Phosphorus input is vital to the maintenance of profitable sugarcane crop production in
Mauritius. The intensive use of some 5,000 tonnes of P annually during the past 50 years is
believed to have built up the P status of the sugarcane soils, perhaps even to excessive
levels. While this accumulation of P is desirable from an agronomic perspective, there is
growing concern in Mauritius about its possible effect on surface water quality. In
response to that concern, a study was initiated with the following specific objectives:
i. To review the usage of P fertilisers in sugarcane production in Mauritius and assess
their resulting impact on the P status of the main soil groups under sugarcane.
ii. To enlarge the scope of the current method used (0.1M H2SO4 extraction) for
agronomic P testing so that it also indicates environmental status of sugarcane soils
in Mauritius.
iii. To determine the environmental threshold P in soils above which the P will
represent a hazard to surface waters.
The five yearly averages of fertiliser P usage by the Mauritian sugarcane industry showed
that from the 790 tonnes of P2O5 (mainly as rock/guano phosphates) consumed at the
beginning of the 20th century, P usage attained a peak of 5,675 tonnes in the 1970s before
declining thereafter as a result of a decreasing land area under sugarcane. During the
period 2005 to 2008, an average of 3,350 tonnes of P2O5 mainly as ammonium phosphates
were applied annually to sugarcane which is cultivated in Mauritius mainly on five soil
groups, namely the Low Humic Latosol (Humic Nitosol)*, the Humic Latosol (Humic
Nitosol)*, the Humic Ferruginuous Latosol (Humic Acrisol)*, the Latosolic Reddish Prairie
(Eutric Cambisol)* and the Latosolic Brown Forest (Dystric Cambisol)*.
A method based on 0.1M H2SO4 as extractant is currently used as a routine soil test to
assess P available to sugarcane in the soils of Mauritius. On the basis of soil P test values,
four soil P fertility classes could be discerned, namely: Examination of the soil test P data obtained in 1997/1998 showed that 48% of the land still
required P fertilisation while approximately 40% had an excess of P (P ⥠100 mg kg-1). Less
than 10% of the soils had an optimum soil P (80 ⤠P < 100 mg kg-1). Moreover, soils with a
highly excessive soil P status (P ⥠150 mg kg-1) rose from 23% in 1997/1998 to 34% in
2005/2006 indicating that with the current P management practice in sugarcane, the P
status of soils in Mauritius will shift more and more towards an excess of P.
In spite of the extensive information available on the soil P status, its significance from the
freshwater protection angle was, prior to this study unknown due mainly to a lack of a
suitable environmental soil P test method. From this perspective, as a laboratory
extraction of soil with 0.01M CaCl2 gives a very reliable representation of the P in runoff,
the P extractable in a 0.01M CaCl2 (0.01M CaCl2-P) solution was determined in 112 soil
samples representing the five main soil groups under sugarcane. The soil samples whose
characteristics of pH, organic matter content, exchangeable bases and cation exchange
capacity were also determined, were selected to cover a range of 10 to 250 mg kg-1 P
extractable by the 0.1M H2SO4 used for agronomic soil P testing in Mauritius. As the
environmental soil test P must be independent of soil properties and the concept of degree
of P saturation (DPS) meets that criteria, the ammonium oxalate DPS (DPSox) was
determined in the 112 soil samples to provide a reliable pointer of P susceptibility to loss
from soils. Since it is very unlikely that ammonium oxalate extraction would be used as a
routine soil test, the relationship between DPSox and 0.1M H2SO4-P was established by
conventional statistical regression techniques.
The results obtained indicate that no single soil characteristic could be said to have a
distinct influence on the amount of P extracted by either the 0.1M H2SO4 or the 0.01M CaCl2 or by the DPSox. Indeed the correlation (r2) between the 0.1M H2SO4-P, 0.01M CaCl2-
P, DPSox with the individual measured soil characteristics was low and never exceeded
0.28 in the case of 0.1M H2SO4-P and 0.52 with 0.01M CaCl2-P. The DPSox exhibited the
poorest relationship with the soil properties with none of the r2 values being above 0.16.
Instead the low r2 values observed indicated as confirmed by multiple regression analysis
that the amount of P extracted by each reagent would be the result of the combined effects
of certain soil characteristics.
The results moreover showed that for soil P not to constitute a hazard to the freshwaters
in Mauritius, the DPSox should not exceed 3.10±0.10% and the 0.01M CaCl2-P must lie
below 18±1µg L-1. Moreover the linear fit regression equation 0.1M H2SO4-P = 17.3 + 23.2
DPSox with r2 = 0.54 was found to most appropriately describe the relationship between
0.1M H2SO4-P and DPSox. From that equation the threshold DPSox of 3.10±0.10% would
correspond to a range of 85 to 95 mg kg-1 of 0.1M H2SO4-P which is henceforth considered
as the threshold range of P in sugarcane soils in Mauritius above which the soil P would
become a hazard to freshwater sources. Using this environmental threshold range of soil P
values as basis, the soils can be divided into the following four environmental classes
namely: [table]. Application of the above criteria showed that in 1997/1998, 58% of the soils did not
represent any hazard to freshwater quality in Mauritius. As much as 42% of the sugarcane
fields in 1997/1998 had from the environmental viewpoint unacceptably high levels of P
(P ⥠95 mg kg-1) in the soils. After one crop cycle in 2005/2006, the number of fields with
unacceptably high levels of P (P ⥠95 mg kg-1) had risen to 53%. The majority (74%) of the
sugarcane fields with an environmentally unacceptable P status were located in the
Latosolic Reddish Prairie and Latosolic Brown Forest soils. In extending the scope of the current agronomic soil test P using 0.1M H2SO4 as an
extractant into an agro-environmental soil P test, this study demonstrated clearly that the
agronomic objectives in P management for sugarcane production in Mauritius are
incompatible with the environmental aims of protecting the freshwater resources in
Mauritius. With the agronomic threshold range of 80 to 100 mg kg-1 P overlapping the
environmental range of 85 to 95 mg kg-1 P, soils in Mauritius that are agronomically
suitable for sugarcane cultivation are on contrary unsafe from the environment protection
viewpoint and vice versa.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-12152010-122234 |
| Date | 15 December 2010 |
| Creators | Mardamootoo, Tesha |
| Contributors | Dr KF NG Kee Kwong, Prof CC du Preez |
| Publisher | University of the Free State |
| Source Sets | South African National ETD Portal |
| Language | en-uk |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | http://etd.uovs.ac.za//theses/available/etd-12152010-122234/restricted/ |
| Rights | unrestricted, I hereby certify that, if appropriate, I have obtained and attached hereto a written permission statement from the owner(s) of each third party copyrighted matter to be included in my thesis, dissertation, or project report, allowing distribution as specified below. I certify that the version I submitted is the same as that approved by my advisory committee. I hereby grant to University Free State or its agents the non-exclusive license to archive and make accessible, under the conditions specified below, my thesis, dissertation, or project report in whole or in part in all forms of media, now or hereafter known. I retain all other ownership rights to the copyright of the thesis, dissertation or project report. I also retain the right to use in future works (such as articles or books) all or part of this thesis, dissertation, or project report. |
Page generated in 0.0021 seconds