RESPONSE OF 'CADOUX' CLEMENTINE TO FOLIAR POTASSIUM FERTILIZATION: EFFECTS ON FRUIT PRODUCTION AND QUALITY

‘Cadoux’ clementine is a citrus fruit well appreciated by consumers. However, this cultivar produces fruits of small to medium size that are fairly accepted commercially. In a field study, we evaluated the effects of rates and frequencies of foliar potassium (K) fertilization, applied either as potassium nitrate (KNO3) or potassium sulfate (K₂SO₄), on fruit production (fruit size, weight and yield) and quality parameters (skin thickness, firmness, color index, maturity index, juice content, acidity and total soluble sugars). Concentrations of foliar fertilizer solutions were 5 and 8% (m/v) for KNO₃, and 2.5 and 4% (m/v) for K₂SO₄, applied either two or three times during fruit growth on orchards with three planting densities (D1: 6×6 m, D2: 5×6 m and D3: 6×3.5 m tree spacing). In all treatments, after potassium sprays, K (% DM) content increased in the leaves of clementine ‘Cadoux’ between 12 and 40% compared to their initial K levels while in the control this content dropped about 15% compared to its initial content (before K sprays). Fruit weight increased with K application rate and frequency. K fertilization treatments in three foliar applications showed the best percentages of fruits of extra size class, whatever the source of K (KNO₃ or K₂SO₄) or plant density. Foliar applications of K also increased fruit color, firmness and rind thickness. Fruit juice content, acidity and total soluble solids slightly increased with foliar K application. Raising K concentration and the number of foliar applications increased tree fruit production. In terms of efficiency of foliar K fertilizers, 4% K₂SO₄ in three applications resulted in maximum gain of fruits per kg of foliar fertilizer.