Proper Application of Fertilizers May Lead to More Consistent Olive Harvests

A recently pub­lished study has found that alter­nate bear­ing char­ac­ter­is­tics in olive trees can be mit­i­gated by appro­pri­ate nutri­ent sup­ple­ments.

The study, which was car­ried out in con­junc­tion by sci­en­tists from the University of Sfax in Tunisia and the University of Bari in Italy, found that well-timed and sys­tem­atic appli­ca­tions of cer­tain fer­til­iz­ers can pre­vent yield fluc­tu­a­tions while min­i­miz­ing resource inputs.

Alternate bear­ing is a com­mon trait of fruit trees and widely con­sid­ered to be a home­o­sta­tic mech­a­nism. In ​‘on’ years, trees pro­duce a higher yield of fruit, which depletes their reserves of nitro­gen, phos­phate and potas­sium. Fruit trees then pro­duce less fruit in ​‘off’ years in order to expend those same nutri­ents on seed pro­duc­tion and leaf and shoot growth.

“Our data of the four-year trial could be very use­ful to reduce alter­nate bear­ing in olive trees by means of an appro­pri­ate fer­til­iza­tion man­age­ment,” Saida Bedbabis, the lead researcher from the University of Sfax, wrote in the study. ​“Nutrients would be applied to olive trees when required, tak­ing into account if trees are in ​‘on’ or ​‘off’ years.”

According to the study, the appli­ca­tion of cer­tain fer­til­iz­ers at par­tic­u­lar times of the year is also a more cost-effi­cient and envi­ron­men­tally friendly way of mit­i­gat­ing the effects of alter­nate fruit bear­ing than the alter­na­tive: super-inten­sive crop­ping.

“Correct fer­til­iza­tion is an appro­pri­ate way to face and reduce alter­nate bear­ing whereas the super-inten­sive sys­tem is a dif­fer­ent way of cul­ti­va­tion,” Giuseppe Ferrara, the cor­re­spond­ing author of the study, said. ​“With an appro­pri­ate sched­ule of fer­til­iza­tion, we try to face the real requests of [what nutri­ents] the trees [need to pro­duce olives]. The super-inten­sive sys­tem forces the trees to pro­duce at a max­i­mum rate.”

The study tracked the sea­sonal changes in nitro­gen, phos­phate and potas­sium in the leaves and stems of trees in a Tunisian olive grove over four years.

The low­est nitro­gen con­cen­tra­tions in the trees occurred in the spring of ​‘on’ years and sum­mer of ​‘off’ years.

During ​‘on’ years, the study rec­om­mended fer­til­iz­ing olive trees with nitro­gen in the late win­ter, then again dur­ing the spring and once more after the har­vest. During ​‘off’ years, fer­til­iza­tion dur­ing the spring and again in the autumn helped to sus­tain veg­e­ta­tive growth.

The low­est con­cen­tra­tions of phos­phate were mea­sured dur­ing the sum­mers of both ​‘on’ and ​‘off’ years. Meanwhile, the low­est con­cen­tra­tions of potas­sium were mea­sured dur­ing the win­ter of both years of the cycle.

“Our data con­firmed that… a sup­ply of phos­phate in late autumn or win­ter and par­tially in the late spring or sum­mer [is nec­es­sary], par­tic­u­larly in an ​‘on’ year,” Bedbabis wrote. ​“A sup­ply of potas­sium in January or February can be nec­es­sary for sup­port­ing the root activ­ity and suc­ces­sive shoot growth of ​‘off’ years and the fruit devel­op­ment of ​‘on’ years.”

The study was car­ried out in Tunisia, which has a hot and dry cli­mate for most of the year. However, Ferrara said the con­cept is applic­a­ble any­where but must be tai­lored to a spe­cific loca­tion.

“It [the fer­til­iza­tion sched­ule] can fit for dif­fer­ent sit­u­a­tions but it is for sure most effec­tive for arid con­di­tions where tem­per­a­ture are higher all year long,” Ferrara said. “[In the study] we only indi­cated the appro­pri­ate times for fer­til­iza­tion with no doses since these depend on the yield, soil char­ac­ter­is­tics, tree vigor and vari­ety and har­vest time.”

Ferrara said he plans to con­tinue study­ing olive fer­til­iza­tion, but also plans to con­duct sim­i­lar stud­ies with grape vines and pome­gran­ate trees.

• Acta Scientiarum Polonorum


• UC Davis Division of Agriculture and Natural Resources