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Theoretical equilibrium considerations explain the failure of the maleic-itaconic copolymer to increase efficiency of fertiliser phosphorus applied to soils.

Introduction

Phosphorus (P) is an essential macronutrient for plant growth and application of fertiliser P is an essential input into agriculture to optimise and maintain crop production in most soils low in available P. The major P fertilisers traded internationally are the water soluble products such as triple superphosphate (TSP), mono-ammonium phosphate (MAP) and di-ammonium phosphate (DAP). However, the agronomic efficiency of these water-soluble P fertilisers can be reduced due to the conversion of water soluble P to less soluble forms such as Fe-Al-P in acid soils and Ca-P in alkaline soils (Syers et al. 2008). The applied recovery of P by plant uptake in the year of application is generally in the range of 10-30% (Chien et al. 2012). Chien et al. (2009) reported that recent research studies have been directed to improving P use efficiency by: (i) coating of P fertiliser with polymers to slow down or reduce P release from the fertiliser granule, (") using additives to chelate soil Fe, Al, and Ca ions that reduces formation of water-insoluble P in soils, and (Hi) use of liquid MAP/DAP, instead of solid granular forms, in calcareous soils. Lombi et al. (2004) and Hettiarachchi et al. (2006) reported that, in these soils, total and labile P from the liquid forms as compared with granular forms diffused farther from the site of P application.

Recently, a P fertiliser additive product consisting of maleicitaconic acid copolymer (trade name: Avail[R]) has been marketed worldwide as a P enhancer. It is claimed that the copolymer can reduce the P-binding by soil exchangeable Ca, Fe, and Al ions and thus enhance fertiliser P use efficiency. The product is reported to increase crop yield by 10-15%. It should be pointed out that all the claims by the manufacturer were based on the data from field trials without having any published fundamental studies to support the field research. The present authors are not aware of any peer reviewed journal articles describing the binding of the copolymer with soil cations. Two recent reports by Degryse et al. (2013) and Chien et al. (2014) showed that the copolymer failed to prevent P-binding by soils. They attributed the failure of the copolymer, in part, to the fact that the commercially-recommended rate of the copolymer by the manufacturer was too low (0.25% of the weight of DAP/MAP).

The purpose of this article is to use the theoretical equilibrium considerations to explain the failure of the copolymer as a P enhancer that has been reported from many field trials (Chien et al. 2014). The study considered the negative equilibrium constants (pK) of various reactions including chelation of cations by maleic acid and itaconic acid versus the P-precipitation reactions between P and the cations in soils. The goal is to determine which of these reactions predominate when the soluble P fertilisers containing the copolymer are applied to the soils. A summary of agronomic review will be first presented as a background to provide information to those who are not familiar with the producer's claim that Avail is an enhancer to fertiliser P availability.

Materials and methods

Summarised agronomic results with the copolymer

In Kentucky, Grove (2011) reported that the copolymer did not improve DAP efficiency for soybean (Glycine max L.) grown on soils at soil test levels ranging from 4 to 8 mg P [kg.sup.-1] (Mehlich 111). Binford (2008) also did not observe any significant effect of the copolymer on P efficiency for com in Delaware (Fig. 1). Degryse et al. (2013) observed no significant effect of the ligands coatings on dry-matter yield, P uptake or shoot P concentration in wheat in a pot experiment with three P-responsive soils. Additional recent published reports that showed the lack of effect of the copolymer on fertiliser P efficiency at significant level (P=0.05) can be found in studies conducted by Dunn and Stevens (2008), Cahill et al. (2013), Dudenhoeffer et al. (2012, 2013) and McGrath and Binford (2012).

The interpretation of results from field experiments is difficult, especially when the potential effects of a given product on plant yield are similar to or less than the normal background variability that occurs in all biological experimentation and is typically ~5-10% (expressed as the coefficient of variation, CV) in well designed, planned and executed trials. The best way to detect the actual treatment effect is to use the so-called 'meta-analysis' as described by Chien et al. (2014). When a given product has been tested many times, the frequency distribution of the measured treatment effects to be examined can be compared with a normal distribution with a mean of zero effect. For convenience, this can be achieved by plotting the % cumulative distribution function against the observed % increase or decrease in crop yield associated with the treatment with respect to that of the control (no treatment). Any displacement of the frequency distribution, either positive or negative, can be taken to indicate a real treatment effect.

The results with the copolymer (Fig. 2) show that yield responses were around a mean of 1.2% (confidence interval 1.1%, n = 92). Importantly, the occurrence of either positive or negative responses was not related to either soil P status. These results suggest that the product has little practical effect on crop production. The range in the observed response simply reflects the background variability which is expressed in all field trials of this nature.

