Evolution of the Agronomic and Environmental Phosphorus Status of Soils in Mauritius After a Seven Year Sugarcane Crop Cycle

The intensive application of P fertilisers in sugarcane production has probably resulted in P levels which could be of concern to freshwater quality in Mauritius. Only very small amounts of P must be lost from the soil to create a P concentration in freshwater ecosystems likely to cause eutrophication. To prevent eutrophication, total P should not exceed 0.05 mg L −1 in streams entering lakes/reservoirs as per directives of the United States Environmental Protection Agency. For this reason, the scope of the current agronomic soil P test (using 0.1 M H 2 SO 4 as extractant) was broadened to be used also in the evaluation of the environmental P status of soils for freshwater protection in Mauritius. The environmental threshold was determined by establishing the relationship between 0.01 M CaCl 2 and 0.1 M H 2 SO 4 soil extractable P. It was inferred that above 85 mg kg −1 0.1 M H 2 SO 4 extractable P, the soil P represents a potential risk for contamination of runoff waters. The number of fields with unacceptably high levels of P ( P ≥ 95 mg kg −1 ) increased from 42 to 53 % in just one sugarcane crop cycle. In 2005/2006 more that 50 % of the sugarcane soils had an unsafe environmental soil P status and the majority of the sugarcane fields with an environmentally unacceptable soil P level were located on the two latosolic soil groups. Hence revision of P management practices or implementation of other corrective measures such as creation of riparian zones should primarily be directed towards those two soil groups.