Calcium is a highly required mineral for increased peanut yield and grade (Cox et al, 1982). Lower concentrations of calcium in the peanut pegging zone can result in low pod yield, grade, and germination. There is little or no transport of calcium from the roots to the leaves and back to the pod via the phloem through the peg, thus calcium uptake must be through the developing pod (Wiersum, 1951; Sumner et al., 1988). This implies that two conditions must occur for calcium uptake to the plant or to the pod; 1) there must be enough soil moisture in the pod zone for water uptake by roots and pods, and 2) calcium must be present in sufficient concentrations to dissolve into the soil water for uptake into the roots and pods. If one or both of these conditions does not occur, then yield and grade may be reduced. Additions of calcium sulfate (gypsum) and irrigation by overhead sprinkler systems assure that neither water nor calcium is deficient. Various researchers have shown the importance of calcium rate and timing with relationship to yield of both runner and Virginia type peanut (Adams and Hartzog, 1991; Adams et al., 1993; Jordan et al., 2000; Wiatrak et al., 2006; Grichar et al., 2004). Most research was accomplished using overhead irrigation or non-irrigated conditions. Currently, there is no research information available describing pod yield and calcium fertilization when irrigated with subsurface drip irrigation (SDI).

Irrigation systems using SDI have the capability of frequently supplying water to the root zone while reducing the risk of cyclic water stress that is typical of other irrigation systems or nonirrigated production (Phene et al., 1987). Subsurface drip systems are adaptable to variations of field shape making them an important consideration in the Southeast. In recent years, drip irrigation has been expanded with good success on field crops such as corn (Zea mays L.) (Lamm et al., 1997, 2001; Mitchell, 1981; Mitchell and Sparks, 1982; Powell and Wright, 1993), cotton (Gossypium hirsutum L.) (Camp et al. 1997; Sorensen et al., 2004), and peanut (Jordan et al., 2002; Sorensen et al., 2001a, b; Zhu et al., 2004). Irrigation SDI laterals have been installed at 0.2 and 0.3 m soil depths (Bucks et al., 1981; Camp et al., 1989; Phene et al., 1987; Tollefson, 1985) on cotton, corn, fruits, and vegetables. Drip laterals have been spaced at 1, 2 and 3 m apart with various crops with yields decreasing as lateral spacing increases greater than 2 m (Camp et al., 1997; French et al., 1985; Lamm et al., 1992; Powell and Wright, 1993). Bosch et al. (1998) showed that 1.83 m lateral spacing in corn-peanut rotations had the highest returns on sandy soils in Virginia-North Carolina region.

SDI systems may be efficient in supplying water and nutrients to the root zone, but may not be efficient in supplying water to the peanut peg zone where water is needed to move calcium into the developing fruit. The objective of this research was to determine the efficacy of current calcium recommendations when using SDI systems by documenting yield response and mineral concentration of peanut plants to three calcium application rates when irrigated with SDI.

A SDI system was installed in the spring of 2000 near Sasser, GA on Tifton loamy sand (fine-loamy, kaolinitic, thermic Plinthic Kandiudults) with 2 to 5% slope. The SDI system had drip laterals buried at 0.3 m deep and spaced at 0.91 m with emitters spaced at 0.3 m. Water flow rate was 5.6 L/min per 100 m or 1.0 L/hr per emitter. Peanut was planted following cotton in adjacent sites during the 2004 and 2005 growing seasons.

The experiment was a factorial design with three calcium (gypsum) rates and four peanut cultivars replicated four times. Calcium rates were none, 560 and 1120 kg/ha. Peanut cultivars selected were C99R, Georgia Green (GG), NCV-11 (NCV), and GA-O2C (O2C). Individual plots were 1.83 m wide by 12 m long for a total of 48 plots.

