Abstract
Extractive tests for determining the plant-availability of soil phosphorus (P) give varying results due to the inherently different characteristics of the extraction solution. Generally, classical soil P tests such as the Olsen or calcium acetate/lactate (CAL) method do not give an indication on the total amount of plant available P, but merely give an indication of the equilibrium between soil and extraction solution. It is also not entirely clear which fractions of P are directly determined through the various methods of extraction, i.e., determined P must not be immediately plant available, as is the case for rock phosphate. It is therefore possible that extraction methods either over or under estimate the amount of P available for plant consumption. In this research, we compared three methods of soil P determination (CAL, Olsen and diffusive gradients in thin films (DGT)) with regards to their ability to determine P species (Ca(H2PO4)2, CaHPO4, Ca3(PO4)2 and Inositol-6-hexakisphosphate) added to soils of high sorption capacity, immediately after as well as two weeks after application. For each of the methods, it could be shown that sorption processes in the soil immediately (0 days incubation) fix P to a point where it is not extractable through any of the described methods. These sorption processes continue over time, leading to a further decrease of determined P. The acidic CAL extraction method gives higher results of extractable P compared to the Olsen method. Due to the extraction of Ca3(PO4)2, the CAL method may overestimate immediately plant-available P. The most suitable methods for the determination of immediately plant available P may therefore be the Olsen and DGT methods. Organic IP6 is not determined by any of the extraction methods. At low concentrations of soil P, the DGT method may fail to give results.
Highlights
In recent years, attention has been brought to the depleting reserves of phosphate rock and the necessity of improving agronomic approaches to make the best use of available phosphorus (P) in agricultural soils, thereby reducing the need for frequent applications of P by farmers [1]
The methods more likely describe an equilibrium between the soil and extracting solution, which is helpful only for classifying soils according to their respective PCAL or PDL concentrations in combination with long-term analyses of yield in dependency of soil P status
P eluted a binding a diffusive of diffusive gradients in thin films
Summary
Attention has been brought to the depleting reserves of phosphate rock and the necessity of improving agronomic approaches to make the best use of available phosphorus (P) in agricultural soils, thereby reducing the need for frequent applications of P by farmers [1]. In order to improve agronomic practices that allow a reduction of P application to soil, it is necessary to relate P application to the P status of the soil, which can be determined through wet extraction of soils with acidic or alkaline extractants. In Europe, the extraction methods for the determination of soil P classification vary. The methods more likely describe an equilibrium between the soil and extracting solution, which is helpful only for classifying soils according to their respective PCAL or PDL concentrations in combination with long-term analyses of yield in dependency of soil P status. Soils classified as low in PCAL lead to high yields despite the omission of P application, while high P soils may respond to the application of mineral P
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