History of phosphorus fertilizer usage and its impact on the agronomic phosphorus status of sugarcane soils in Mauritius

Abstract

The intensive use of phosphorus (P) fertilizers during more than 100 years of sugarcane production in Mauritius is believed to have built up the P status of the soils to excessive levels. Hence there may be a scope for reducing P fertilizer usage in order to lessen production costs in sugarcane cultivation and alleviate its hazard to surface water quality. A review of P fertilizer usage in sugarcane showed that the amount of P fertilizer used by the sugarcane industry rose from 790 tons P₂O₅ (mainly as rock/guano phosphates) in the early 20^th century to a peak of 5 675 tons P₂O₅ in the 1970s before declining thereafter to 3 350 tons P₂O₅ in 2008, mainly in the form of ammonium phosphates. The currently adopted 0.1M H₂SO₄ extraction method was used to assess soil P available in the sugarcane soils. Soil test P data showed that in 2005/2006, 36% of the land still required P fertilization (P < 80 mg kg⁻¹) while approximately 52% had an excess of P (P ≥ 100 mg kg⁻¹). Fields with excessive P belong to all soil groups and can be encountered throughout the island. Less than 15% of the soils had an optimum soil P (80 ≤ P < 100 mg kg⁻¹). With only 31% of the soils having a highly excessive P status (P ≥ 150 mg kg⁻¹), this study indicates the belief that past P fertilization had invariably led to an excessive soil P status in Mauritius is unfounded. Also variation in soil P status as a result of differences in management between small and large planters could also not be discerned.