqent reduction of cold tollerance.

ACCOMPLISHMENTS:

Starting in August 1993, a replicated N timing and rate trial was placed in a well managed commercial, mature, seedling root, 12 x 20, low nitrogen status Golden Delicious apple orchard. It is important to note that this orchard was marginally N deficient at the beginning of the trial. The soil was low in organic matter, and had very low residual N.

In an effort to bracket the growers past application rates, 0.3, 0.4, or 0.5 lbs. of actual N per tree were applied (55, 73 and 91 lbs/A). Application treatments included 0.5 lb/tree on August 20th, four weekly applications of 0.1 lb / tree starting on August 20th, 0.3 lb / tree on August 20th , .the growers method of 0.25 lb. in November and 0.15 lb. in May, and 0.15 lb / tree on August 20 plus 015 lb / tree in May. This final treatment developed serious nitrogen deficiency, and was converted to 0.4 lb N / tree in May of 1996 and 1997.

During the four growing seasons of 1994-97, the following measurements were taken: percent N and weight of flower clusters and early season leaves, percent blossom return, percent N in August leaves, yield, and various fruit quality measurements (skin and flesh color, firmness, acids, soluble solids, starch).

The test orchard has been alternate bearing for several seasons, and was in a light bearing year in 1993, when test N applications began. 1994 and 1996 were heavy cropping years, with orchard yields exceeding 100 bins / acre (100 tons / hectare, 2200 bushels per acre). 1995 and 1997 were very light cropping seasons, with yields of about 15-20 bins/acre. Nitrogen application timing had some effect on return bloom and subsequent yields. As current season yield has profound effect on percentage N in tissue tests, it has been difficult to compare test results in successive seasons. The alternate seasons of 1994 and 1996 with a heavy crop load, or 1995 and 1997 with light cropping may provide best comparative year data.

ACCOMPLISHMENTS SUMMARY:

Measurements of soil solution nitrogen indicated that none of the August applied nitrogen apparently moved deeper than the second foot of soil, and that soil solution N levels peaked during the two weeks after application and returned to pre-application levels by 4 weeks post-application.

As compared to the traditional November plus May N timing, the 0.5 lb. / tree on August 20 and the 0.1 lb / tree for four weeks starting August 20 resulted in greater return bloom in light cropping seasons, larger, higher % N blossom clusters, larger, higher % N early season leaves, similar or lower % N August leaves, higher yields on light cropping years, and no significant negative effect on fruit quality.

The most visible difference was in the size of flower clusters and early season leaves. Those trees treated in late August were much “improved” as compared to trees treated with similar rates of N, but at other times of the growing season. This early season boost in vegetative and floral size was not followed by higher August leaf N levels and greener fruit.

An August application of 0.3 lbs / tree had an effect roughly equivalent to 0.4 lbs / tree split between November and May.

Late summer application of nitrogen fertilizers did not induce any new vegetative tree growth in the months leading up to winter dormancy. August fertilized trees’ leaf color and leaf fall timing was similar to other orchards in the vicinity. The trial trees that were not fertilized in August dropped their leaves earlier than other orchards in the vicinity.

Nitrogen timing had no visible effect relative to winter cold damage following the -20 F temperatures in January, 1996. Some orchards in the test orchard vicinity were damaged severely, and many apple trees in the area grew poorly in 1996, and did not appear to resume normal tree growth until 1997.

The financial cost of hand application of nitrogen fertilizers on large trees (about 2 hours of labor per acre) is roughly the same as labor plus machinery costs for mechanical application.

Comparison of effect on tree and fruit summary

NOTE:
There was no practical difference in fruit quality caused by the various treatments. The fruit quality within the trial remained generally high, with seasonal variation. The crop load had greatest over-all effect on fruit quality. Over-cropping reduced fruit firmness, acids and sugars, and resulted in greener skin and flesh, illustrating the importance of maintaining moderate crop load each season (if possible).

RESULTS:

Numbers followed by the same letter should not be considered different from one another. Read the

comparisons only vertically, not from one season to another.

NITROGEN LEVEL IN FLOWER CLUSTER (%)

FLOWER CLUSTER WEIGHT (Grams/100 clusters)
PERCENT OF POTENTIAL BLOOM

NITROGEN LEVEL IN EARLY SEASON LEAVES (%)

WEIGHT OF EARLY SEASON LEAVES (Grams/100 Leaves)

LEVEL OF NITROGEN IN AUGUST MID-TERMINAL LEAVES (%)

APPROXIMATE YIELD (910 lb. Bins /Acre)

FRUIT FIRMNESS ( Lbs. At Harvest )

FRUIT STARCH LEVELS ( At Harvest )

FRUIT ACIDS ( At Harvest )

FRUIT SOLUBLE SOLIDS ( “Sugars” At Harvest )

FRUIT SKIN GREENNESS ( Higher Number is Greener )