Darrell Sparks
Department of Horticulture
The University of Georgia
Athens, GA 30602-7273

The premium pecan market demands a large nut that is well filled with kernel. Factors governing pecan nut volume and percentage kernel, have been reviewed (Sparks, 2000). Nut volume appears to be primarily a function of a single factor, soil moisture. In contrast, percentage kernel is principally controlled by three factors, crop load, nut volume, and soil moisture. Crop load does not appreciably affect kernel percentage on young trees just coming into production or on mature trees with a light fruit set. However, once crop load increases to the point that leaf area per nut becomes limiting, kernel percentage declines with crop load (Sparks and Weber, 1993). Preliminary data (Sparks, 2000) suggest percentage kernel decreases as nut volume increases. That is, a large nut is more difficult to fill with kernel than a small nut. Observations across cultivars suggest the same. For example, ‘Mahan' produces a large nut with poor kernel development; ‘Elliott' produces a small nut with excellent kernel development. At a given nut volume and crop load, kernel percentage is a direct function of soil moisture. Most of the kernel develops within a 4-week period. Soil moisture is especially critical during the first two weeks of this period. The current study reexamines some of these relationships.

Materials and Methods

The study site was near Leary, Georgia in a pecan orchard that had blocks of trees growing under contrasting conditions during the 2001 season. The contrasting conditions were flat vs. sloping topography, irrigation vs. no irrigation, light vs. increasing crop load. These conditions were expected to differentially affect nut volume and/or percentage kernel. The conditions afforded an opportunity to evaluate the effects of these factors on nut volume and percentage kernel and possibly reconfirm or substantiate proposals and recommendations made earlier (Sparks, 2000). Consequently, ‘Stuart' nuts from the contrasting blocks were sampled and analyzed for nut volume, percentage kernel, and other characteristics.

Topography affects runoff of water and thus soil moisture. For this reason, topography would be expected to indirectly affect nut volume and kernel percentage. Blocks 1 and 2 were used to test this contention (Table 1). Block 1 was sloping whereas block 2 was flat. The blocks were nonirrigated; thus, any differences in soil moisture between the two blocks would be due to differential runoff following rain. Rainwater from the sloping block did not run onto the flat block as the blocks were separated by a wide and natural drainage area. Crop load was similar and light in both blocks; therefore, differences in crop load did not complicate the evaluation of the effect of slope on nut volume and percentage kernel. As a result, differences in nut volume and percentage kernel between the two blocks could be attributed to differences in slope.

Blocks 3, 4, and 5 had a gradation in crop load (Table 1). This gradation was utilized to evaluate the effect of crop load on nut volume and percentage kernel. Soil moisture among blocks would be expected to be similar as the blocks were irrigated. Topography was flat in all three blocks, thus, minimizing differential soil moisture among the blocks due to runoff from either rain or irrigation. These conditions allowed a valid comparison of crop load on nut volume and percentage kernel.

Blocks 2 and 3 were used to evaluate the effect of irrigation on nut volume and percentage kernel. The flat topography and light crop load in both blocks (Table 1) allowed a valid comparison between nonirrigation and irrigation.

Results and Discussion

Nut length to width ratio and nut flatness were characters used to evaluate nut shape. These two characters did not vary among the blocks regardless of crop load, irrigation, or topography (Table 1). The uniformity for these characters indicates nut shape was similar among blocks and, thus, differences in soil moisture levels among blocks remained relatively steady during the nut sizing period, June to mid-August.

Nut volume was greater in irrigated than in nonirrigated blocks regardless of crop load (Table 1, blocks 1 and 2 vs. blocks 3, 4, and 5) and reconfirms the conclusion (Sparks, 2000) that nut volume is dependent on soil moisture. In addition, these data substantiate the thesis that nut volume is not influenced by crop load because nut volume did not decrease with increasing crop load (Table 1, blocks 3, 4, and 5). Within the nonirrigated blocks, nut volume and nut weight were greater in block 2 with flat topography than in block 1 with sloping topography (Table 1). The differences suggest less runoff and thus more efficient utilization of rainfall on flat than on sloping topography.

Percentage kernel decreased with increasing nut volume under light crop loads (Table 1, blocks 1, 2, and 3). These data confirm the proposal (Sparks, 2000) that a small volume nut is easier to fill with kernel than a large volume nut. The decrease in percentage kernel, in spite of a light crop, suggests the percentage kernel to volume relationship is independent of crop load. Thus, nut volume is a factor in itself in the ability of the tree to fill the nut.

Weight per nut increased with nut volume when the crop was light (Table 1; blocks 1, 2, and 3). Nut weight increased due to an increase in both shell and kernel weight. As indicated by the decrease in percentage kernel, kernel weight increased to a lesser extent than shell weight. However, weight per nut decreased as crop load increased from light, to moderate, to good (Table 1, blocks 3, 4, and 5, respectively). The decrease in nut weight was due to a decrease in both shell and kernel weight. Kernel weight decreased more than did the shell; consequently, percentage kernel decreased. The decrease in percentage kernel (blocks 3, 4, and 5) shows that filling a fixed nut volume (12.1 to 12.3 cc) with kernel becomes increasingly difficult with increasing crop load. Percentage kernel steadily decreased with crop load indicating that at some point increasing crop load will result in a commercially unacceptable kernel, as often occurs.

