OPTIMAL COMBINATION OF CHEMICAL COMPOUND FERTILIZER AND HUMIC ACID TO IMPROVE SOIL AND LEAF PROPERTIES, YIELD AND QUALITY OF APPLE (MALUS DOMESTICA) IN THE LOESS PLATEAU OF CHINA

Abstract

A two-year field experiment was conducted to investigate the effects of different application rates of chemical compound fertilizer (CF) (control, CF0; low, CF1; medium, CF1.5 and high, CF2 rates) without or with humic acid (no HA, HA0; with HA, HA) on soil chemical properties, growth of apple trees and their fruit yield and quality. With the increasing rates of CF, soil available nitrogen (N), phosphorus (P), and potassium (K) contents increased regardless of HA treatment. The soil organic matter (OM) content increased with the addition of HA. Most of the tree response parameters i.e. leaf N, P, K concentration, one-year-old shoot length, thickness and weight of 100 leaves, leaf chlorophyll index, fruit firmness, soluble solid and sugar, titratable acidity, and vitamin C contents, increased with increasing CF rates. Generally, the values of all these parameters as well as fruit yield peaked at CF1.5 and CF2 rates without and with HA, respectively. Addition of HA to CF increased the fruit yield by 12-35%. The results of this study demonstrated beneficial effects of HA addition to CF to improve soil properties, growth of apple trees, and finally fruit yield and quality. The CF combined with HA may be considered as a moderate and economical model of fertilizer regime for apple orchard in the Loess Plateau of China.