SoilUse and Management doi: 10.1111/j.1475-2743.2011.00353.x
Soil Use and Management, September 2011, 27, 283–293
Predicting the degree of phosphorus saturation using the ammonium acetate–EDTA soil test D. Houben, C. Meunier, B. Pereira & Ph. Sonnet Earth and Life Institute, Universite´ catholique de Louvain, Croix du Sud 2 ⁄ 10, 1348 Louvain-la-Neuve, Belgium
Abstract As a result of the important role played by phosphorus (P) in surface water eutrophication, the susceptibility of soils to release P requires evaluation. The degree of phosphorus saturation, assessed by oxalate extraction (DPSox), has been used as an indicator. However, most laboratories do not include DPSox in routine soil tests because of cost and time. This study evaluates the suitability of the ammonium acetate extraction in the presence of EDTA (AAEDTA), the standard soil test P (STP) in Wallonia (Southern Belgium), to predict DPSox; we also compared it with the Mehlich 3 extraction. Ninety-three topsoil samples were collected in agricultural soils throughout Wallonia. Good correlations were found between the AAEDTA and the Mehlich 3 methods for P, Fe and Al (r = 0.85, 0.77 and 0.86, respectively). An exponential relationship was found between PAAEDTA and DPSox. Results of principal component analysis and regression demonstrated that STP can be used to predict DPSox (r = 0.93) after logarithmic transformation. Soil test Al was also a good indicator of the P sorption capacity (PSCox) of soils (r = 0.86). Including the clay fraction in regression equations only slightly improved the prediction of PSCox (r = 0.90), while other readily available data (such as pH or organic carbon) did not significantly improve either DPSox or PSCox predictions.
Keywords: Phosphorus, saturation, sorption, iron oxides, regression, Belgium
Introduction In the past decade, soil phosphorus (P) has received increasing scientific interest. Although it is essential to maintain adequate P in agricultural soils for economic crop production, the accumulation of P in these soils increases the risk of P loss to surface waters (Breeuwsma et al., 1997; Sibbesen & Sharpley, 1997; Foy, 2005). Phosphorus loss is a major cause of degradation of surface freshwaters because it contributes to eutrophication (Pote et al., 1996; Carpenter et al., 1998; Sharpley et al., 2003a). Therefore, it is important to be able to monitor the release of P by agricultural nonpoint sources (Delgado & Torrent, 2001; Kronvang et al., 2005). As a result of high manure-P loading, many soils have become increasingly P saturated (Beck et al., 2004). Therefore, knowing the P saturation of the soil is essential to take appropriate measures to prevent P accumulation. For non-calcareous soils, it has been shown that phosphorus sorption capacity (PSC) is mainly governed by the content of
Correspondence: D. Houben. E-mail:
[email protected] Received July 2010; accepted after revision April 2011
amorphous aluminium (Al) and iron (Fe) oxides (van der Zee & van Riemsdijk, 1988; Lookman et al., 1995a; Guo & Yost, 1999). The degree of P saturation (DPS), which is the total amount of sorbed P divided by the soil PSC, has been suggested as an indicator of the susceptibility of agricultural soils to release P (Hooda et al., 2000; Nair et al., 2004; Vadas et al., 2005). The first and the most common measure of DPS is DPSox, which is based on P, Fe and Al extracted by acid ammonium oxalate in the dark (referred to as ‘oxalate extraction’ in this paper and many others) which are related as shown in equation (1) (Schoumans, 2000): DPSox ¼
Pox Pox 100 ¼ 100 PSCox aðFeox þ Alox Þ
ð1Þ
where Pox is the oxalate-extractable P (mmol ⁄ kg), PSCox is the phosphorus sorption capacity, which is the sum of oxalate-extractable Fe and Al (Feox and Alox, respectively; mmol ⁄ kg) multiplied by an empirical a coefficient representing the proportion of Feox and Alox that is effectively sorbing P (van der Zee et al., 1987; van der Zee & van Riemsdijk, 1988). Oxalate-extractable P is the total
ª 2011 The Authors. Journal compilation ª 2011 British Society of Soil Science