Land preparation was the same both years with disk harrowing two times followed by an experimental bedder (USDA-ARS-National Peanut Research Laboratory, Dawson, GA) used to make 1.83 m beds. Peanut was planted in a twin-row orientation using a commercial vacuum type planter (Monosem planter, ATI Inc, Lenexa, KS). Twin-row orientation was planted at 1.17 m outside rows by 0.7 m inside rows with 0.22 m between the twin-rows with four rows on a bed. Seeds were planted at rates recommended for reducing the risk of Tomato Spotted Wilt Virus (Brown et al., 2004). Pest management practices, i.e., disease, insect, and weed control, followed University of Georgia Agricultural Extension Service recommendations for peanut production (Prostko, 2004). Decisions to apply pest management practices were determined by field scouting. Irrigation water was applied daily based on replacement of crop water use for peanut described by Stansell et al. (1976) except when precipitation amounts exceeded estimated water use.

A composite soil sample (0 to 30 cm) was taken in early spring for fertility recommendations. Initial pH for 2004 and 2005 was 6.3 and 6.2, respectively. The initial Ca concentration was 405 (2004) and 397 mg/kg (2005). Gypsum was applied by hand over individual rows at first flower or 40 days after planting (DAP), whichever came first. Soil and plant samples were taken 80 DAP and analyzed for mineral nutrition (Waters Agricultural Laboratories, Inc. Camilla, GA). A composite of 6 sub-samples in each plot were taken at the 0 to 5 cm soil depth. Three plants were randomly selected in each plot for chemical analysis. Plants were partitioned into leaves, pegs, and pods. At the 80 DAP sampling period, the pods were immature such that peanut kernels were not separated from the hulls before analysis.

Cultivars were dug individually at maturity determined by the hull scrape method (Williams and Drexler, 1981). Yield rows were dug with a 2-row inverter and combined with a stationary plot combine (Kingaroy Engineering Works, Ltd., Kingaroy, Australia). Pod yield was determined after being mechanically dried, weighed, and adjusted to 7% moisture (wet basis). Since Virginia type peanuts require different screen openings compared with runner type peanuts for market grade, farmer stock grade and kernel size distribution for all peanut cultivars were determined using screens specified in USDA grading procedures (USDA, 1993) for a runner type peanut. It is understood that Virginia type peanut will not have a valid market grade with this procedure while runner type peanuts will have a valid market grade. Mineral analysis was performed on kernels that rode an 8.3×19 mm slotted screen. Germination, vigor (cold test germination), and calcium concentrations were determined from a 250 g kernel subsample (riding a 6.4×19 mm slotted screen) by the Georgia Department of Agriculture (Tifton, GA).

Plant, kernel, and soil analysis along with pod yield and farmer stock grade were analyzed using a general analysis of variance procedure (Statistix8, 2003). Tukey's HSD least significant difference range test was used to show differences among means (P ≤ 0.05) when ANOVA F-test showed significance.

Precipitation during the peanut growing season was less in 2004 (494 mm) compared with 2005 (695 mm) (Fig. 1). Cumulative irrigation amount shows less water applied through the drip system in 2005 (136 mm) compared with that applied in 2004 (181 mm). The disparity between irrigation totals for 2004 and 2005 was not as great as the difference between precipitation amounts for those same years. Cumulative precipitation patterns in 2005 shows about a 30 day period with little precipitation followed by a large precipitation event (day 152 to 182). This pattern of drought followed by precipitation events occurred both years and is typical of the southeast. In 2005, one large storm pattern accumulated over 180 mm in a ten day period (day 184 to 194) which was more than 25% of the total precipitation for the growing season.

Pod yield for peanut cultivars in this research are comparable with those reported by Wiatrak et al. (2006) and Jordan et al. (2000). Kernel calcium levels were higher than calcium levels described by Grichar et al. (2004). Germination and vigor percentages were within acceptable limits. Fields that are irrigated with SDI have similar pod yield, farmer stock grade, and quality as reported fields with overhead irrigation; however, there was no yield response to increasing Ca rates in this study. Based on this study, there is no evidence indicating a change in calcium recommendations for subsurface drip irrigation on peanut is needed.