A plant according to claim 1 in which the means 16, 17, 18 are for automatically selecting the amount A dosed by the dosing means 4 in response to the measured density D and the volume per unit time V to provide a preselected dose A where A=kM and k is a constant, and for automatically varying the volume per unit time V in response to the measured density D to provide a dry mass per unit time M wh