Studies on the irrigation of sugar-cane

Abstract

Eleven replicated irrigation trials on sugar-cane, together with some preliminary observation tests, were conducted in the Bundaberg region of southern Queensland during four seasons, 1961–62 to 1964–65. Irrigation control was through actual measurements of potential evapotranspiration of cane (E_tc) or grass (E_tg) with lysimeters. In conjunction with the lysimeter work, pan evaporation (E_p), mean air temperature (T_a), and sufficient data to calculate Penman's potential evaporation (E₀) were recorded.

E_tc consistently exceeded E_p, and E_tg slightly exceeded E_p in summer. E_t was fairly well correlated with E_p and T_a, but less well correlated with E₀. The inclusion of T_a in regressions of E_t on E_p or E₀ improved these regressions; T_a presumably reflects the additional external source of energy which was causing the high E_t values.

Yield responses to irrigation were very different on two soil types: on a deep red volcanic loam they were 10 times as great as those on a heavy alluvial clay. The reasons for the poor response on the alluvial clay appear to be threefold: (i) a permanently moist (but saline) subsoil maintained some growth in dry weather; (ii) growth in soils kept moist by irrigation virtually stopped when heavy rain fell; (iii) heavy rain falling on previously dry soil resulted in an abnormal increase in growth rate, an effect which was associated with a dramatic increase in soil mineral nitrogen.

The most satisfactory indicator of water use by cane was E_tg; growth rate data indicated that 100% E_tc is required, but yield data suggested that 66% E_tc is adequate. The latter value approximates to 100% E_tg or 100% E_p in the summer months.

Growth, and therefore growth response to irrigation, is very severely curtailed when mean temperatures fall below 70°F; soil temperature appears to be the critical factor. Growth rate was too susceptible to factors such as temperature to be of any value as an indicator of the need for irrigation.