Response of olive trees to foliar application of humic substances extracted from leonardite

The effect of foliar applied humic substances extracted from leonardite on olive growth was studied in greenhouse experiments using cuttings and young olive plants, some of which were irrigated with a nutrient solution, and also in field experiments selecting mature trees growing in the drylands or under irrigation. Foliar application of leonardite extracts to young olive plants stimulated shoot growth when they were growing without the addition of mineral elements to the irrigation water, but did not promote growth when applied to plants watered with a nutrient solution, although growth of fertilized plants was greater than that of unfertilized ones. Under field conditions, foliar application of leonardite extracts stimulated shoot growth and promoted the accumulation of K, B, Mg, Ca and Fe in leaves, However, when leaf N and leaf K values were below the threshold limit for the sufficiency range, foliar application of humic substances was ineffective to promote accumulation of these nutrients in leaves.