Field study.

Studies were established on drip-fertigated and plastic-mulched bell peppers during 2006 in the coastal plain (eastern region) and the western Appalachian Mountains of North Carolina. Experiments were conducted at the Horticultural Crops Research Station in Clinton, North Carolina (lat. 35°N, long. 78.3°W, altitude 158 ft), and at the Mountain Horticultural Crops Research and Extension Center in Fletcher, North Carolina (lat. 35.4°N, long. 82.5°W, altitude 2067 ft). Bed dimensions were 8 in high × 5 ft wide, and beds were pressed and covered with polyethylene black plastic mulch (1.25 mm thick). Drip irrigation tubes (standard 5/8-inch drip tape) were installed 1 inch deep in the bed center under the mulch film. Drip irrigation was used to establish the crop and to maintain soil moisture at −25 kPa or greater to a 6-inch depth. Recommended production practices for fresh-market bell peppers were followed (Sanders, 2006). Bell pepper cultivars were chosen for this experiment based on differences in the soil characteristics at the two sites. Soil temperatures were not recorded.

‘Heritage’ peppers were planted in a loamy sand soil (Norfolk fine-loamy, siliceous thermic Typic Paleudults) in Clinton. The average monthly and total rainfall during the 2006 growing season was 3.25 and 15.71 inches, respectively, with an average temperature of 71.2 °F (April to July). Seedlings about 3 inches tall were mechanically planted in double rows spaced 12 inches apart on beds 30 ft long on 18 Apr.

‘Aristotle’ peppers were planted in a fine sand loam soil (Comus fine-loamy, mixed mesic Fluventic Dystrochrepts) in Fletcher. The average monthly and total rainfall during the 2006 growing season was 4.96 and 26.28 inches, respectively, with an average temperature of 69.9 °F (June to September). Seedlings about 3 inches tall were manually planted in double rows spaced 12 inches apart on beds 30 ft long on 22 June.

A block design was used with eight treatments and five replicates in Clinton and four replicates in Fletcher (Table 1). Granular and liquid methylene urea formulations were investigated. The treatments included a granular formulation (G-43) consisting of 43% water-soluble N with 30% of the N in the form of methylene urea polymers and 70% urea. Additional liquid treatment formulations consisted of a 5% N formulation (L-5) and a 30% N formulation (L-30) with 60% of the N as methylene urea polymers and 40% of the N as urea. All treatments received 500 lb of 10N–4.4P–16.6K preplant fertilizer to provide 50 lb/acre N from ammonium nitrate (NH₄NO₃), 50 lb/acre phosphorous (P) from ammonium phosphate [(NH₄)₃PO₄], and 100 lb/acre potassium (K) from potassium chloride (KCl). Preplant fertilizer accounted for the increase of 50 lb/acre N in total N applied for all treatments. G-43 was applied in a band at rates of 100 and 150 lb/acre N about 3 inches below the soil surface and 15 inches from the center of the plot before the plastic mulch was installed. L-5 was applied at 150 lb/acre N in Clinton and 100 lb/acre N in Fletcher because of differences in soil texture between these sites. L-30 was applied at 100 and 150 lb/acre N at both sites. A combination treatment that included L-30 and KNO₃ was applied at 150 lb/acre N in Clinton and 100 lb/acre N in Fletcher. In this treatment, only 40% of the N came from L-30, with the remaining N from potassium nitrate (KNO₃). It received the same preplant fertilizer as was applied to the other treatments. Adjustments were made to balance the K supplied by KNO₃. L-30 and L-5 treatments were applied weekly, starting 3 WAT, through drip irrigation over a 6- and 9-week period, respectively. Two conventional fertilizer treatments (a mix of potassium nitrate and calcium nitrate) were injected during the season to supply 150 or 250 lb/acre N for total N rates of 200 (NC-200) and 300 lb/acre N (HI-300). The extension recommended N rate is 200 lb/acre N (Sanders, 2006) and reflects the N rate typically used by commercial bell pepper growers in North Carolina. The conventional treatments were applied via fertigation for a 12-week period starting 3 WAT. Potassium sources (KCl or KNO₃) were added to granular and liquid fertilized plots through the drip tube to keep the ratio of 1N:0P:2K from the beginning of the study until the 12th week. Potassium fertilizer rates were based on soil test results taken before planting. Weekly irrigation and fertigation were applied using independent lines for each treatment.

Table 1.

