Split application of humic acid significantly improves the yield, quality and nitrogen utilization efficiency of ‘Fuji’ apple

Objectives Split application times of humic acid were studied for the efficient use in ‘Fuji’ apple production.

Methods Five-year-old apple trees (Fuji 3/M26/Malus hupehensis) and ¹⁵N trace technique were used in the field experiment. There were four treatments: CK (No humic acid), one-time application (HA1), twice application (HA2) and three-times application of humic acid (HA3) in rate of 1.5 kg per tree. Destructive sampling of the whole plant was carried out at fruit maturity stage, the N content and ¹⁵N abundance in different organs were determined, the yield and fruit quality were investigated. The soil samples were collected in 0–120 cm dept of soil with an interval of every 20 cm, for the measurement of available N content and ¹⁵N abundance.

Results Compared to CK, the single fruit weight of HA1, HA2 and HA3 were significantly increased by 4.1%, 8.8% and 13.6%, and the fruit yield per tree were increased by 5.4% and 11.9%, 17.8%, respectively, the fruit hardness, soluble solid and soluble sugar contents and the sugar-acid ratios were all significantly increased, and the difference among the three treatments was significant with each other, with the effects of HA3 treatment better than HA2 and than HA1. The HA1, HA2 and HA3 treatments all improved the Ndff values, and the Ndff values in all the organs showed the same order as HA3 > HA2 > HA1 > CK. Compared to CK, the ¹⁵N utilization rates in HA1, HA2 and HA3 treatment were increased by 5.08–13.34 percentage points, while the ¹⁵N lose rate were decreased by 10.27–20.17 percentage points; the differences in both the ¹⁵N utilization and loss rates among the three HA treatments were significant, and in descent order of HA3 > HA2 > HA1. The ¹⁵N residual amount among the treatments were significantly different. ¹⁵N residual amount in 0–60 cm soil layers of the three HA treatments were significantly higher than those of CK, while ¹⁵N residual amount in 60–120 cm soil layers were opposite, and ¹⁵N residual amount of every layer in 0–60 cm showed the same order as HA3 > HA2 > HA1 > CK.

Conclusions Humic acid could improve the yield and quality of ‘Fuji’ apple, promote ¹⁵N uptake of plant, and reduce the leaching of fertilizer N into deep soil. Application of humic acid in late March, middle June and middle Augest could achieve the best effect.