Resumo. ABSTRACTThe quality of seedlings produced at forest nurseries is related to several factors, including irrigation andthe used substrate. Both have as their purposes the production of quality seedlings in a short time period,but over-irrigation can negatively affect the substrate nutrient content. This work aimed to evaluate thechemical characteristics dynamics of a forest substrate subjected to successive irrigation at sub-irrigationtanks in two plastic tubes sizes. To this end, plastic tubes of 110 and 300 cm³ were placed, containing forest substrate without the presence of new seedlings at two sub-irrigation tanks with recirculation, one foreach tube size, built at Universidade Federal de Lavras. It was set up completely randomized design withtwo replications, being the treatment levels 0,6, 12, 18, 24 and 30 irrigations, five per day. The substratechemical characteristics were evaluated: pH, phosphorus, potassium, iron, zinc, manganese, copper,calcium, magnesium, aluminum, boron, sulfur, effective CEC, organic matter and electrical conductivity.Additionally, before the irrigations counted as treatment, water samples were taken to check for possiblechanges in the quality of irrigation recycled water after successive irrigations. The sub-irrigations providedonly changes in the potassium concentration in the substrate in both tubes, and most of the chemicalcharacteristics assessed on the substrate were unchanged as a sub-irrigation function.