Darrell Sparks¹

Among pecan growers there is a wide variance in timing fertilizer applications but, mostly, fertilizer is applied during dormancy, near budbreak, or about a month following budbreak. In Georgia, some growers split the application with one application in late April and the second application in June. Recently, there has been renewed discussion as to the best time to apply fertilizer in pecan orchards (Goff et al., 2001; Wood, 2001). Goff et al. tentatively concluded late fertilizer applied during “on-crop” years can improve nut quality and the return crop. These workers further concluded that August application favors return crop while September or October applications result in higher kernel quality. Wood theorized, without citing supporting data, that most of the nitrogen applied to “off” trees should be applied in the spring while during the “on” year most of the nitrogen should be applied in late summer near the beginning of kernel filling. The purpose of this paper is to summarize two long term studies on timing fertilizer application in pecan orchards that apply in today’s production. The results in the two papers have been available for a long time. The papers were published in1947 and 1964.

The first study (Hunter and Hammer, 1947) reports the results of timing fertilizer application in Georgia to ‘Moore’ over a 10-year period from 1935 to1944. Timing treatments and subsequent responses are summarized in Table 1. Responses are presented as an average for “on” years and for “off” years. “Moore’ was an excellent choice for the study because it is very prolific and, hence, a severe alternate bearer.

Fertilizer, compared with no fertilizer, dramatically increased pounds of nuts produced during “on” years (Table 1) by a factor of 2.0 to 2.5. Highest production occurred when fertilizer was applied in 1/6 increments. Second highest production occurred when half of the fertilizer was applied in April. Production from fertilizer applied at other times of the growing season, including late season fertilization, was very similar to trees in which all the fertilizer was applied in February.

During the “off” years, the most striking effect, other than low production as such, was that trees fertilized in 1/6 increments had the lowest production (Table 1). Production during “on” years was highest from this treatment and, thus, the lower “off” production simply reflects enhanced alternate bearing and not a direct effect of time of fertilizer application. There was no definitive evidence that time of fertilizer application minimizes alternate bearing. In fact, minimal alternate bearing occurred in the non fertilized plots; that is, the percentage decrease from the “on” years was less than from any fertilizer treatment.

Number of nuts produced per tree is a more fundamental measure of fertilizer response than is pounds of nuts per tree. The reason number of nuts is a better indicator is because nuts per tree continue to increase with nitrogen application whereas pounds of nuts per tree may not. That is, at excessive nut numbers, production is reduced due to decreased nut quality (Sparks, 1989). For this reason, response to timing of fertilizer application was examined as number of nuts produced per tree during “on” years (Table 2).

Number of nuts per tree was highest when fertilizer was applied throughout the year (Table 2). When application was based on specific months, April application produced the largest number of nuts. As application was delayed from April through October, number of nuts produced steadily decreased. Fertilizer application during dormancy produced the lowest number of nuts (17% less than the 1/6 increments treatment).

During “on” years, the best nut quality (nuts per pound) occurred from the treatments (April and 1/6 increments) producing the highest production (Table 1). This effect is contrary to the norm as nut quality typically decreases with increasing production. Lack of a decrease in nut quality with elevated production suggests a promotive fertilizer effect from the April and 1/6 increments treatments. Best nut quality was obtained with no fertilizer but this was at the expense of suppressed nut production. There was no definitive evidence that late season fertilization improves nut quality during the “on” year.

Nut quality during the “off” years was substantially better than during the “on” years (Table 1). The higher quality was due to low production during the “off” years. Nut quality during “off” years was not affected by time of fertilizer application.

In the second study, Hunter (1964) compared nitrogen application in December vs. February to ‘Stuart’ in Georgia (Table 3). The treatments were initiated in December 1956 and February 1957. The trees were deficiency in nitrogen at the beginning of the study. Nitrogen application greatly increased production (2.6 times). There was no effect from time of application. As with ‘Moore’, the best nut quality occurred with no fertilizer. However, as was also the case with ‘Moore’, the better quality was at the expense of low nut production. Beginning in1958, the December treatment was discontinued while trees in the February treatment continued to receive varying amounts of nitrogen. Four years later, production and nut quality were compared (Table 4). These results show that nitrogen in the soil and/or trees remained partially effective for at least 4 years after the December treatment was discontinued. Because of a strong carryover effect, the chances of getting a dramatic increase in nut production from extra nitrogen application (Goff et al.,2001) in an orchard with existing adequate nutrition (as determined by leaf analysis) are indeed slim. The carryover effect is especially obvious when a nitrogen study is initiated in a pecan orchard that has been well fertilizer in the past. Years are required to induce nitrogen visual deficiency symptoms in the non fertilized plots. The same is true for phosphorus, potassium, and magnesium.

