Potassium is an essential nutrient for plant growth, and its availability as well as its various forms can be influenced by humic substances, including humic acid. The present study investigated the effects of the type and application rate of humic acid (HA) extracted from cattle and sheep manures on soil potassium forms. Humic acids were applied to the soil at two rates of 0.2% and 0.4%. The control treatment received no humic acid. Soil samples were incubated for 90 days under laboratory conditions at field capacity moisture. At the end of the incubation period, the forms of potassium including soluble-K, exchangeable-K, fixed-K, and structural K were determined. Results showed that in all treatments the distribution of potassium followed the order: soluble-K < fixed-K < exchangeable-K < structural-K. Application of humic acid increased soluble and exchangeable potassium while decreasing fixed potassium; structural potassium remained unaffected by either the type or the rate of humic acid. Application of 0.4% HA isolated from sheep-manure (SHMHA) increased soluble potassium by 40%, whereas the same rate of HA isolated from cattle-manure (CMHA) produced a 20% increase. The increases in exchangeable potassium under the 0.4% SHMHA and CMHA treatments were 37% and 33%, respectively, indicating a stronger effect of SHMHA on the more labile forms of potassium. These findings demonstrate that humic acid extracted from animal manures, particularly sheep manure, has considerable potential to enhance solution and exchangeable potassium and may be recommended as an effective approach for improving potassium availability in soil.