A method for monitoring and/or modifying an active agent concentration in a spray liquid of a spray device including a plurality of spray nozzle units for deploying the spray liquid for agricultural purposes. The method includes receiving one property signal each including a piece of property information of the spray liquid detected with the aid of a sensor unit of the spray device in a through-flow area of one each of the spray nozzle units, an active agent concentration in the spray liquid being ascertainable using the piece of property information； and outputting an information signal to a display unit and/or a control signal to an intermixing unit and/or a delivery unit of the spray device as a function of the received piece of property information of the spray liquid and/or of an active agent concentration in the spray liquid ascertained using the piece of property information.