Shells were slightly thicker for nuts from trees growing in the flat vs. sloping topographic sites (block 2 vs. 1), on irrigated vs. nonirrigated sites (block 2 vs. block 3) and on trees with light vs. moderate crop load (blocks 3 and 5). Shell thickness varied by about 4% between blocks 1 and 4 which differed in crop load, topography, and irrigation (Table 1). Rainfall was moderate during the year of the study. During a year of severe drought, irrigation had a greater effect on shell thickness and thickness was increased 11% (Sparks, 2000). The current study suggests that during a season of moderate rainfall, shell thickness does not vary significantly with soil moisture and crop load even though shell thickness for a given cultivar can vary considerably among seasons and geographical areas (Smith et al., 1948; Sparks, 1992).

With regard to nut growth, crop load decreases nut weight but not nut volume. Nut size is usually reported as weight per nut and not as nut volume. For this reason, crop load is generally considered to suppress pecan nut size (Sparks, 1987). However, whether or not crop load decreases nut size depends on whether size is measured as volume or as weight.

Implications for Commercial Production

The influence of nut volume on percentage kernel has implications for commercial
production. Under nonirrigated conditions and a similar crop load, nuts produced on a slope will have a higher percentage kernel and, thus, better quality than nuts produced on a flat. Better quality will occur because nut volume will be less on a slope than on a flat. A smaller nut and better quality is expected on a slope regardless of the amount of rainfall during the nut sizing period (June - mid August). A smaller nut is anticipated because of the runoff effect. Under conditions of irrigation a similar effect is expected, again because of a runoff effect following either irrigation or rain. Although nut quality is expected to be better on a slope, long term production of nuts is greater on flats than on slopes (Middleton et al., 1968). Higher production occurs on flats than slopes because of two factors. Trees growing on flats produce a heavier nut and trees are larger resulting in more fruiting surface.

The data demonstrate that kernel percentage increases with decreased nut volume (Table 1) providing confirmation for a water management system called deficit irrigation (Sparks, 2000). Deficit irrigation is a practice of water management system proposed for pecan orchards equipped with drip irrigation systems that cannot deliver the 5000 gal/acre/day required during the period of kernel development. The purpose of deficit irrigation is to reduce water inputs during development of nut volume, thereby producing a small nut. Then at the time of kernel development, the irrigation system is set to deliver its maximum water capacity. Thereby a small nut with a high kernel quality is produced. Kernel development in the Southeastern United States occurs mostly in September (Sparks, 2000), the second driest month of the year. Deficit irrigation was designed for use mainly in drip irrigation orchards, but may be adapted for use in orchards with adequate irrigation. If crop load is excessive and a portion of the fruit is not removed by mechanical thinning, deficit irrigation should be employed during nut sizing and irrigation maximized during kernel development.

Deficit irrigation should not be used in years the crop is "off" but commercial. Instead, nut volume should be maximized by irrigation in order to partially compensate for less than desired number of fruit per tree (Sparks, 2000). When the crop load is light, there is little risk in maximizing nut volume. A large nut can usually be filled to a commercial acceptable level even with marginal soil moisture provided by under designed drip irrigation. The recommendation to maximize nut volume when crop load is light (Sparks, 2000) is supported by comparing nut volume vs. percentage kernel in blocks 1, 2, and 3 (Table 1). Kernel percentage did not fall below 48 which is within the range of a commercially acceptable ‘Stuart' nut (Sparks, 1992) in spite of the decrease in percentage with nut volume. Maximizing nut volume will increase kernel production by as much as 13% if crop load is light. This percentage is calculated from the increase in kernel weight per nut in a non irrigated block 1 vs. irrigated block 3, 4.0 g/nut vs. 4.6 g/nut, respectively (Table 1).

In summary, nut volume is a function of soil moisture and is unaffected by crop load. A small volume nut is easier to fill with a kernel than is a large volume nut. Filling nuts with a fixed volume becomes increasingly difficult with increasing a crop load. When a crop load is light, nut weight increases with nut volume due to increases in weight of both shell and kernel. As crop load increases, nut weight decreases due to a decrease in both shell and kernel weight. Nuts produced on slopes have smaller nuts and higher a percentage kernel than nuts produced on flats. The rationale for deficit irrigation in orchards with inadequate drip irrigation systems is that small nuts produce better kernel quality than large nuts. This can be accomplished by minimizing water inputs during the period of nut elongation and expansion and maximizing water during kernel development. Deficit irrigation is also recommended in sprinkler-irrigated orchards during years of excessively high crop loads if the excessive fruit is not mechanically removed. During seasons of light crop loads, nut volume should be maximized by irrigation to compensate as much as possible for the light crop year.

Literature Cited