Utilisation of Sludge from Ditches to Maintain Soil Nutrients and Increase Rice Yield in Rice-Shrimp Systems in Vietnam ^†

An integration of a shrimp production cycle into the traditional system of rice farming is an adopted practice by farmers due to saline intrusion and changes of environmental condition in Vietnam. Aquacultural pond sludge or biological waste has the potential to cause environmental pollution if it is not managed well. However, the sludge from shrimp ponds in rice-shrimp farming systems can provide nutrition and maintain soil fertility for the integrated rice crop production. A randomized block field experiment was conducted on a rice-shrimp farm to test the fertilizer value of sludge for rice over two consecutive seasons in Vietnam. Five treatments of sludge and fertilizer rates were applied in field experiments including control, sludge application alone, recommended fertilizer rate, reduced fertilizer rate, and combination of sludge and reduced fertilizer rate. The results consistently proved that the pond sludge can provide large quantities of plant available nutrients such as ammonium nitrogen, total nitrogen and organic matter. The use of sludge either alone or in combination with the reduced fertilizer rate produced optimal rice yields while fertilizer cost of rice production was reduced. This finding indicates that the use of sludge is able to save cost of production to rice-shrimp farmers and reduce adverse impacts of pond sludge released on the surrounding environment.