The nature of soil P is a problem that has concerned soil scientists for many decades. A puzzling aspect of the problem is that reaction between soil and P does not seem to reach equilibrium, but rather continues at a rate that is proportional to a fractional power of time. This has been measured in terms of the decline in effectiveness of P fertilisers for plant growth (Barrow 1974), and in terms of the declining concentration in the solution (Barrow and Shaw 1975). A further component of the puzzle is that the rate of reaction increases with increasing temperature (Barrow 1974, Barrow and Shaw 1975). This is a component that must be included in any explanation; it also provides a useful experimental tool. By using increased temperatures, the rate of reaction can be increased thereby producing effects in a few days that would take months or years at lower temperatures.
Further, the behaviour of phosphate is not peculiar; many other reactants behave similarly. These include molybdate, fluoride, sulfate, selenate, selenite, zinc, cadmium, nickel, and cobalt. (For references see Barrow (2022).)
Two views about the nature of soil P exist. One can be traced to the work of Hall and Amos (1906). They extracted soil with dilute acid and concluded that “soil contains compounds of phosphoric acid of varying solubility”. This was disputed by Russell and Prescott (1916) who wrote “something more is concerned than a mere mixture of phosphates…”. Nevertheless, the idea that there was a mixture of P compounds persisted and was further developed, for example by Lindsay (1979). Although frequently quoted, there are two problems with Lindsay’s work. One is that he did not calibrate his theories against observations; the other is that he dealt entirely with equilibrium and therefore had no means of describing the slow, continuing reaction. According to this view of the nature of soil P, the decreasing effectiveness of P with time is caused by conversion from one compound to another. Such conversions are thought to be reflected by soil P chemical fractionation schemes.
Alternative theories depended on developments in surface chemistry which largely occurred in the 1960s and 1970s. It is central to these theories that the adsorption of P (and other specifically sorbed ions) is strongly dependent on the surface potential of variable charge materials in soil. This potential decreases as the pH increases and also decreases with increasing reaction with (negatively charged) P ions. By taking these effects into account, it is possible to quantitatively describe many aspects of the reaction of specifically sorbed cations and anions with soil (Barrow 1999). However, in order to explain the continuing reaction it is necessary to make a further assumption: that after reaction with the surface, the adsorbed ions penetrate the reacting material by solid-state diffusion. The development of this idea is chronicled by Barrow (2022) and evidence is presented there from which it is argued that it is the only one that is consistent with the observations. For example, the activation energy for the forward reaction is similar to that for the back reaction; this would be very unusual for chemical reactions but is characteristic for diffusion. When this assumption is incorporated into models, it quantitatively explains observed effects (Barrow 1999).
However, some prefer to explain the continuing reaction in terms of rearrangement of surface molecules. Frossard et al. (2011) wrote that “on the basis of long-term sorption experiments, (we) concluded that inorganic P ions located on the solid phase of the soil are distributed along a continuum of solubility”. Zhang and Selim (2007) proposed that adsorbed ions react with sites that are described as “equilibrium sites”, “kinetic sites”, “consecutive irreversible sites”, and “concurrent irreversible sites”. Shi et al. (2008) proposed that for zinc there was a fast reaction associated with monodentate binding sites and a slow reaction associated with bidentate binding sites. It is difficult to understand how surface reactions could be slow enough to explain the observed behaviour. Others (Penn and Camberato 2019) do not consider the continuing reaction in their review of soil phosphate chemistry.
The proposal that P ions penetrate the reacting surfaces and thereby change their properties is essential to understanding the increased effectiveness of subsequent P applications (Barrow 1999, 2022). In order to provide further evidence, we have exploited a technique pioneered by Strauss et al. (1997). They reacted P with samples of goethite ranging from well-crystallised to poorly-crystallised and treated the samples with 5M HCl. They found that, for a well-crystallised sample of goethite, most of the P was dissolved when the little of the iron was dissolved, but for a poorly-crystallised sample the P was not fully dissolved until about 60% of the iron was dissolved. They interpreted this, together with other evidence, as showing that P penetrated poorly-crystallised goethite, but mostly remained on the surface of well-crystallised goethite.
Dissolution with acid is certainly not new; it was used by Hall and Amos (1906). The difference is that by simultaneously measuring iron and aluminium dissolution, we thought that much could be learned about the nature of soil phosphate.
In the work reported here, we used this technique, and also dissolution using alkali, to study the effects of period of reaction. We included a treatment with alkali because we wanted to compare the effects of continuous treatment with both acid and alkali with the results obtained from soil fractionation techniques in which a treatment with acid follows a treatment with alkali. The P extracted by these treatments has been interpreted as representing different pools of P.
We compared the dissolution procedures with the fractionation procedures of Chang and Jackson (1957) and of Chen et al. (2000), a variant of Hedley et al. (1982). We included the method of Chang and Jackson (1957) partly for historical reasons because it was the first widely-adopted fractionation procedure. Although widely deprecated, it still has a large influence. Between January 2020 and May 2022, this method was cited 320 times (Google scholar). We wanted to compare it with an updated method and chose the method of Chen et al. (2000) because it was developed after consideration of previous chemical-fractionation methods.
We studied a sample of goethite, a sample of aluminium oxide, and a soil. We reacted P with these materials at 70°C in order to accelerate the reaction and produce, within periods measured in days, effects equivalent to those requiring years at lower temperature. Our aim was to test whether changes through time in P, Fe and Al were consistent with P diffusion into the adsorbing particles. We also chose to compare the results with those obtained from chemical fractionation schemes.