Characteristics of phosphorus adsorption by upland red soils different in fertilization and their affecting factors were studied with X_m-value calculated out of the Langmuir curve as index.The results show:(1) X_m-value is always lower in soil fertilized with inorganic or organic phosphorus than in soil not fertilized with phosphorus.Livestock manure is an effective fertilizer because it makes X_m-value lower than other fertilizers.(2) Organic matter and available-Al significantly influence soil P absorbing capacity,which rises with the increase in available-Al content in soil,but it declines with the increase in organic matter content.pH and available-Fe are minor factors affecting X_m-value in this research.The above-listed results are testified by data reduction-analysis, indicating the content of organic matter was the most influential factor found.(3) Because of the good relationship between X_m-value and ff-value, which can be used as a basis for recommendation of P fertilization in practice,X_m-value may be an index to judge the amount of P fertilizer needed in the field primarily. In a word,these results contribute to research on transformation of phosphorus and determination of P application rate in red soil regions.