Seasonal variations in the crop water-use coefficient of banana (cultivar ‘Williams’) in the subtropics

Abstract

At Levubu, in the subtropical north-eastern Transvaal, soil samples were used to determine, gravimetrically, the crop water-use coefficients ( E t E o ) of cultivar ‘Williams’ banana in summer, autumn, winter and spring. Sampling was conducted in a mature plantation at a density of 1666 plants ha −1, irrigated by undercanopy sprinklers, and replicate soil samples were taken after and before successive irrigations. Total water loss ( E t) from a profile of 500 mm (5 × 100 mm divisions) was related to total evaporation loss ( E o) between sample times. Seasonal E t E o varied from 0.57 in winter to 1.01 in summer at a 54% level of available water depletion. Vertically, approximately 40% of the total water loss was from the upper 100 mm of soil and 80% from the upper 300 mm, indicating a shallow extraction pattern for banana roots. In summer, a schedule based on 80% depletion of available moisture induced proportionally more water extraction at deeper levels (300–500 mm) than did a 16% depletion. In a root laboratory, a high correlation was found ( R 2 = 84%; P < 0.001) between minimum soil temperatures at 200 mm depth and the rate of ‘Williams’ primary root extension. The reduced E t E o in a subtropical winter could be partly explained by low soil temperatures leading to poor root growth and water extraction potential. The implications of these results are discussed in relation to irrigation practices.