RESPONSE OF OLIVE TREES TO FOLIAR APPLICATION OF HUMIC SUBSTANCES EXTRACTED FROM LEONARDITE

The effect of foliar applied humic substances extracted from leonardit e 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 wa ter, but did not promote growth when applied to plants watered with a nutrient solution, although growth of fertilized plants was greater th an that of unfertilized ones. Under field conditions, foliar applicati on of leonardite extracts stimulated shoot growth and promoted the acc umulation of K, B, Mg, Ca and Fe in leaves, However, when leaf N and l eaf K values were below the threshold limit for the sufficiency range, foliar application of humic substances was ineffective to promote acc umulation of these nutrients in leaves.