Fractionation and availability of phosphorus in acid soils of Hagereselam, Southern Ethiopia under different rates of lime
© The Author(s) 2017
Received: 14 December 2016
Accepted: 22 April 2017
Published: 4 September 2017
Phosphorus (P) availability is commonly assumed to limit productivity in many tropical soils, yet there is relatively little information on the phosphorus chemical forms, distribution, and transformations that P undergoes in Ethiopian soils. We used a sequential fractionation scheme to assess phosphorus fractions of acid soils of Southern Ethiopia. The study area called Hagereselam is characterized by high soil acidity, and the availability of P in the study area is relatively low. As crop production in Ethiopia is dominated by low external input practices, native P remains the main contributor to plant P nutrition in many locations.
Although the total P concentration is 829.7 mg kg−1, the organic P content was relatively low (58 mg kg−1) and constituted on average less than 7% of the total P. The ratio of organic carbon to organic P was generally greater than 240, suggesting the potential for immobilization of P. The Al + Fe-associated P was the dominant inorganic P pool. The majority of the P occurred in recalcitrant form (568.3 mg kg−1). Readily available, exchangeable phosphate, as extracted by anion-exchange resin membranes, was present in very low concentrations (3.3 mg kg−1); moreover, labile P accounted for less than 2% (9.9 mg kg−1) of the total soil P.
KeywordsInorganic phosphorus Labile phosphorus Liming Occluded phosphorus Phosphorus fraction Sequential phosphorus extraction Resin membrane
Crop productivity on most tropical soils is limited by low availability of essential nutrients. This is especially true for soil P in sub-humid areas. The reasons include low native soil P concentrations and strong soil P retention associated with weathering and other soil properties. The problem is further worsened by nutrient mining due to the low-input agriculture practices. Yet many small scale subsistence farmers in Ethiopia are unable to make regular applications of P fertilizer because of economic constraints.
Soil P chemistry is complex with numerous forms of P present and possible transformations that P may undergo depending on the physical, chemical, and biological environment. Soil P occurs in organic forms as well as in inorganic compounds in association with aluminum (Al), iron (Fe), and calcium (Ca). The strength of association affects the availability of P to plants . This has generated attention in identifying the pools of soil P, and in quantifying their contribution to plant P nutrition. This is because an understanding of soil P forms and the transformations that affect P availability can be useful when making decisions about how to improve soil P availability for crop production. Phosphorus is among the most limiting nutrients for food production in the sub-humid and humid tropical highlands of East Africa . This deficiency is mainly caused by the inherent low P content characteristics of the parent material and by the strong sorption of PO4 3− to Al and Fe oxides. Limited availability of soil P to plants may be attributed to the inherent low soil phosphorus content and high degree of phosphorus fixation. Significant phosphorus fixation and its precipitation as iron and aluminum phosphates are very common in intensively weathered and sesquioxide-rich acidic soils . There are two ways of addressing such low P availability problems in acidic soils: soil liming for pH neutralization and application to P fertilizer. With liming, added carbonate (CO3 2−) is readily hydrolyzed and produces hydroxide ions (OH−), increasing soil solution pH. This severely impairs Al oxide activity reduces the ability of the oxide surfaces to retain P and consequently increases P availability . The second way is a more generous P applications to addresses the low P availability rather than addressing the root of the problem but it is not economical.
Although most soils of the highlands are characterized by soil acidity and high P fixation due to intensive weathering and leaching attributed to high rainfall , there is limited knowledge about soil P distribution, transformations, and fertility in Ethiopia, but the general impression about the soils is that availability of P in the dominant soils of in the country is relatively low [2, 7]. Although much of the Ethiopian soils have a high potential for crop production, no detailed work has been done on the P status, forms, and dynamics . Most past studies have focused only on the plant-available inorganic P fraction [15, 28, 34]. Because inorganic phosphorus (Pi) and organic phosphorus (Po) differ in their rates of phosphorus release, it is necessary to separate them into different biologically meaningful pools. This can be accomplished by sequentially fractionating soil P into different inorganic and organic pools. This study is therefore intended to quantify the various labile and stable pools of inorganic and organic P in acid soils of southern Ethiopia and the influence of reclamation of acid soils via application of lime on soil P fractions and availability.
Description of the study sites
Soil sampling and analysis
Composite surface soil samples of the 0–15 cm depth were collected using random sampling technique from representative (15) crop fields using auger. In the remediation pot study, 4 kg of soil was crushed to pass through a 2 mm sieve and were placed in plastic pots. The treatments were arranged in a completely randomized design (CRD) with five replications. The lime requirement (LR) was determined according to a method described by Shoemaker et al. . This method is frequently used in Ethiopia and southern Ethiopia in particular because it is suitable for soils, requiring lime correction of more than 1 ton ha−1 and soils with pH less than 5.8 Pansu and Gautheyrou . Accordingly, the lime requirement was found to be 6.5 ton ha−1. Four different rates of lime (0, 3.25, 6.5, and 9.75 ton ha−1) were used and the soil was incubated for 3 months at field capacity moisture.
