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 requires information’s regarding existing soil N and K residues, dynamics in soil, and tree N and K requirements. However, more understanding of the fate of nutrients in the tree root zone soil matrix provides useful information to develop fertilization management practices with an adequate balanced N and K citrus needs that maximize tree uptake efficiency by minimizing losses of water soluble nutrients. The objectives of this work, was to assess the effects of nitrogen and potassium rates applied through drip irrigation system, on the citrus trees nutrient uptake, fruit nutrient removal and seasonal dynamics of fertilizer-N and K in the root zone. A field trials were conducted from 2005 to 2007 in the citrus production area located in the North East of Tunisia (El Gobba, Cap Bon); on sandy soil with 25 years old ‘Clementine mandarin’ trees (C.reticulata) on ‘Sour orange’(C.aurantium) rootstock. Nitrogen and potassium rates from 160 to 232 kg ha~(-1)yr~(-1) and 200 to 290 kg ha~(-1)yr~(-1), respectively, were applied as fertigation through a drip irrigation system. Irrigation was scheduled based on soil water content monitoring by daily tensiometer readings at the root zone. Fruit yield was positively associated with N (r~2=0.91~(**)) and K (r~2=0.84~(**)) rates, indicating that application of 192 and 200 kg ha~(-1)yr~(-1) of N and K_2O (N:K=0.9), respectively, are required for Clementine mandarin trees to support optimal fruit yield of 43 T ha~(-1)yr~(-1). These responses suggest that N and K tree use efficiency varied from 4.5 to 3.9 kg T~(-1) of Clementine mandarin fresh fruit, respectively. On the other hand, tree nutrient removal indicated that both total N (r~2=0.97~(**)) and K (r~2=0.98~(**)) in fruit were positively linear correlated with fruit load, and there was also, a strong linear relation (r~2=0.97~(**)) between N and K in fruits, which supports the needs to maintain 1:1 ratio between the rates of N and K_2O applications. Further more, nutrient uptake.