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, folk 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.