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.