Foliar application of humic acids on strawberry (cv Onda)

In June-bearing strawberries (Fragaria x ananassa L.), soil nutrient uptake has two major peaks during fall and spring growth. After the beginning of fruit maturation in late spring, nitrogen moves directly from leaves to fruit and its soil uptake is negligible. As a result, in organic farming, soil nutrients may be suboptimal and unable to support these short periods of high absorption. The present field trial was undertaken with the cultivar Onda in an organic production system and was designed to improve fruit quality using foliar application of humic acids applied between bloom and fruit harvest. An experimental, dilute mixture of humic and fulvic acids (Zymo) extracted from earthworm humus was compared with a mineral solution containing a similar amount of nitrogen. Foliar applications, performed weekly starting from bloom and until fruit maturity included: 1) control, sprayed eight times with tap water; 2) mineral solution, sprayed eight times; 3) Zymo, sprayed eight times; 4) Zymo, sprayed four times (only to the beginning of fruit maturity). Humic acid sprays significantly reduced fruit set and commercial production. Prolonged applications (8 weeks) stimulated higher photosynthetic pigment accumulation and greater photosynthetic efficiency starting from the 5th application at the beginning of fruit harvest. These same physiological parameters decreased two weeks after bloom in the control treatment, four weeks after bloom with the mineral solution treatment, and after five weeks in the humic acid treated plants. Prolonged humic acid application had a positive effect on fruit quality, reducing the number of misshapen and rotten fruits, and increasing the sugar content. These positive effects on fruit quality are likely due to an indirect positive physiological effect of the humic acid foliar applications on the whole plant and may not relate to any curative action.