| || || Goundar, Muni Sangeeta|
| || || Phosphorus status of different soils in the sugarcane belt of Fiji |
Author:Goundar, Muni Sangeeta
Institution: University of the South Pacific.
Subject: Sugarcane -- Soils -- Fiji
Call No.: pac S 597 .S84 G68 2013
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: The availability of P in soil is perceived to be one of the limiting factors to sustainable sugarcane production in Fiji. Knowledge about the available forms of P in the soil is fundamental in determining the appropriate required amount of P fertilizers to apply. It is possible to measure the bioavailable forms of P in the soil and adding P fertilizer to attain the level of P required by the crop. P is easily fixed by the soil and many factors that contribute to the P sorptivity of soils need to be considered in order to assess a proper estimation of P requirements. The main objective of this research was to ascertain the amount of bioavailable P in the soils which will be useful in determining the required amount of inorganic fertilizer to be applied to the soil. In this study, twelve different soils were selected from the sugarcane belt of Fiji and the PBI and P isotherm experiments were performed. The PBI analysis was carried out using the Australasian Soil and Plant Analysis Council (ASPAC) Method 9I2 extractions with a Murphy and Riley analytical finish. Other physical and chemical soil tests were also performed for each soil type and Pearson‟s correlation tests were used to identify the patterns and trends. The basic properties were investigated as these factors are considered to affect the sorption of P in different soil types under varying conditions. The overall results of the soils analysed from the sugarcane cultivated belts showed that P availability and P fixation vary with the soil types. These results indicate that a common dosage of fertilizer will not meet the P requirements for the different soils. It was found that the Oxisols had the highest PBI values ranging from 134 to 217 while Inceptisols had the lowest ranging from 33 to 54. Examination of the soil types shows that the Alfisols and the Ultisols have generally intermediate values and the Mollisols values are quite wide ranging. The highest mean PBI of the Oxisols implies that P will be fixed strongly in the Oxisols rendering it unavailable for the plant uptake. It is expected though for the Oxisols to show a high affinity for P since they are dominated by the oxyhydroxides of Al and Fe and kaolinite in the subsoil. This study showed a positive correlation between P fixation and Al and Fe content in the soils (R = 0.65, p = 0.04). The Oxisols, Mollisols, Alfisols and Inceptisols from the four sampling locations viii showed a wide range of PBI values and this is expected as each soil has different soil pH, soil texture, fertilizer application rates, chemical constituents, soil temperature, flooding, and the degree of weathering and vegetation which are known to affect the P sorption capacities in the soils. The P isotherm data for each soil type were fitted to two extensively used sorption models, Langmuir and Freundlich. The sorption P maxima obtained from the Langmuir equation ranged from 480 to 1314 mg/kg, showing a significant positive correlation with the PBI of R= 0.70 and p < 0.01. The PBI data followed a similar pattern to the generated isotherm curves of the different soil types. The soil pH, clay content, exchangeable calcium, organic matter and Al and Fe oxyhydroxides seem to perform an important role in affecting the P fixation in the soils as significant correlations were found for these factors with the PBI. The soil pHs ranges from 4.7 to 6.8 and it varied with soil type. Pearson‟s correlation showed PBI has a significant moderate negative correlation with pH (R = -0.67, p = 0.017). The particle size analysis showed that all soils studied have relatively high clay content ranging from 54 - 84%. In the present study, the Inceptisols had the lowest clay content with the lowest PBI with relatively high available P. In contrast, high PBI and very low available P with high clay content are evident in the Oxisols as these soils are commonly highly weathered and often clay rich. Clay content showed a very strong positive correlation with the PBI (R = 0.76, p ≤ 0.005). There is a strong association of P fixation in the soil with the increasing level of Al and Fe ions in the soil. The use of PBI as an indicator of the P status of the sugarcane soils in Fiji will reduce the risk of over-fertilization and the loss of P to waterways, hence protecting the environment. The soils with a low PBI have a greater potential for the P leaching through the runoff into the waterways. Further research using the field experimental trial is still required to develop the improved fertilizer recommendations based on the soil types and also considering other factors affecting P availability