Development of the theoretical equilibrium considerations Derivation of equilibrium constants of various reactions in soils

The free energy of reaction ([DELTA][G.sup.o]r) at standard state is related to the equilibrium constant (K) in the following equation:

[DELTA][G.sup.o]r = -RT ln K (1)

where, R is the gas constant, T is the absolute temperature and K is the same as the solubility-product constant for water-insoluble compound or the dissociation constant for cation-chelate. The negative logarithm of K is then defined as pK.

Thermodynamically, a larger pK value indicates more stable formation for the reaction products either as solid precipitation or cation-chelate because of more negative [DELTA][G.sup.o]r which is related to more favourable spontaneous reaction (but not necessary for reaction kinetics). For example, the solubility of P compounds follows: dicalcium phosphate dihydratc (DCPD) > octacalcium phosphate (OCP)> (3-tricalcium phosphate (TCP) > hydroxyapatite (HA) regardless of the solution composition such as pH, Ca, and P concentration, etc. This is because their pK values follow: DCP (6.6)<OCP (24.7)<P-TCP (29.5) < HA (58.6) (Lindsay 1979), the same order of negative [DELTA][G.sup.o]r.

To further explain, it is known that EDTA can chelate Ca at high pH and prevent CaC[O.sub.3] precipitation because pK of Ca-EDTA (9.3) is larger than that of CaC[O.sub.3] precipitation (pK = 8.2). In the opposite reaction, EDTA can be used to extract Ca out of CaC[O.sub.3] in soils (Lindsay 1979). This demonstrates that pK. can be used to suggest the preference chelation versus precipitation reactions in soils.

Likewise, a comparison of the pK of soil P-binding reactions with Ca, Fe, and Al ions versus the pK of chelation reactions of Ca, Fe, and Al ions with maleic acid and itaconic acid would indicate whether maleic-itaconic acid copolymer could prevent the P-precipitation reactions. The selected data of the pK values relevant to the present study are shown in Table 1. Due to the lack of information on the equilibrium dissociation constants of Fe-itaconic acid and Al-itaconic acid complexes in the literature, their pK values will be indirectly estimated in the present study. Ramamoorthy and Santappa (1969) reported that, both maleic acid and iatconic formed only 1 : 1 complexes with uranyl (U) ion. However, U-maleic acid complex (pK = 5.15) was slightly more stable than that of U-itaconic acid complex (pK=4.86) (Table 1). Similarly, Ca-maleic acid complex (pK = 2.43) is also more stable than that of Ca-itaconic acid complex (pK = 1.20) as shown in Table 1. Therefore, it may be assumed that the Fe-maleic acid complex (pK = 5.15) is more stable than that of Fe-itaconic acid complex and Al-maleic acid complex (pK = 5.47) is more stable than that of Al-itaconic acid complex. In other words, pK of Fe-itaconic acid complex must be <5.15 and pK of Al-itaconic acid complex must be <5.47 as shown in Table 1.

A well-known strong chelating agent, EDTA, is also included in Table 1 as a standard reference for comparison purpose.

Results and discussion

Comparison of equilibrium constants of various reactions in soils

It should be pointed out that a given soil has its own specific amount of amorphous Fe-Al oxides in an acid soil or CaC[O.sub.3] in a calcareous soil that can fix water-soluble fertiliser P. A typical acidic Oxisol could have 70.4 x [10.sup.3] cmol [ha.sup.-1] of oxalate-extractable (Fe+Al) or amorphous (Fe+Al) oxides and a typical alkaline soil without CaC[O.sub.3] could have 16.5 x [10.sup.6] cmol [ha.sup.-1] of exchangeable Ca (Chien et al. 2014). The recommended rate of Avail to be added to P fertilisers is 0.25% of MAP/DAP by weight. At 100 kg P [ha.sup.-1] applied, the CEC of Avail is only 0.12 enrol [ha.sup.-1] (Chien et al. 2014). All these values are fixed to test the efficacy of Avail at a given rate of fertiliser P coated with Avail that is applied to a given soil, and therefore there are no other variables. The purpose of this study is not intended to compare the efficacy of Avail in different soils varying widely in soil properties, e.g. soil pH, CEC, etc. or changing soil properties such as pH due to liming. The issue is whether the P precipitation by oxalate-extractable (Fe+Al) or exchangeable Ca would dominate over the chelation of Avail with those ions in terms of P-fixation of soluble P fertilisers by soils.

