Twenty-First Annual Technical Report
pH is above 7 or total alkalinity is above 50 mg l-1.
Aquaculture ponds normally are drained for harvest, bottoms are allowed to dry, and liming materials are applied. The liming material frequently is blended with the bottom soil by tilling.
In spite of the widespread use of liming materials, studies to compare the effectiveness of different methods of application have not been completed. In this study, conducted on fish farms in Brazil and South Africa, three methods of applying agricultural limestone to ponds were compared. These methods were as follows: (1) application over the pond water surface,(2) application to the bottom of the dry pond, and (3) application to the bottom of the dry pond followed by tilling.
Methods and Materials
Trial in Brazil
The experiment was conducted at the Santa Barbara Fish Farm in Itupeva District, Sao Paulo State, Brazil. The farm is situated at coordinates 23o 11.211' South and 47o 02.013' West and is near the town of Itupeva which is about 50 km North of Sao Paulo. Soils in the area usually are of sandy clay or clayey texture and members of the ultisol (red-yellow podzols) and oxisol (red-yellow latosols) groups. The pond bottoms were clayey consisting of 30 to 40% clay, 40 to 50% silt, and 10 to 20% sand. The water supply for the farm is a small, natural stream, and all ponds are located on the flood plain of this stream.
Ponds used in this study were rectangular, 1,000 m2 in area with average depth of 1.0 m. The ponds were each stocked with 320,000 to 450,000 of Oreochromis nilotica fingerlings. Feeds and fertilizers were applied according to the judg
ment of the farm manager. The commercial fish feed contained 32% crude protein and each pond received 170 to 200 kg per month. The farmer did not keep records of fertilizer additions or provide data on fish production. However, based on an assumed feed input of 2% body weight per day, standing crops of fish were probably around 2,800 to 3,200 kg ha-1.
Nine ponds were available for use, but it was not possible to initiate the entire experiment at one time. One replication of each of the three treatments was installed in groups of three ponds in December 2001, 7 March 2002, and 18 May 2002. Each group of ponds was in production for about four months before fish were harvested. The agricultural limestone dose was based on the lime requirement of the pond bottom soil and ranged from 4,000 and 5,000 kg ha-1. The treatments were as follows: agricultural limestone applied over the water surface at the beginning of the crop; agricultural limestone spread uniformly over the bottom soil before filling ponds with water; agricultural limestone spread uniformly over the bottom soil and soil tilled to a depth of 10 cm with hand tools before filling ponds with water.
South Africa
The experiment was conducted at Q-Koi Farm (Pty) Ltd. on the Boesmansrivier Farm in the Breede River Valley in the Western Cape Province of South Africa. The farm is situated at coordinates 34o 02 south and 19o 57 west and is 20 km south of the town of Bonnievale and 180 km east of the city of Cape Town. Soils in the area of the farm are of the Glen Rosa series. These are highly weathered alfisols. The terrain is gently sloping and the pond bottoms were constructed of a soil that contained a large amount of fractured shale. The soil, excluding the shale fragments, was 26.8% clay,
Table 1. Data on pond sizes and depths, treatments, dates of treatments, lime requirements, and doses of agricultural limestone (ag lime) for experiment conducted at Q-Koi Farm near Bonnievale, South Africa.