Fig. 4 Percentage of the whole ³¹P-nuclear magnetic resonance spectral area allocated to each particular P form in the size separates of the Askov soils. Apart from these quantified P forms, traces of phosphonates were found in the unfertilized (0) and inorganic fertilized (NPK) treatment clays, and traces of inorganic orthophosphate were found in NPK and animal manure (AM) treatment clays