[en] The uptake ratio of supplying P³² labelled double superphosphate and the hastening efficiency of the uptake by addition of magnesium sulfate to the fertilizer were studied on the xPopulus albaglandulosa planted with 0/1 cutting in 1975. The results are summerized as follows. Average 13% of supplying double superphosphate was absorbed into xPopulus albaglandulosa planted on the reddish heavy clay soil in Institute of Forest Genetics. The accumulation of absorbed magnesium was more amount in leaf than in stem. The uptake ratio of supplying double superphosphate was able to increase up to 16%-33% by the addition of magnesium sulfate to the fertilizer. It might be possible to increase the tree growth following the acceleration of photosynthesis due to the increasing amount of magnesium known to be a component of chlorophyll in leaf as well as to hasten the efficiency of uptake of phosphorus by the addition of magnesium to double superphosphate. (Author)