Studies on response of crops to potassium, and use of fertilizer-and soil-potassium in soils with different potassium supply potentials were made with successive cropping. The availability of potassium,varied with different potentials of potassium in soils, but both available and, slowly available potassium contents decreased in varying degree with the uptake of potassium by crops. The potassium in plant was partly the potassium released from minerals which constitutes only 0.04-1.58% of total mineral-potassium. The mineral potassium is released too slow to meet the requirement of a crop during the growing season. Comparison between the canon-exchange resin bag method and chemical or biotic methods in determining potassium release from minerals showed that the results obtained by resin bag method and biotic method were quite similar. The amount of potassium extracted by resin bag method was also very close to that taken up by rice or Sparrina artglica (r=0.916^**).It was also indicated that resin had method is more convenient than the biotic one.