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.