Introduction
Higher yields and improved root quality are essential for processing carrot growers to remain competitive. Nitrogen fertilizer applications usually range from 50 to 100 pounds N/acre with most between 50 and 80 pounds. More research is needed to clarify yield response to N, especially at higher rates, and the influence of N on such root characteristics as diameter, length, splitting, and rots. Yield response to application of banded P fertilizer is also poorly understood.
In a trial in 1986, carrot yields tended to be greatest at 85 pounds N/acre, but did not vary greatly with N rate. Incidence of soft rot and root cracking increased with increasing N rate. Yield of large roots increased with banded P but this may have been caused by reduced stand on the banded P plots. The objective of the 1987 trial was to evaluate the effect of five N rates, both with and without banded P fertilizer, on yield and root characteristics.
Methods
A base fertilizer of 500 pounds/acre of a ION-8.7P-8.3K fertilizer was disked into a Willamette silt loam on 27 April, 1987, and five-footwide beds were formed by rotary tillage. 'Nantes' carrot was seeded on 28 April at 15 seeds/foot, three rows per bed, with Planet Jr. seeders. Concentrated superphosphate was banded at 50 pounds P/acre approximately two inches beneath and one inch to the side of the row of appropriate plots at planting. Plot length was 15 feet. Linuron was applied at 1.0 pound/acre immediately after planting and was reapplied at 0.5 pounds/acre on 1 June. Fluazifop was also applied on 1 June at 0.25 pounds/acre.
The N rate variable was established on 5 June when ammonium nitrate was sidedressed at 30, 60, 90, or 120 pounds N/acre on the appropriate plots with four replications of each treatment in randomized block design. The P variable was restricted to three rates of N, for a total of eight treatments. Roots were harvested on September 24 and graded into three size categories (over 2-inch shoulder diameter, 1 to 2 inch, and less than 1 inch). Each root was visually inspected for cracking, and rots and defective roots were counted separately.
Results
The yield of roots in the large size category increased significantly with increasing N rate but most of the increase occurred between 50 and 110 pounds N/acre (R2 = 0.80 linear, 0.89 quadratic). The increased yield was due to larger number of roots in this size category (Table 1). The increase in numbers of large roots also occurred mostly between 50 and 110 pounds N (R2 = 0.84 linear, 0.92 quadratic). Yields of other size categories did not vary significantly with N rate. Total yield varied significantly with N rate, reaching a maximum at 140 pound; N/acre (R2 = 0.70 quadratic, P = 0.04). The total number of roots recovered did not vary with N rate. Mean root weight also increased with N rate to a maximum at 140 pounds N/acre (R2 = 0.72 quadratic, P = 0.03). Unlike 1986, the percentage of cracked and rotten roots did not vary significantly with N rate, but there was a strong trend for greater numbers of cracked roots at the higher N rates. The percentage of cracked roots was about tenfold higher in 1987 than in 1986, reflecting reduced plant population and the resulting increase in root size and maturity at harvest.
Banded P had no effect on any component of yield or quality. This is in contrast to 1986, when banded P reduced both yield and the percentage of rotted roots. Banded P reduced stands in 1986 but not in 1987.
These results indicate that, although root yields at low plant density may be increased by applying higher N rates, the increases are not of large magnitude and may be offset by increases in cracking. There was a trend, although not statistically significant, for fewer defect-free roots at higher rates of N.
Table 1. Main effects of N rates and banded P on yield and root characteristics of carrot, 1987 No. Mean % No. of Root yield (tons/acre) roots/ No. large root cracked % soft- defect-free Treatment Large Med. Small Total foot roots/ft. wt. (g) roots rotted roots roots/foot N (lb/acre) 50 2.4 27.7 2.9 33.0 11.4 0.3 131 21.3 8.0 8.8 80 3.5 31.9 2.6 38.0 12.1 0.5 139 19.8 5.6 9.8 110 6.6 28.7 2.5 37.8 11.0 0.9 148 32.2 10.4 8.1 140 7.0 34.6 1.7 43.3 11.4 0.8 167 33.6 15.8 7.8 170 7.2 29.7 3.4 40.3 12.0 1.0 150 33.6 8.8 7.8 Linear *z NS NS NS NS * NS NS NS NS Quadratic * NS NS * NS ** NS NS NS NS P (lb/acre) 0 5.5 28.6 2.9 37.0 11.5 0.7 142 30.1 9.4 8.2 50 4.9 29.4 2.3 36.6 11.0 0.6 146 25.9 7.9 8.3 NS NS NS NS NS NS NS NS NS NS Z*, **, NS: significant differences at 5% and 1% levels, and no significant differences, respectively.