Fertilizer treatments applied on bell pepper at two locations in North Carolina.

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Plant quality and yield measurements.

Bell pepper plants were rated twice during the season at 4 and 8 WAT, according to vigor, canopy shade, and overall quality in the eastern experiment (Clinton). Categories were established based on the following scale: 1 = very good stand (100% growing plants, 100% canopy shade); 2 = good stand (90% growing plants, 90% canopy shade); and, 3 = fair stand (80% growing plants, 80% canopy shade). These data were transformed to square roots before statistical analysis. Bell peppers were harvested three times at 1-week intervals at both sites. Mature fruit from 20 of 60 plants were harvested from the center of each plot. Fruit number and weight per plot and per class were recorded. Bell peppers were mechanically sized and graded into U.S. Fancy, U.S. No. 1, and unmarketable according to the U.S. Standards for Grades of Sweet Peppers (U.S. Department of Agriculture, 2005). Cull grade consisted of fruit that were misshapen or damaged by insects or diseases. Early marketable yield (the first harvest), total marketable yield, cull yield, and total yield were recorded. Nitrogen use efficiency (NUE) is defined as the number of pounds of fruit produced by each pound of added N (Hutchinson et al., 2003). In this study, NUE was calculated by dividing the total yield and marketable yield by the N rate (lb/acre) for each treatment.

The General Linear Models procedure of analysis of variance in SAS (version 9.1; SAS Institute, Cary, NC), was used to test differences between treatments. Treatments means were separated using the Fisher's protected least significant difference (lsd) test. Data from each location were analyzed independently because of differences in cultivar, planting time, and environmental conditions.

Clinton.

Statistical differences among treatments were found for total, marketable, and early marketable yield at the Clinton site (Table 2). G-43 (150 lb/acre N), L-30 (150 lb/acre N), and NC-200 produced the lowest total and marketable yield. These treatments had statistically lower yield than G-43 (200 lb/acre N), which produced the highest yield. All slow-release treatments (granular and liquid) at 200 lb/acre N produced 17% to 38% more total yield than NC-200. Early marketable yield for L-30 and G-43 at 200 lb/acre N accounted for 44% and 46%, respectively, of the total yield. No significant differences among treatments were detected during second or third harvests.

Table 2.

Effect of fertilizer treatments on pepper yield and plant quality at Clinton, NC.

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No statistical differences were found among fruit size classes. U.S Fancy bell peppers were the main contributors of marketable yield in this study. The percentage of culls was generally low across all treatments (Table 2). The most frequent reason for culling fruit was sunscald. Sunscald occurs when fruit are exposed to high levels of sunlight, probably because of a poor foliage cover (Roberts and Anderson, 1994). Canopy shade was poorest with treatment G-43 (150 lb/acre N; Table 2). NUE values for all slow-release treatments were higher and, in some cases, significantly different from the conventional fertilizer treatments (Table 3). L-30 (150 lb/acre N) and G-43 (200 lb/acre N) differed by more than 53 lb of fruit per pound of fertilizer compared with NC-200. This suggests that more N was made available by slow-release fertilizers to meet the needs of the plant for growth and fruit production. Similar results have been reported for slow-release fertilizers on potato (Solanum tuberosum; Hutchinson et al., 2003) and tomato (Koivunen and Horwath, 2005).

Table 3.

NUE for total and marketable pepper yield at Clinton, NC.

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Fletcher.

No significant differences were detected between treatments in total and marketable yields, fruit size, and percentage of cullage (data not shown). The low rate of slow-release fertilizers performed as well as NC-200, probably because of the better retention of water and nutrients in loam soil at this location. Ristimaki (2000) reported similar results in fertigated onion (Allium cepa), tomato, and watermelon (Citrullus lanatus), with equal or slightly higher yields using methylene-urea slow-release fertilizer. U.S. Fancy bell peppers were the main contributor to yield, which represented at least 60% of the marketable yield in all treatments. The percentage of fruit rated culls was generally low (4.6%–8.3%) among treatments.

NUE values for all low rate (150 lb/acre N) slow-release liquid treatments were higher and, in some cases, significantly different from the conventional fertilizer treatments for total and marketable yield (Table 4). L-30 (150 lb/acre N) treatments differed by more than 64 lb of fruit per lb of fertilizer compared with NC-200. Marketable NUE values for NC-200 and HI-300 were low, at 190 and 132, respectively.

Table 4.

NUE for total and marketable yield at Fletcher, NC.

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