Higher yields and improved root quality are essential for processing carrot growers to remain competitive. Nitrogen fertilizer applications range from 50 to 150 pounds N/acre with most between 50 and 80 pounds. More research is needed to clarify yield response to nitrogen, especially at higher rates, and the influence of N on such root characteristics as diameter, length, splitting, and rots. The response to N should be investigated at plant densities typically used for slicing and dicing. The objectives of this trial were to evaluate the effects of five N rates and 2 plant populations on yield and root characteristics and to evaluate the effects of banded P fertilizer at two rates of N.
A base fertilizer of 300 pounds/acre of 10-20-10 was disked into a Willamette silt loam on April 30, 1986, and 5-foot wide beds were formed by rotary tillage. 'Chantenay' carrot was seeded at 50 or 80 seeds/foot, three rows per bed, with a belt planter. Concentrated superphosphate was banded at 50 pounds P/acre beneath the seed row of appropriate plots. Plot length was 15 feet. Linuron was applied at 1.0 pounds/acre immediately after planting. The nitrogen variable was established on May 30 when half the remaining N was sidedressed as ammonium nitrate. The final N application was on July 3 when the largest roots were 0.5 to 0.8-inch diameter. The two seeding rates and five N rates were in factorial combination. Plots were in randomized block design with 4 replications. Nitrogen rates were 50, 85, 120, 155, and 190 pounds/acre. The P variable was added at N rates of 85 and 155 pounds/acre and the higher seeding density. Leaves were sampled for tissue analysis on July 30 and plots were harvested on October 16 and 17. Good roots were graded into two size categories (greater than 1.5 inch diameter and 5-inch length and all others). Each root was visually inspected for cracking and rots and defective roots were weighed and counted separately.
The yield of roots in the large size category did not vary significantly with N rate but decreased from 1.71 tons/acre to 0.24 tons/acre with the increase in plant population from 46/foot to 77/foot. Yield of large roots increased significantly with banded P but this may have been caused by a reduction in stand on the banded P plots. Total yield did not vary significantly with either N rate or plant population but the yield tended to be highest with 85 pounds N/acre. Phosphorus tended to reduce total yield because of the reduced stand.
Mean root weight did not vary significantly with N rate, when averaged over plant populations, but at the high plant population, root weight increased to a maximum at 155 pounds N/acre. Root weight decreased with increasing population and increased with banded P. Root length did not vary significantly with any treatment (data not shown).
The percentage of rotten or pitted roots increased with plant population and increased linearly with N rate (r2 = 0.225, p = 0.002) but decreased with banded P. Increasing plant population decreased the percentage of cracked or split roots but banded P had no effect on cracking. Cracking increased linearly with increasing N rate at high plant population (r2 = 0.0978, p = 0.049) but not at the low population.
The leaf tissue analysis confirmed that the higher rates of N were available to the plants as reflected in the increased N concentration of leaves from heavily fertilized plots.
These results indicate that the commonly applied rates of N are adequate for good carrot yields. The plant populations used in this experiment were higher than normal, however, and at lower plant densities the response to N rate may be different. Incidence of rot and other disorders also may be lower at reduced plant density.
Table 1. Effect of N rate, plant population, and banded P on yield and root characteristics of carrot, 1986 Roots/ Root yield (T/A) Mean root Cracked Rot Leaf N Treatment foot Large All wt. (g) % % % 50 lb N, low density 38.3 2.3 41.8 48.0 2.6 2.8 2.47 85 56.3 1.4 58.4 43.1 1.8 4.8 2.75 120 43.9 1.7 44.2 41.8 2.1 4.0 2.97 155 49.5 1.0 51.7 43.5 2.4 13.8 3.39 190 44.1 2.2 48.5 45.7 2.9 5.5 3.56 50 lb N, high density 85.7 0.6 44.5 22.4 0.8 1.7 2.37 85 82.2 0.0 47.5 23.9 0.8 6.5 2.65 120 78.1 0.0 48.6 27.0 1.2 9.4 2.84 155 68.9 0.4 47.0 28.8 2.4 12.6 3.20 190 68.4 0.1 44.6 27.6 2.3 24.7 3.11 85 lb N, high densityZ 55.3 2.4 38.2 31.8 1.4 2.8 2.73 155 lb N, high densityZ 40.9 1.6 38.7 40.4 1.0 8.0 3.21 LSD(0.05) 21.1 1.1 9.8 10.2 1.3 4.4 0.38 Significance: N rate NSY NS NS NS NS ** ** Plant density ** ** NS ** * * ** N x density NS NS NS * * * NS P rate * * * * NS * NS ZPhosphorus banded at planting. YNS,*, **: no significant differences among means; significant differences at 5% and 1% levels, respectively.