Why is Fe-P in NaOH part of Hieltjes & Lijklema (1980) P fractionation?

**Background:**


In order to separate the different phosphorus (P) species in lake or marine sediments, one can use sequential P fractionation method. Its main idea is to use extractants with weak or strong affinity to P to extract the sediment sample step by step. By doing this, different operationally defined P fractions are extracted sequentially.


There are many methods available, and my question is about the method proposed by Hieltjes & Lijklema (1980). Its procedure is shown in the first screenshot below.


**Question:**


I'm wondering why Fe-P is in the "NaOH extraction" part, which is "17 hours with 50 ml 0.1N NaOH".


In the Table 2 (see second screenshot below) of [the original paper](https://acsess.onlinelibrary.wiley.com/doi/abs/10.2134/jeq1980.00472425000900030015x) (Hieltjes & Lijklema, 1980), we can see that 98.9% of Fe-P is extracted from the 0.1 N NaOH solution. The authors mentioned that this synthetic Fe&P compounds is "*... a suspension of freshly precipitated ironhydroxy-phosphate, aged by standing or moderate heating.*" However, they didn't explain why this synthetic Fe hydroxides can be dissolved in the NaOH solution.


I understand that Al oxides can be dissolved by NaOH, so Al-P is in the "NaOH extraction" part. But Fe oxides can not be dissolved by NaOH, for example [in this posted question](https://chemistry.stackexchange.com/questions/13901/can-i-use-sodium-hydroxide-naoh-to-remove-oxides-from-metals). Then why Fe-P is in this extration?


[![Fig.1](https://i.stack.imgur.com/UQDUP.png)](https://i.stack.imgur.com/UQDUP.png)


[![Fig.3](https://i.stack.imgur.com/NCUYO.png)](https://i.stack.imgur.com/NCUYO.png)


