NITROGEN AND POTASSIUM CITRUS TREE UPTAKE, FRUIT REMOVAL AND SEASONAL DISTRIBUTION IN THE ROOT ZONE UNDER IMPROVED FERTIGATION MANAGEMENT PROGRAM

摘要

Optimum application of nitrogen and potassium fertilizers to citrus grove requiresrninformation’s regarding existing soil N and K residues, dynamics in soil, and tree Nrnand K requirements. However, more understanding of the fate of nutrients in the treernroot zone soil matrix provides useful information to develop fertilization managementrnpractices with an adequate balanced N and K citrus needs that maximize tree uptakernefficiency by minimizing losses of water soluble nutrients. The objectives of this work,rnwas to assess the effects of nitrogen and potassium rates applied through driprnirrigation system, on the citrus trees nutrient uptake, fruit nutrient removal andrnseasonal dynamics of fertilizer-N and K in the root zone. A field trials were conductedrnfrom 2005 to 2007 in the citrus production area located in the North East of Tunisiarn(El Gobba, Cap Bon); on sandy soil with 25 years old ‘Clementine mandarin’ treesrn(C.reticulata) on ‘Sour orange’(C.aurantium) rootstock. Nitrogen and potassium ratesrnfrom 160 to 232 kg ha~(-1)yr~(-1) and 200 to 290 kg ha~(-1)yr~(-1), respectively, were applied asrnfertigation through a drip irrigation system. Irrigation was scheduled based on soilrnwater content monitoring by daily tensiometer readings at the root zone.rnFruit yield was positively associated with N (r~2=0.91~(**)) and K (r~2=0.84~(**)) rates,rnindicating that application of 192 and 200 kg ha~(-1)yr~(-1) of N and K_2O (N:K=0.9),rnrespectively, are required for Clementine mandarin trees to support optimal fruit yieldrnof 43 T ha~(-1)yr~(-1). These responses suggest that N and K tree use efficiency varied fromrn4.5 to 3.9 kg T~(-1) of Clementine mandarin fresh fruit, respectively. On the other hand,rntree nutrient removal indicated that both total N (r~2=0.97~(**)) and K (r~2=0.98~(**)) in fruitrnwere positively linear correlated with fruit load, and there was also, a strong linearrnrelation (r~2=0.97~(**)) between N and K in fruits, which supports the needs to maintainrn1:1 ratio between the rates of N and K_2O applications. Further more, nutrient uptake.