First Ratoon Results of Two Irrigation Experiments at Pongola

Introduction Approximately 53,000 acres of sugarcane are grown under irrigation in the low-rainfall regions north of Mtubatuba, mainly at Pongola and in the Eastern Transvaal. The number of acres which limited supplies of irrigation water should serve in these areas has not been clearly defined. The area in acres to be irrigated with one cusec of water pumped or available at the source, for a given proportion of the total time, is conventionally termed the "WATER DUTY ". In experiments on the Natal coast, maximum farm productivity on a Clansthal sand was achieved using 1 cusec per 200 acres, when water was available 24 hours per day and overhead irrigation was assumed to be 80% efficient. On a Windermere clay, total farm productivity increased up to a water duty of at least 442 acres per cusec in a dry year (Thompson et a14; Thompson and de Robillards). Economic assessment of the experimental results showed that maximum farm RETURNS were obtained when a limited amount of water was used to serve a larger acreage than could be irrigated if maximum yields of sucrose per acre were aimed at. When, however, the area of irrigable land is limited but water supplies are plentiful, maximum farm productivity will be achieved by irrigating to produce maximum yields of sucrose per acre. The plant crop results of the two irrigation experiments referred 'to in this paper were described by Thompson and Boyce6. In a season with a welldistributed summer rainfall of 27.5 inches, the application of 2 inches of effective water every 19 days to this crop, which was grown on a deep Makatini series soil, was sufficient to produce maximum yields of cane and sucrose per acre. This treatment represents a water duty of 182 acres per cusec with continuous delivery and 80% efficiency or 98 acres per cusec at Pongola, where water normally is available for only 5 days each week, and the furrow irrigation employed is assumed to be 60% efficient. Drying off sugarcane which initially had been irrigated to estimated maximum requirements, produced a statistically significant increase in both sucrose per cent cane and tons sucrose per acre. Although complete suspension of irrigation for two months prior to harvesting gave a significantly . greater yield of sucrose than gradual drying off, it was suggested that gradual drying off might prove to be superior if water applications just prior to harvesting were avoided. The rainfall for the first ratoon crop was welldistributed in summer but the total was only 13.92 inches compared with the mean of 25.1 inches at Pongola for the previous 17 years. The results presented here therefore, apply to a season of relatively low rainfall.