This study was carried out in the experimental orchard, Faculty of Agriculture, Assiut University, Egypt, to investigate the possibility of replacing nitrogen fertilization of Balady mandarin orchards by humic acid or seaweed extract during 2014, 2015 and 2016 seasons.
The experiment was arranged in a randomized complete block design with three replications per treatment, one tree each. Mineral nitrogen fertilization was splitted into three equal batches and added in March, May and August, while liquid nitrogen as organic form was added once in March. Seaweed extract spraying and humic acid as soil dressing were applied three times on mid of March, June and August.
The obtained results could be summarized as follow:
Using recommended nitrogen dose (RND) via liquid organic-N, as well as a combination of inorganic or organic plus humic acid or seaweed significantly increased the growth traits as well as N, P, K and total chlorophyll of leaves compared to the use of RND via mineral-N (check treatment).
The maximum values of these traits were recorded on trees that fertilized with the triple form (33% mineral and 33 g humic acid plus 33% seaweed). No significant differences due to fertilize via triple form using either organic or mineral form, as well as via double form using either humic acid or seaweed extract.
Fertilization via either double or triple forms could achieve a beneficial improvement to the nutrient status and C/N ratio of trees since it increased the yield/tree compared to the use of RND via mineral-N only.
Using double or triple forms improved the fruit quality in terms of increasing fruit weight, pulp percentage, total soluble solids, sugar and vitamin C contents and decreasing the total acidity compared to the use of RND via mineral-N source only. The best fruit quality was recorded due to triple forms.

It is evident from the foregoing results that replacing 25 to 66% of nitrogen fertilization by humic acid or/and seaweed extract improved the tree nutrient status, yield and fruit quality. In addition, it minimized the production costs and environmental pollution which could be occurred by excess of chemical fertilizers. Theremore, the growers are able to produce organic farming products.