Analysis of variance and mean separation were carried out for the selected soil physicochemical properties and P fractions using SAS software  to differentiate the extent of relations and associations between parameters and treatments. The significance was then evaluated using the least significant difference (LSD) at 5% level significance.
Results and discussion
General properties of soils of the study area
General characteristics of the soil for the study areas
33.90 C molc kg−1
Soil phosphorus fractions
Sequentially extracted soil phosphorus fractions (mg P kg−1) for the soils of the study site
% of total
Effect of liming on phosphorus fractions
P concentration (mg P kg−1) in P fractions extracted from Hagereselam soil with different rates of lime application
LR (ton ha−1)
Hagereselam soil with different rates of lime application
Liming at a rate of 9.75 ton ha−1 significantly increased the amount of Ca phosphates as revealed in HCl extractable Pi. It increased from 14.2 to 53.1 mg kg (Table 3). Similar results were reported by Hartono et al.  and Jokubauskaite et al. . It is likely related to an increase in adsorption and precipitation of P with the addition of excess Ca derived from lime. Our data are consistent with those obtained in recent studies conducted in acid soils by [6, 23]. Liming at lower rates creates optimal conditions for plant growth as it has low Ca–P compared with higher rates of lime .
Soil organic P occurs in a variety of chemical forms and can be released through mineralization processes mediated by soil organisms and plant roots. These processes are highly influenced by the soil pH. Organic P transformation has a great influence on the overall bioavailability of P in soil . Analysis of the organic P fractional composition showed that the major portion of the total extractable organic P was extracted by NaOH solutions (Table 3). These phosphorus compounds are more readily mineralized because they are constituents of the newly incoming organic residues as well originating from cells of microorganisms . Although not statistically significant, the organic P contained in both NaOH and HCl fractions decreased with addition of lime. A possible explanation for the decrease of organic P might be related to the intensified microbial activity at higher pH values, enhancing the mineralization processes in soil . In general, as the amount of lime increases, the available P form increases from 9.8 to 35.3 (Table 3) and become adequate to plant indicating the benefit of liming.
The total P concentrations in the acid soils of southern Ethiopian are generally comparable to those found elsewhere. Among the fractions extracted and for which separation into organic and inorganic P forms was conducted, there appears to be a low partitioning of soil P into the organic pool. The inorganic Al + Fe and Ca associated pools are the most affected fractions by the addition of lime. The preponderance of a large pool of “residual” P in these soils brings forward the issue of how this P may be brought back into a more actively cycling available pool. The low available P supply creates crop production limitations in these soils. This is despite the reasonably good total P contents. This constitutes a real challenge to crop production systems based upon low-input management. From the results of this study it appears that the addition of lime to the acidic soils is an effective strategy to mobilize and render more phosphorus available for plant uptake and management of soil pH which further have impact on the availability of nutrients.
AK: researcher and author. SB and SJ: editor and advisor. All authors read and approved the final manuscript.
This research was supported in part by IDRC.
The authors declared that they have no competing interests.
Availability of data and materials
We declare that all the data are available in the manuscript.
I am enclosing herewith a manuscript entitled “[Availability and Chemical fractionation of phosphorus in Acid soils of Hagereselam, Southern Ethiopia under different rates of lime] for possible evaluation.
With the submission of this manuscript, I would like to undertake that the above mentioned manuscript has not been published elsewhere, accepted for publication elsewhere or under editorial review for publication elsewhere; and that my Institute’s [Hawassa University] representative is fully aware of this submission.
The type of Submitted manuscript is original article. For the Editor-in-Chief, I would like to disclose the following information about the project in that the research project was conducted under the supervision of Beyene Sheleme (Major Advisor) and Jeff Schoenau (Co-advisor) and the project was run as my Ph.D. project.
This research project was conducted from September 2013–December 2015. My Research Project was fully sponsored by IDRC-CRID Hawassa University.
I would also like to share the following information with Editor-in-Chief in that I do not have the following similar manuscripts already published from this project:
Here in this paper, the importance of addition of lime for soil phosphorus fractions is quantified. Accordingly although the soil has high total phosphorus addition of reclamation materials like lime will increase available phosphorus which is very important for plant production. This implies the need for site specific soil studies especially for problem soils where there is limited fertility status. In addition the findings of this result will be used as a base for further researches on the soils to alleviate similar problems and initiate ideas for other types of soils with different problem.
A paragraph explaining why the manuscript is appropriate with the scope of the journal.
The journal is a highly reputable journal publishing articles related soils and plant. These research is about the relationship between plant-available soil phosphorus and soils. By this paper, it is tried to explain about the relationship between available plant phosphorus and the different fractions with different rates of lime. Therefore, it is highly related with the scope of the journal.
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This research was supported by IDRC with Project no: 200623033.
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