Soil  richness   in  available magnesium,   phosphorus  and  potassium  forms is one of the key  factors  of fertility,  which  ensures   the  potential   of soil  for satisfying nutritional  requirements   of plants.   The  aim  of the  present research   has been to  determine  the  effect  of crop rotation  and  varied mineral   and  organic fertilisation on the  content  of available  Mg, K and  P forms.  Soil  was  sampled from  a  long-term  experiment,  carried   out  on Luvisol formed from  sandy loam (soil  valuation class  IVa, very  good rye complex).  The  experiment was performed  in  a  3-factor  design,  which included  two  types  of crop  rotation  as well  as FYM and  nitrogen fertilisation. Basic  physicochemical   properties  of soil were  determined. The  content  of available  forms  of magnesium   was  defined with  Schachtschabel method and  the  content  of potassium  and  phosphorus  - with  Egner-Riehm method  (DL). The  reaction  in  the  arable-humus horizon  of the  soils  ranged from  4.7  to 6.2.  It  was  found  that the  Corg  to  Nt  ratio  in  the arable-humus  horizon  of soils  of all  the  experiment   variants was  typical   of biologically  active  soils.  The  content  of available  magnesium   ranged  from 27.8 to 58.3  mg kg-1  of soil,  while  its water-soluble  forms  varied  from  3.5 to 6.8 mg kg-1 of soil.  The  highest  content  of magnesium,   potassium  and phosphorus forms  available  to plants  was  observed after  the  application  of FYM in the doses of 60 and  80 t  ha-1 combined  with  mineral   nitrogen fertilisation, in both crop rotation  regimes.  The  content  of magnesium  and phosphorus  forms available  to  plants   was  significantly  positively  correlated with   the  content  of organic  carbon.  Soil  sampled  from  those  plots demonstrated  higher classes  of the  content  of that  element.  The  research  data  proved  that  the  analysed  soils showed moderate  and  high  richness  in  nutrients available to plants.  High  doses of organic and  mineral   fertilisation,  however, did  not increase  significantly the electrolytic   conductivity  of the soil  solution.