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284 D. Houben et al. potentially desorbable P (Lookman et al., 1995a) and oxalate-extractable Fe and Al are generally considered to represent the active amorphous or weakly crystallized Fe and Al oxides (van der Zee et al., 1990). Finally, reported values for a are usually between 0.3 and 0.7. An a value of 0.5 is frequently used, often without experimental justification (Pautler & Sims, 2000; Schoumans, 2000; Maguire et al., 2001). Soluble P released by the soil solid phase can, in some circumstances, be strongly related to the DPS (Lookman et al., 1996; Pote et al., 1996; Hooda et al., 2000; Pautler & Sims, 2000; Schoumans & Groenendijk, 2000; Vadas et al., 2005; Little et al., 2007). Based on this assumption, the DPSox approach (equation (1), with a = 0.5) has among others been integrated in Dutch environmental legislation and fertilizing guidelines to predict potential P transfer by runoff or drainage. In Flanders (Northern Belgium, 44% of the total Belgian surface area), DPSox has been integrated in surveys to estimate the local P saturation (Lookman et al., 1995b, 1996; De Smet et al., 1996; Schoeters et al., 1997). Unlike in Flanders, DPSox is not a routine test in Wallonia (Southern Belgium, 55% of the total Belgian surface area) or in many other countries. Agricultural recommendations are usually based on the value of the soil test phosphorus (STP), which estimates the content of phytoavailable P in soils and, thus, the need for P additions to reach the optimum range for plant growth (Bundy et al., 2005). In addition, STP is increasingly being used for environmental recommendations. The rationale is that, unlike the expensive and timeconsuming DPSox approach, STP is a fast and cheap routine method that can be implemented on a large scale and thus provides widely available data. However, STP provides less information than DPSox about the status of P in soils and cannot be used to consistently predict the susceptibility of soils to release P (Kleinman et al., 1999). An additional disadvantage is the great variety of P extraction methods used in different countries, thereby hampering comparisons of extracted P values between studies. This is particularly true for Wallonia, which uses (for historical reasons) ammonium acetate + EDTA (AAEDTA) extraction (pH 4.65) as the soil P test. Although this method is seldom used elsewhere, it offers the advantage, compared with other STP tests such as the Olsen or Bray, of allowing the analyst to measure the exchangeable cations using the same extract (Hons et al., 1990). As it would be unfeasible to include the measure of the DPSox in routine soil tests, Schoumans & Groenendijk (2000) attempted to establish relationships between STP values and chemical soil P characteristics and proved that STP could be used as a good indicator of P loss risk. Likewise, Kleinman et al. (1999) and Beck et al. (2004) suggested developing pedotransfer functions or models to link soil test data to DPS. The pedotransfer technique consists in relating, often empirically, readily available soil properties to other less accessible ones.
The main objective of this study was to investigate the relationship between PAAEDTA and DPSox and to derive mathematical expressions to predict DPSox based on soil data that are readily available in Wallonia such as pH, organic carbon (OC), texture and P, Fe and Al extracted by AAEDTA. Because AAEDTA extraction is seldom used as an STP in other parts of the world (Grzebisz & Oertli, 1992), our second objective was to relate our results from AAEDTA extraction with the data obtained from the more commonly used Mehlich 3 extraction.
Materials and methods Soil sampling Ninety-three surface composite soil samples (0–20 cm deep) were collected from pasture lands (34 samples) and crop lands (59 samples) with a cylindrical hand auger. The selection of sampling sites was made to ensure an accurate representation of Wallonia (Figure 1) and to best represent the main soil types. All samples were air-dried and sieved through 2 mm and plant roots and particles larger than 2 mm were discarded.
Soil analysis Chemical elemental composition was measured by induced coupled plasma – atomic emission spectrometry (ICP-AES; Jarrell Ash) after reflux aqua regia digestion (Kjeldatherm SMA8A, Gerhardt). Soil pH was measured in deionised water (1:2.5 soil:water ratio). The OC content was determined using the Walkley and Black method that oxidized OC in the presence of excess dichromate (Nelson & Sommers, 1982). The N (Kjeldahl) content was determined using the procedure outlined by Bremmer & Mulvaney (1982). The cation exchange capacity of the soil (
