Effect of high rates of potassium chloride fertiliser on banana leaf conductance, plant growth, nutrient concentration and root death under contrasting watering regimes

Abstract

Weak root systems, which predispose banana plants with a bunch to topple, are common in the Australian subtropical banana industry. Many banana growers consider the problem could be associated with the use of potassium chloride (KCl) fertiliser under dry soil conditions. We assessed the effect of high rates of KCl on banana leaf abaxial conductance (g_ab), plant growth including root weights, and whole plant nutrient concentration under a range of watering regimes using banana plants growing in troughs of soil in a glasshouse. The trial factorially combined watering after 15 (W₁), 30 (W₂), 60 (W₃) and 120 (W₄) mm evaporation from nearby bins with 4 rates of KCl—78 (T₁), 376 (T₂), 617 (T₃) and 796 (T₄) g/m².

At the end of the trial exchangeable potassium on the T₄ treatment was 5.8 cmol(+)/kg with 31% of this in the soil solution. A substantial proportion of exchangeable calcium, magnesium and sodium were also displaced into the soil solution. Banana leaf abaxial conductance peaked at 2–2.5 cm/s on the T₁ W₁ treatment combination, but decreased as soil matric potential decreased or KCl rate increased. The linear relationship for effect of KCl rate on g_ab indicated that g_ab was reduced by an average of 80% on the T₄ treatment across a wide range of matric potentials. Increasing KCl and withholding irrigation generally had similar effects on the concentration of nutrient elements in the plant. The high concentration of KCl in the soil did not appear to interfere with plant uptake of calcium and magnesium.

Plant size was reduced 42% on the T₄ treatment, but at harvest most plant components were reduced more by infrequent watering than by the highest KCl rate.

Dead root weight as a percentage of total root weight ranged from <30% for combinations of T₁ or T₂ with W₁ or W₂ to >50% for other treatment combinations. It was concluded that the interaction of dry soil conditions and reasonable fertiliser application rates was unlikely to contribute greatly to the death of banana roots in the field.