Processes and Factors Affecting Phosphorus Sorption in Soils

Samuel Kwesi Asomaning

## Abstract

The subject of this chapter is soil chemistry. The chapter is entitled Processes and Factors Affecting Phosphorus (P) Adsorption in Soils. The chapter aims to give an overview of the major mechanisms responsible for phosphate sorption (i.e., adsorption and absorption of phosphate) in soils, particularly of acid soils. According to studies conducted by some soil scientists, the major soil factors affecting P sorption are time, soil pH, soil organic matter, and iron and aluminium oxides of soils. Studies conducted indicated that adsorption of the P increases as the P ages in the soil. Soil pH affects phosphate adsorption but the effect is limited for adsorption by soils in the pH range of 4–8. Organic matter may affect P adsorption in two ways: indirectly by inhibiting iron oxide crystallisation and directly by competing for adsorption sites. On per mole basis, oxalate extractable aluminium oxides adsorb nearly twice as much P as oxalate extractable iron oxides due to poorer crystallinity (higher specific area) of the aluminium oxides compared to the iron oxides and also to a higher charge of the former. The chapter also highlights the pedotransfer functions (PTFs) of Borggaard, which can be used to calculate for the P adsorption in highly weathered acid soils.

Keywords: sorption, adsorption, adsorbent, adsorbate, specific adsorption, amorphous oxides, pedotransfer functions

### 1. Introduction

With increasing demand of agricultural production and as the peak in global production will occur in the next decades, phosphorus (P) is receiving more attention as a non-renewable resource [1, 2]. One unique characteristic of P is its low availability due to slow diffusion and high fixation in soils. Few unfertilized soils release P fast enough to support the high growth rates of crop plant species. In many agricultural systems in which the application of P to the soil is necessary to ensure plant productivity, the recovery of applied P by crop plants in a growing season is very low, because in the soil more than 80% of the P becomes immobile and unavailable for plant uptake because of adsorption, precipitation, or conversion to the organic form [3]. All of this means, that P can be a major limiting factor for plant growth.

Phosphate in soils tends to react with soil components to form relatively insoluble compounds, many of which have limited availability to plants [4]. Major factors which influence these reactions include: phosphorus concentration in solution, amount of free oxides of iron and aluminium, type and amount of clay, soil pH, and organic matter [5].

Aluminium oxides, iron oxides and clay silicates are well known phosphate adsorbents in soils [6]. According to Borggaard et al. [7], aluminium and iron oxides are the main phosphate adsorbents in sandy soils. Close correlations have been found between a soil's capacity to adsorb phosphate and the content of aluminium and iron oxides, in the soil suggesting these oxides to be the main phosphate adsorbents in soils [8].

The single–coordinated OH groups are those surface hydroxyl groups onto which specifically adsorbable anions are adsorbed [7]. The single–coordinated hydroxyl groups can be quantitatively replaced or exchanged by the phosphate anions. This results in the formation of a binuclear or surface complex for the phosphate iron oxide system, where one phosphate ion occupies two surface sites. This is accompanied by a release of hydroxyl (OH) and H2O groups (Figure 1).

Processes and Factors Affecting Phosphorus Sorption in Soils

DOI: http://dx.doi.org/10.5772/intechopen.90719

The three kinds of hydroxyl groups occurring on the goethite surface denoted (A) single-coordinated, (B) triple -

coordinated and (C) double-coordinated. Source: Borggaard and Elberling [6].

Examples of phosphate adsorption mechanisms. Source: Syers and Cornforth [17].

Figure 1.

Figure 2.

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In acidic soils, P can be dominantly adsorbed by Al/Fe oxides and hydroxides, such as gibbsite, haematite, and goethite [9]. P can be first adsorbed on the surface of clay minerals and Fe/Al oxides by forming various complexes. The nonprotonated and protonated bidentate surface complexes may coexist at pH 4–9, while protonated bidentate innersphere complex is predominant under acidic soil conditions [10]. Clay minerals and Fe/Al oxides have large specific surface areas, which provide large number of adsorption sites. The adsorption of soil P can be enhanced with increasing ionic strength.

Phosphate is strongly adsorbed by the number of adsorption sites, which vary greatly among soils [11]. With further reactions, P may be occluded in nanopores that frequently occur in Fe/Al oxides, and thereby become unavailable to plants [10].

Therefore, the availability of soil phosphate as well as the soil solution concentration of phosphate will depend on the degree of phosphate saturation, rather than on the total phosphate content [12]. Phosphate saturation is the proportion of adsorption sites occupied by phosphate, which is normally taken as the ratio between adsorbed phosphate and the phosphate adsorption capacity (PAC) of the soil [13].
