Kernel Development
in Pecan - A Function of Soil Water
Darrell Sparks
Department of Horticulture
University of Georgia, Athens 30602
In another
paper located on this web site, the role of water in the sizing of the
nut was discussed. Nut size is a direct function of soil moisture. In the
southeastern United States, soil moisture during the months of June, July,
and early August is critical as this is the period during which nut size is
determined. In the present paper, the role of soil moisture in the next stage
of nut growth, kernel development, is discussed.
Barring damage from insect
and disease, kernel development depends on two factors, crop load on the tree
and soil moisture. Under southeastern United States conditions, 2000 pounds
of nuts per acre is near the upper limit that the tree can fill with an acceptable
kernel (Fig. 1). Above 2000 pounds per acre, kernel percentage
steadily decreases. Decrease in kernel quality with increasing crop load is
normally not a major problem with Desirable, Schley, and Sumner. However,
Stuart is much more subject to over fruiting in some years. Over fruiting
with prolific cultivars (for example, Mahan, Success, Wichita, Moore, Cheyenne,
etc.) is often the rule. Over fruiting has spurred an interest in mechanical
fruit thinning (Reid et al., 1993; Smith et al., 1993; Sparks et al., 1995).
The role of soil moisture
on kernel percentage is dramatic (Fig. 2). Depending on
the level of soil moisture, kernel quality may range from very poor to excellent.
Additionally, the data in Figure 2, along with other data not presented here,
indicate that a critical period for kernel development in the southeastern
United States occurs during the first two weeks in September.
The correlative data in
Figure 2 is confirmed by experimental data (Table
1). Application of water at the beginning of the first two weeks in September
resulted in substantially better kernel quality compared with no irrigation.
However, application of 1.5 inches of water (0.75 inches per week for the
two-week period) did not result in maximum kernel quality. A high quality
Stuart should have a kernel percentage of about 48, not 46.5. Thus, insufficient
water was applied. The data in Figure 2 and other data not
presented here indicate the tree needs about 1.25 inches of water per acre
per week during the period of kernel development. Drip systems designed to
deliver 2400 gallons per acre per day provide only about one half of the water
needed (See "Nut sizing period in pecan and soil water," in this issue, for
a strategy for coping with submarginal irrigation systems).
The two-week period in
which water is especially critical for kernel development is about four to
five weeks before nut maturity. The four or five-week rule can be used to
detect the critical period of kernel development for pecans grown in geographical
regions other than the southeastern United States. Within the southeastern
United States, the critical two weeks will deviate somewhat from September
1-15. The deviation will vary depending on whether the growing season is early
or late but should not vary much more than plus or minus 5 days in most years.
Thus, September 1-15 remains a good target date.
Although the first two
weeks in September are especially critical for kernel development, this does
not mean that adequate soil water is not important outside this period. Adequate
soil moisture up to the time of nut maturity is absolute for proper shuck
opening. There have been years in Georgia where sufficient soil moisture was
available for kernel development but the shuck did not open properly because
the soil became dry after the critical period of kernel development had passed.
In such years, shuck removal ( which is not an easy task and particularly
so when the shuck has dried onto the shell ) is required before the nut can
be cracked and shelled. Improper shuck opening, due to inadequate soil moisture,
becomes especially important in pecan growing regions with a hot climate and
low elevation (southwest Texas, Visalia area of California, and parts of Arizona
and Mexico). In these regions, premature germination is a perennial problem
that is greatly accelerated by poor shuck opening (Sparks, 1993).
In the southeastern United
States, September is the most critical month of the pecan growing season.
This is the case because most of the kernel develops during this month. Thus,
the crop (kernel) is made or loss to varying degrees within a very short period.
The chance of losing the crop or else producing a crop with submarginal quality
is greatly increased in non-irrigated orchards by the fact that September
is the driest month of the year. Long term, there is a 50% chance that rainfall
received during the first two weeks in September will produce only marginal
kernel quality. Twenty-five percent of the time, rainfall during this period
is so low that submarginal quality is produced. Year in and year out, inadequate
soil moisture during the critical two week periods of kernel development is
the number one reason for poor quality. In Georgia alone, maintaining adequate
soil moisture during the kernel development would easily increase that state's
long term annual pecan production by 25%.
In summary, soil moisture
has a pronounced effect on kernel quality. If pecan orchards are to be profitable,
soil moisture must be maintained at an adequate level.
Table
1. Effect of irrigation on percentage kernel of Stuart pecans.
From
Alben (1957).
a Last irrigation (1.5") was August 31; no rain occurred in September.
Fig.
1. Change in kernel percentage of pecan with increasing crop load. From
Sparks and Weber (1993).
Fig.
2. Relationship of percentage kernel to rainfall during the first
15 days in September. From Sparks (1992).
Literature
Cited
Alben, A. O. 1957. Results
of an irrigation experiment on Stuart pecan trees in east Texas in 1956. Proc.
Texas Pecan Growers Assn. 36:23.
Reid, W., S.M. Husling,
M.W. Smith, N.W. Maness, and J. Whitworth. 1993. Fruit-removal time influences
return bloom in pecan. HortScience 28:800-802.
Smith, M.W., W. Reid,
B. Carroll, and B. Chary. 1993. Mechanical fruit thinning influences fruit
quality, yield, return fruit set, and cold injury of pecan. HortScience 28:1081-1086.
Sparks, D. 1992. Pecan
cultivars - the orchard's foundation. Pecan Production Innovations, Watkinsville,
Ga.
Sparks, D. 1993. Growing
pecans at a low elevation in a hot climate. Annu. Rpt. Northern Nut Growers
Assn. 84: 127-141.
Sparks, D., W. Reid, I.E.
Yates, M.W. Smith, and T.G. Stevenson. 1995. Fruiting stress induces shuck
decline and premature germination in pecan. J. Amer. Soc. Hort. Sci. 120:43-53.
Sparks, D. and R. E. Weber.
1993. Nut quality versus nut production in pecan. Annu. Rpt. Northern Nut
Growers Assn. 84:146-147.
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