Interaction of pH amendment and potassium fertiliser on soil chemistry and banana plant growth

Abstract

Two glasshouse trials were conducted at Alstonville, NSW, to investigate the effects of rates of potassium (K) fertiliser and pH amendment on soil chemistry, plant nutrient content, and growth of tissue-cultured banana plants. The first trial used 5 rates of lime combined with 5 rates of KCl with plants in 1.5-L pots, and the subsequent trial used 4 rates of pH amendment (CaCO₃ + MgO) combined with 4 rates of K₂SO₄ with plants in 140-L troughs. The soil used in both trials was the A horizon of a Yellow Kurosol.

In the pot trial, very high rates of KCl caused a reduction in plant weight, and heaviest plants were produced at pH 4.5 5 (pH_Ca, measured with CaCl₂). Analysis of exchangeable and soluble cations indicated that increasing rates of added KCl displaced both Ca and Mg off cation exchange surfaces into the soil solution, from where they could be lost by leaching. Liming caused a marked decrease in plant Zn and Mn concentrations to levels that may have limited growth when soil pH_Ca exceeded about 5.0.

In the trough trial, plant weight was greatest at pH_Ca 4.3. Plant weight responded negatively to increasing K₂SO₄ at pH_Ca 3.5, positively at pH_Ca 4.3, and was generally unresponsive at the higher pH_Ca values of 5.0 and 5.8. A multiplicative Mitscherlich model was used to relate plant weight to leaf nutrient concentrations and indicated that plant weight was most limited by Mg deficiency at pH_Ca 3.5 and Mn deficiency at pH_Ca 5.8, with K availability having a moderate effect on growth at intermediate pH_Ca levels. Mg deficiency occurred at low pH despite regular foliar applications of this element, indicating the inefficacy of the foliar pathway for fertilising bananas with macro-nutrients. The study indicated that attempting to raise the pH_Ca of this soil above 5.0 for banana growing may not be appropriate, and due attention should be paid to Mg requirements and possible effects of liming on trace element availability.