Traditionally the annual irrigation water requirements are determined considering necessities of agricultural crops. However another approach should be applied also in order to estimate the impact of irrigation on agricultural soil quality. In general, the irrigation causes direct and indirect impacts on soil. The direct impact is related with influence of irrigated waters on soil properties depending on chemical and biological composition of irrigation water. Such impact is rather well known and realized during relatively short periods (several years). The indirect impact is related with alteration of natural hydrothermal conditions of soil formation processes. This impact is less investigated and is realized during longer term (several decades). Because of long term the prediction of such impact using mathematical modelling is rather difficult. The methodology of indirect impact assessment (different to mathematical modelling) were developed during last years and it consists of establishment of quantitative relationship between some regional properties of virgin not used in agriculture soils and Budyko's radioactive index of dryness RID typical for natural (without irrigation) conditions in each site of soil sampling. It is considered that irrigation modifies RID and should cause corresponded positive o negative changes of agricultural soil fertility. The integral soil fertility index was used for assessment of irrigation indirect impact. The preliminaries results obtained for Mexico show that in some regions the annual irrigation requirements should be reduced up to 30% in order to avoid or reduce expected negative irrigation indirect impact on soil fertility during long terms even sacrificing partially crop yields but conserving soil quality.