In alkaline or calcareous soils, the initial reaction product precipitated from the applied TSP/MAP/DAP is DCPD in the form of CaHP[O.sub.4] x 2[H.sub.2]O (Bell and Black 1970a, 19706). Along with time, DCPD gradually transforms to other less soluble Ca-P compounds such as OCP, [alpha]-TCP, and eventually to HA (Lindsay 1979). The formation constant of DCPD (pK = 6.60) which is larger than those of Ca-malcic acid complex (pK = 2.43) and Ca-itaconic acid complex (pK = 1.20) (Table 1). The implication is that when TSP/MAP/DAP fertilisers containing the maleic-itaconic acid copolymer are applied to alkaline soil, binding of P by exchangeable Ca or free CaC[O.sub.3] to form DCPD precipitation will always take place until soil solution P concentration is in equilibrium with the solubility product constant for DCPD before the remaining Ca is chelated with the copolymer even the amount of the copolymer used is very high. Consideration of these chemistry relationships may explain why there was no effect of the copolymer on the P-binding from MAP in a highly calcareous soil (59% of CaC[O.sub.3]) as observed by Degryse et al. (2013). The MAP granule they used contained a very high rate of the copolymer (2.5 g of Avail[R] or 1.0 g of the copolymer per 1.0 g of MAP; since Avail" contains 40% of the copolymer by weight). At this rate MAP granule contained the copolymer that was 1000 fold higher rate to the commercial rate (0.25% Avail[R] of MAP or 0.1 g of the copolymer per 100 g of MAP) recommended by the manufacturer.

In contrast to the copolymer, the equilibrium dissociation constant of a Ca-EDTA complex (pK = 10.70) (Table 1) is larger than that of DCPD (pK = 6.60) suggesting that EDTA can prevent P from reacting with Ca ions. The amount of EDTA required to prevent DCPD formation in highly calcareous soil, however, can be extremely high. The percentage of EDTA in the P fertilisers required to prevent DCPD formation increases with increasing CaC[O.sub.3] content in the calcareous soils. In general, most agricultural calcareous soils contain CaC[O.sub.3] content in the range of 1-25%. The corresponding amounts of EDTA required prevent DCPD formation will be in the range of 2-50% of EDTA based on the procedure for calculations as described by Chien et al. (2014). This may explain the failure of EDTA to increase P solubility of MAP granules enriched with 1% EDTA by weight in two calcareous soils, containing 1.2% and 59% CaC[O.sub.3] as reported by Degryse et al. (2013). In addition, these calculations only considered CaC[O.sub.3] without including exchangeable Ca. If the Windthorst calcareous soil which had 10cmolc kg 1 of exchangeable and 0% of CaC[O.sub.3] (Chien et al. 2014) was used, it would still require ~2% of EDTA in the MAP/DAP granule to prevent DCPD formation resulting from the reaction of exchangeable Ca with P from MAP/DAP. Even a mere 2% of EDTA enriched in MAP/DAP is cost prohibitive to fertiliser companies as well as to farmers.

In acid Oxisols and Ultisols, it is known that amorphous oxalate-soluble Fe-Al-oxide minerals, not exchangeable Fe and Al, are mainly responsible for the binding of fertiliser P (Schwertmann 1964; Chien et al. 1987). In general, P adsorption on the surface of Fe-Al-oxidcs, instead of bulk precipitation of Fe-P and Al-P, is often accepted by researchers (Olsen and KJiasawneh 1980; Sanchez and Uehara 1980; Degryse et al. 2013). Due to the lack of equilibrium constants associated with this type of surface P-binding mechanism, it is assumed that Fe-P and Al-P precipitation could be considered as a two-dimensional process (monolayer) on the surface of Fe-Al-oxidcs rather than as a three-dimensional process for the discrete P precipitates. This concept has been strongly advocated by Corey (1981) and Sposito (1987) who tried to reconcile the controversy of P adsorption versus P precipitation by considering that the two-dimensional precipitation on the solid surface is the same as three-dimensional discrete P precipitation. Based on this concept, the equilibrium constants of AlP[O.sub.4] x 2[H.sub.2]O (variscite) and FeP[O.sub.4] x 2[H.sub.2]O (strengite) shown in Table 1 are used for the present discussion.

The equilibrium constants of FeP[O.sub.4] x 2[H.sub.2]O (pK = 15.0) and AlP[O.sub.4] x 2[H.sub.2]O (pK = 20.0) are larger than those of Fe-maleic acid complex (pK <5.15), Al-maleic acid complex (pK <5.47), Fe-itaconic acid complex (pK <5.15), and Al-itaconic acid complex (pK <5.47). These comparisons suggest that Fe and Al ions would first react with P forming FeP[O.sub.4]-2[H.sub.2]O and Al P[O.sub.4] x 2[H.sub.2]O on the surface of Fe-Al-oxides, instead of the chelation of Fe and Al ions with maleic acid and itaconic acid when fertiliser P mixed with the copolymer is applied to the acid soils. In other words, the copolymer is not able to prevent P-binding by Fe and Al ions in acid soils, even a large amount of the copolymer is mixed with P fertilisers. This may explain the failure of the copolymer to increase P solubility after the MAP enriched with the copolymer even as high as 100% of MAP by weight was applied to an Oxisol (Degryse et al. 2013).

