Vinasse is an important agribusiness by-product frequently reused in sugarcane crop as an alternative potash (K) source. When appropriately applied, the added nutrients can be deduced from conventional fertilization, decreasing the production cost. On the other hand, an excessive proportion of vinasse can induce the lateness maturation and the damage to the sugarcane technological quality, including reduction in sucrose content. Some areas in Aleluia Farm, Araras/SP, Brazil, have shown serious problems with K lixiviation, as a consequence of high vinasse in the last decade. Sugarcane crop require 185 kg K₂O ha^-1 for high productivity. An oversupply, when the K supply from vinasse surpass the sugarcane nutritional requirement, describe an excess and characterize a situation of potential pollution, according to standards for use and disposal of vinasse (P4.231 rule), established by CETESB, the most important brazilian environmental protection agency. The objectives of this study were to evaluate soil fertility changes, especially with regard to K dinamic, and sugarcane technology quality, after sucessive applications of vinasse. In two distinct sampling periods (february and november/2004), samples were taken from differents layers (0-0.2; 0.2-0.4; 0.4-0.6; 0.6-0.8 and 0.8-1.0m) of a Rhodic Eutrudox, wich were representative of areas under sugarcane cultivation submited to followed management: (i) five areas with vinasse fertirrigation along ten consecutive years; (ii) one area with conventional mineral K fertilization; (iii) one area of native vegetation adjacent to agricultural area; (iv) and samples from no covered channels that conduct vinasse to the farming. The results from native vegetation area attested the high fertility of undisturbed systems and showed that K lixiviation is a natural phenomenon. Samples from fertilized areas presented lower K content (122 kg K₂O ha^-1) than native areas (273 kg K₂O ha^-1), as expected due to agricultural practices. The summation of K contents of 0-1.0m samples from vinasse fertirrigated areas, for the first sampling period, varied between 385 and 2369 kg de K₂O and was 2-19 fold higher than the K content of areas with no vinasse, confirming the potential pollution of K by residue. For the second sampling period, the results indicated intense K lixiviation, mainly from 0-0.6m. Approximately 4630 kg K ha^-1 were added in fertirrigated areas and 1280 kg K were leached from the 1.0m layer. This K content is suficient for manuring of seven sugarcane full cycle. Independently of both sampling depth and period, the proportion of K soil saturation exceed the advisible 5%. At superficial layer, the K content exceeds about 11 fold the maximum dose recommended for sugarcane. About 9830 kg K₂O ha^-1 were detected in the 0.8-1.0m layer under vinasse channels, indicating the necessity of covering ducts as an attempt of decrease K leaching. Macro and micronutrients levels were satisfactory or even excessive in fertirrigated areas. Sugarcane areas of Aleluia Farm are receiveing high vinasse burden. High K levels instigated a sugar productivity decay, leaching of K mobilized to deep layers. In this case, sugarcane cultivation can not be considered a sustanaible system, representating for the environment a serious pollution situation, since the excess of K added will be leached and will not be recovered.