Evaluation of the Egner–Riehm DL and Mehlich 3 Tests for the Determination of Phosphorus: The Influence of Soil Properties on Extraction Efficiency and Test Conversion

The leaching of phosphorus (P), together with nitrogen (N), into deep water and runoff from fields into surface water has caused the dangerous phenomenon of eutrophication, which threatens the Baltic Sea. This process has led to a revision of fertilizer recommendations for farmers in most European countries. The basis for proper recommendations is the determination of soil P using a soil test. There are many different soil tests used in Europe for the determination of plant-available P in soil, which is primarily the result of the different soil conditions in the individual countries. In Poland, two soil P tests are currently used: the Egner–Riehm DL (DL) test and the Mehlich 3 (M3) test. The aim of our study was to determine the extraction efficiency of the DL and M3 tests, to explore the possibility of converting the results of one test to another, and to compare the diagnostic value of the tests. For this purpose, a collection of 237 soil–plant sample pairs was taken from maize fields in Poland. The phosphorus content was determined in all the soil and plant samples, with two methods used in the soil samples: DL and M3. The results of our study show that it is not possible to state unequivocally which test extracts more P from the soil. The extraction efficiency of the tests depends on the specific soil properties, particularly pH and texture. The most reliable conversion of DL test results to M3 is possible for soils that contain a 21–35% fraction characterized by particles less than 0.02 mm in diameter, regardless of soil pH. Furthermore, the DL test has better diagnostic value than M3, especially for alkaline soils.