The equilibrium constant of Fe-EDTA complex (pK = 25.0) is larger than that of FeP[O.sub.4] x 2[H.sub.2]O (pK = 15.0) but the equilibrium constant of Al-EDTA complex (pK=15.9) is smaller than that of AlP[O.sub.4] x 2[H.sub.2]O (pK = 20.0). This suggests that while EDTA cannot prevent the formation of AlP[O.sub.4] * 2[H.sub.2]O, it may prevent the formation of FeP[O.sub.4] x 2[H.sub.2]O. However, even if an Oxisol has only oxalate extractable Fe without oxalate Al (very rare case); the amount of EDTA required to block the formation of FeP[O.sub.4] x 2[H.sub.2]O can still be rather high. For example, if we ignored the oxalate Al (2310 mg kg'1) and considered only the oxalate Fe (2560 mg [kg.sup.-1]) in the Oxisol (Kingaroy) used by Degryse et al. (2013), the amount of EDTA required to prevent the FeP[O.sub.4] x 2[H.sub.2]O formation was calculated as high as 81% of EDTA by weight of DAP/MAP granule based on the procedure as described by Chien et al. (2014). This high rate of EDTA, of course, is not economically feasible.

Conclusions

The results from the solubility considerations and agronomic greenhouse or field trials with the copolymer (maleic-itaconic acid) clearly have shown that the product has no positive effect to enhance fertiliser P efficiency. Therefore, the maleic-itaconic acid copolymer should not be recommended to farmers if their intention is to improve P use efficiency and/or increase crop production as concluded by Chien et al. (2014).

http://dx.doi.org/10.1071/SR15057

Received 24 February 2015, accepted 26 May 2015, published online 15 January 2016

References

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Binford G (2008) Com response to starter fertilizer and Avail on soils with high soil test phosphorus. University of Delaware Coop. Exp. Bull. 118.

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Chien SH, Sikora F, Gilkes RJ, McLaughlin MJ (2012) Comparing of the difference and balance methods to calculate percent of fertilizer phosphorus applied to soils: a critical discussion. Nutrient Cycling in Agroecosystems 92, 1-8. doi:10.1007/sl0705-011-9467-8

Chien SH, Edmeades D, McBride R, Sahrawat KL (2014) Review of maleic-itaconic copolymer purported as urease inhibitor and phosphorus enhancer in soils. Agronomy Journal 106, 423-430. doi:10.2134/agronj2013.0214

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Patel PR, Gregory TM, Brown WE (1974) Solubility of CaHP[O.sub.4]. 2[H.sub.2]O in the quaternary system Ca[(OH).sub.2]-[H.sub.3]P[O.sub.4]-NaCl-[H.sub.2]O at 25[degrees]C. Journal of Research of the National Bureau of Standards. Section A. Physics and Chemistry 78A, 675-681. doi:10.6028/jres.078A.043

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S. H. Chien (A,C,D) and C. W. Rehm (B)

(A) International Fertilizer Development Center (IFDC), Muscle Shoals, Alabama, USA.

(B) Department of Soil, Water and Climate, University of Minnesota, St. Paul, Minnesota, USA.

(C) Present address: 1905 Beechwood Circle, Florence, Alabama, USA.

(D) Corresponding author. Email: nchien@comcast.net

Table 1. Selected equilibrium constants of various chemical species

Chemical species                 Negative     Reference
                               logarithm of
                               equilibrium
                                 constant
                               (K (A)) (pK)

Al-EDTA                           16.1        Martell and Smith (1974)
Mg-EDTA                            7.31       Martell and Smith (1974)
[([AlF.sub.6]).sup.-3]            23.7        Martell and Smith (1974)
CaHP[O.sub.4] x 2[H.sub.2]O        6.60       Patel et al. (1974)
Ca-Maleic Acid                     2.43       Martell and Smith (1974)
Ca-itaconic Acid                   1.20       Martell and Smith (1974)
Fe-Maleic Acid                     5.15       Martell and Smith (1974)
Fe-itaconic Acid                  <5.15       Present study
Al-Maleic Acid                     5.47       Martell and Smith (1974)
Al-Itaconic Acid                  <5.47       Present study
AlP[O.sub.4] x 2[H.sub.2]O        22.1        Lindsay(1979)
FeP[O.sub.4] x 2[H.sub.2]O        19.2        Lindsay (1979)
U-Maleic Acid                      5.15       Ramamoorthy and Santappa
                                                (1969)
U-Itaconic Acid                    4.86       Ramamoorthy and Santappa
                                                (1969)

(A) 'K' refers to dissociation constant of cation-chelate or
solubility-product constant of solid water-insoluble compound.
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Date:Feb 1, 2016
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