In crop production phosphorus (P) is an essential nutrient for crop growth, and hence P
fertilization is necessary to achieve optimum yields. However, this can induces in soil a P
concentration which may contributes to eutrophication of fresh water bodies. Soil P tests are
therefore considered very useful in setting threshold values important for both agronomic and
environmental management purposes. Soil P tests developed from a water pollution protection
point unlike agronomic P tests are not easily adapted for use on a routine basis because they
are not considered, for this purpose, and this could make agronomic P tests more practical for
routine environmental P assessment also. Determination of appropriate agronomic P tests for
this purpose however, involves evaluating the potential use of the tests for environmental
purposes. Hence, the objective of this study was to review the current methods used to
determine the agronomic and environmental P status of soils, and to establish whether P
extracted from a range of soils by various agronomic and/or environmental P determination
methods are related or not.
Soil samples from the orthic A horison were collected in three cropping areas in the Free State
province, namely Jacobsdal, Bloemfontein, and Ficksburg. These samples were treated with
K2HPO4 to induce different phosphorus concentration levels and then incubated at room
temperature for three months. During incubation the samples were subjected to several wetting
and drying cycles to ensure that the applied phosphorus equilibrated. The samples were then
analysed for P using the extractants of Olsen, Bray 1, Truog, ISFEI and citric acid commonly
employed for routine analysis to establish the agronomic P status of soils. In order to establish
the environmental P status of the soils, the samples were analysed for using the extractants
calcium chloride (CaCl2) and ammonium oxalate [(NH4)2C2O4.H2O]. The latter was used to
calculate the degree of phosphorus saturation (DPSox).
The results showed significant relationships among agronomic P tests when data of individual
soils were analysed separately (r2=0.65-0.99) and, when data of all soils from a sampling area
were pooled (r2=0.52-0.87). All the relationships were significant for the Ficksburg soils
(r2â¥0.55) and for the Bloemfontein soils (r2â¥0.82) but not for the Jacobsdal soils. For the latter
soils the Truog-P correlations with Olsen-P (r2=0.44), Bray 1-P (r2=0.42) and ISFEI-P (r2=0.35)
were not significant, probably due to that they are calcareous. Significant relationships were also obtained for P extracted by the environmental P tests when
regression analysis was done for each individual soil (r2â¥0.80). However, when data of soils
from a sampling area were pooled significant relationships were obtained for Bloemfontein soils
(r2=0.92) and Ficksburg soils (r2=0.56) while Jacbosdal soils (r2=0.33) showed an insignificant
relationship. Pooling data of all soils from the three sampling areas also resulted with a lower
correlation coefficient (r2=0.40) implying a poor relationship between the environmental P tests.
The correlation between P extracted by the agronomic tests and CaCl2-P showed positive
relationships (r2 â¥0.57) except in a few instances. Truog-P and citric acid-P showed a poor
correlation with CaCl2-P when the Jacobsdal soilsâ data were pooled (r2=0.22 and 0.35
respectively). Pooling of all soilsâ data resulted also in a poor correlation between CaCl2-P and
Truog âP (r2= 0.28). The DPSox correlated significantly with the extractable P of all agronomic
tests when the individual soilâs data were analysed separately (r2 â¥0.73). However, when data of
all soils from a sampling areas were pooled for regression analysis, all relationships were
significant for the Bloemfontein soils (r2 â¥0.70), but not for the Jacobsdal soils, and Ficksburg
soils. Pooling data of all soils from the three sites resulted with a positive relationship between
DPSox and the extractable P of all agronomic tests (r2 â¥0.50), except ISFEI (r2 â¥0.45).
The threshold values estimated for agronomic tests with regression equations from CaCl2-P
DPSox threshold values varied greatly between individual soils and even the soils groups of a
sampling area. The threshold values for all soils when based on CaCl2 implied that if the
extractable P status of cropped soils are maintained at optimum levels for Bray 1, Truog, ISFEI
and citric acid the soils may be a threat to water pollution. The opposite is true with the
estimated threshold values when based on DPSox. The results therefore showed that agronomic
tests can be used also for environmental management of P although only the Olsen test showed
the potential for developing a single threshold value for all soils.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ufs/oai:etd.uovs.ac.za:etd-05242013-154819 |
| Date | 24 May 2013 |
| Creators | Nthejane, 'Mabatho Margaret |
| Contributors | Prof CW van Huyssteen, 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-05242013-154819/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. |
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