The fitted multivaliate quadratic regression equation can better express the relation between nodulation and nitrogen fixation of alder and physical-chemical factor of the soils.The best mathematical model indicates that soil pH is the main factor affecting nitroge fixation amount.The associated effect of the available phosphorus with available potassium,the CO₂ with organic matter,or the CO₂ with available molybdenum is the secondary factor.Deficiency of the available phosphorus is the limitation to nitrogen fixation of the alder.The highest N₂-fixing capacify of the symbiont occurs in the alluvial soil and yellow earth (except the calcareous parent material); and higher in the purplish soils on Shaximiao parent material,but low in strongly calcareous and acid purplish soil.