Abstract: 【Objectives】 Soil organo-mineral complexes contain most of soil nutrients that have become a most important part of soil nutrients. Change of soil potassium contents affects the contents of potassium in different sizes of organo-mineral complex. So long-term experiments of red soil started from 1990 (Qiyang) and 1986 (Jinxian) were carried out to investigate distribution of exchangeable, non-exchangeable and total potassium (K) in different sizes of organo-mineral complex. 【Methods】 The soil samples of 0-20 cm depth were collected from the four treatments in the long term experiment at both sites: No fertilization (CK), mixed application of chemical nitrogen and phosphorus fertilizer (NP), mixed application of NP combined with chemical potassium fertilizer (NPK), mixed application of NPK combined with manure (NPKM) treatments. Stokes law was used to calculate sedimentation rates of different sizes of organo-mineral complex which were separated using siphon. The soil exchangeable, non-exchangeable and total potassium were determined. 【Results】 In the two sites, 96.4%-98.9% of the exchangeable K, 87.9%-96.7% of the non-exchangeable K and 95.1%-96.1% of the total K are in 0-50 m sizes of organo-mineral complex. Especially, 76.3%-92.3% of the exchangeable K, 45.8%-73.7% of the non-exchangeable K and 49.4%-70.6% of the total K at both sites are mainly in 2 m size complex. Comparing with CK, the contents of non-exchangeable K under NP are decreased by 5.4 to 8.3 mg/kg, and the decreased ratios are from 8.2% to 16.3% at both sites. The contents of the exchangeable and non-exchangeable K in 2 m complex are mostly influenced by the potassium fertilization, and compared with NP at Jinxian site, the contents under the treatments of NPK are increased by 36.7 and 9.5 mg/kg with the increased ratios of 64.5% and 15.7%, and the contents under the treatments of NPKM are increased by 147.4 and 91.2 mg/kg with the increased ratios of 258.9% and 151.1%. Compared with NP at Qiyang site, the contents of exchangeable and non-exchangeable K under the treatment of NPK are increased by 52.9 and 20.3 mg/kg with the increased ratios of 104.9% and 47.6%, and the contents under the treatment of NPKM are increased by 219.5 and 41.3 mg/kg with the increased ratios of 435.9% and 96.9%. The contents of total K in different sizes of organo-mineral complex have little changes under the long-term fertilization. The main sources of K which plants uptake are 2, 2-10 and 10-50 m sizes of organo-mineral complex. Both the exchangeable and non-exchangeable K in 2 m size of organo-mineral complex have significant positive correlations with crop yield (P0.05). 【Conclusions】 The organo-mineral complex of 2 m is the major K pool. The long-term fertilization without K induces the decreases of the contents of exchangeable and non-exchangeable K in 2 m size of complex, while K fertilization is beneficial to increasing the contents of exchangeable and non-exchangeable K in this size of organo-mineral complex. The organo-mineral complexes of 2, 2-10 and 10-50 m contain most of the potassium, which are the main sources of potassium absorbed by plants. There are positive correlations between the contents of potassium in different sizes of organo-mineral complex and crop yield in red soil.