The use of humic substances in agriculture has increased in recent years, and leonardite
has been an important raw material in the manufacture of commercial products rich in humic and
fulvic acids. Leonardite-based products have been used to improve soil properties and to help plants
cope with abiotic and biotic stresses. In this study, the effects of two commercial leonardites and
an organic compost, in addition to a control treatment, were assessed for pot-grown olive plants
over a period of fourteen months on soil properties, tissue elemental composition and dry matter
yield (DMY). Three organic amendments were applied at single and double rates of that set by the
manufacturer. The study was arranged in two experiments: one containing the seven treatments
mentioned above and the other containing the same treatments supplemented with mineral nitrogen
(N), phosphorus (P) and potassium (K) fertilization. Overall, organic compost increased soil organic
carbon by ~8% over the control. In the experiment without NPK supplementation, N concentrations
in shoots and P in roots were the highest for the compost application (leaf N 12% and root P
32% higher than in the control), while in the experiment with NPK supplementation, no significant
differences were observed between treatments. Total DMY was ~10% higher in the set of treatments
with NPK in comparison to treatments without NPK. Leonardites did not affect significantly any
measured variables in comparison to the control. In this study, a good management of the majority
of environmental variables affecting plant growth may have reduced the possibility of obtaining
a positive effect on plant nutritional status and growth from the use of commercial leonardites.
The leonardites seemed to have caused a slight effect on biological N immobilization. This is not
necessarily an advantage or a drawback; it is rather a feature that must be understood to help farmers
make better use of these products.
This research was funded by the Foundation for Science and Technology (FCT, Portugal)
and FEDER under Programme PT2020 for financial support to CIMO (UIDB/00690/2020). The
research was integrated in the activities of the operational group “Novas práticas em olivais de
sequeiro: estratégias de mitigação e adaptação às alterações climáticas”, funded by PT2020 and
EAFRD (European Agricultural Fund for Rural Development).