In summary, several conclusions and implications can be drawn from the data in Tables 1-4.

1) When pecan trees are deficient in nitrogen, nitrogen application results in a dramatic increase in nut production.

2) However and because of the nutritional carryover effect, the expectation of obtaining a dramatic increase in nut production by application of above normal amounts of nitrogen in an already well fertilized orchard is unrealistic.

3) Fertilizer can be effectively applied any time of the year, but some application dates are more effective than others.

4) Under humid conditions, application of fertilizer during dormancy is the least effective time. Presumably, a portion of the fertilizer is leached from the soils via winter rains.

5) Multiple fertilizer application throughout the growing season is the most effective fertilization program. Multiple applications, mainly nitrogen, can be well managed via injection in flood, microjet or drip irrigation. The same is true for sprinkler systems; however, ammonium nitrogen will burn pecan foliage in the path of the sprinkler.

6) Under humid conditions of southeastern United States, and if application is limited to one time per year, April is the most effective time to apply fertilizer. April application is especially suited for non irrigated and drip irrigated orchards (assumes fertilizer is not injected via irrigation) because rains in April are usually adequate to move the fertilizer into the root zone.

7) Although late season fertilizer applications are effective, they are less effective than multiple or April applications. Consequently, there is no support for the recommendation (Goff et al., 2001; Wood, 2001) that late application will increase production, return bloom, or nut quality over that obtained from earlier application dates.

8) In sprinkler-orchards, the current Georgia practice of splitting fertilizer into two applications within the mid-April to June interval has merit. The April application is made for general maintenance of the tree’s nutrition. Decision for the June application is based on fruit set. If fruit set is commercial, the second half of the fertilizer is applied with the primary goal of increasing return bloom. If fruit set is not commercial, no additional fertilizer is added for the season. During the “off” year, the odds are good that there will be ample or excessive fruit set the following year even if no additional fertilizer is applied. Consequently, additional fertilizer during an “off” year further increases the chance for an excessive return crop with the associated challenge of trying to fill the extra nuts with kernel.

9) Annual applications of a constant amount of fertilizer will not minimize alternate bearing, instead alternate bearing is intensified. In theory, reduced nitrogen application during the “off” year should minimize alternate bearing but the theory lacks proof. The best means of ensuring annual nut production is management of crop load by mechanical fruit thinning. If mechanical fruit thinning is properly employed, fertilization is simplified. Crop load becomes more or less stable, thus, minimizing the need to fertilizer based on crop load.

10) Although a mixed fertilizer will help maintain nut quality, kernel quality is primarily governed by crop load and by soil moisture during the kernel development period, not by fertilizer application. If soil moisture is deficient and/or if fruit load is excessive, extra fertilizer, regardless of the amount applied, will not prevent poor kernel quality. The best means of ensuring kernel quality is to mechanical fruit thin during seasons of excessive crops and, regardless of crop load, maintain adequate soil moisture during kernel development (Sparks, 2001).

---

Literature Cited

Goff, B., M . Nesbitt, and C. Browne. 2001. Late season fertilization: An exciting new development for the pecan industry. Proc. Southeastern Pecan Growers Assn. 94:91-93.

Hunter, J. H. 1964. Time of applying nitrogen to pecan trees in sod. Proc. Southeastern Pecan Growers Assn. 57:18-22.

Hunter, J. H. and H. E. Hammer. 1947. The results of applying different fertilizers to the Moore variety of pecan over a ten-year period. Proc. Southeastern Pecan Growers Assn. 40:10-32.

Sparks, D. 1989. Pecan nutrition - A review. Proc. Southeastern Pecan Growers Assn. 82:101-122.

Sparks, D. 2001. Managing pecan nut growth. Proc. Southeastern Pecan Growers Assn. 94:129-147.

Wood. B. W. 2001. Managing nitrogen in pecan orchards. Proc. Southeastern Pecan Growers Assn. 94:153-159.