in order to aid in an understanding of the invention , fig1 illustrates a cross - section through the ground over a portion of a field that is to be fertilized . beneath the surface 2 of the ground is a layer 4 of soil , and beneath the soil is a formation 6 of rock . the soil is a loose surface material consisting of disintegrated rock with an admixture of organic matter and soluble salts , and is able to support growth of plants . the depth d of the rock formation beneath the surface 2 of the ground varies over the field from as little as a few inches to several feet . a fertilizer applicator 10 is used to apply fertilizer to the ground . the fertilizer applicator 10 is of a conventional type , and has a valve 14 ( fig2 ) for controlling the rate of application of the fertilizer to the ground . the setting of the valve is controllable by means of a control signal applied to the valve over a control line 16 . if , for example , the valve is adjusted by rotating a shaft , the control signal may be in the form of a train of pulses that drive a stepping motor to rotate the shaft through a predetermined angle to the desired setting . the direction of drive of the motor depends on the stator winding to which the control signal is applied . the fertilizer applicator is towed behind a tractor 8 . the tractor carries a ground penetrating radar unit 12 . the ground penetrating radar unit 12 is of the kind that is currently used in geophysical surveys , and operates by emitting a pulse of electromagnetic energy into the ground and detecting reflections of electromagnetic energy received from the ground . the electromagnetic energy penetrates the soil but is partially reflected from the interface between the soil layer 4 and the rock formation 6 . some of the reflected energy is detected by the radar unit , and by measuring the interval between emission of a pulse and detection of the reflected energy , the radar unit is able to provide an output signal that represents the depth d . a control unit 18 receives the output signal of the radar unit 12 and provides the control signal to the valve 14 . in the case of the example mentioned above , the control unit receives the signal representing the value d and for a selected change in the value d the control unit generates a pulse suitable to cause the rotor of the stepping motor to rotate by one angular step . the winding to which the pulse is applied depends on whether the value d increased or decreased . the control unit receives additional signals from a user console 20 representing maximum and minimum rates of application of fertilizer . the user console is also to provide a signal that overrides the output signal provided by the radar unit and allows the rate of application of fertilizer to be manually controlled by the user through the console . it will be appreciated that the present invention is not restricted to the particular embodiment that has been described with reference to the drawings , and that variations may be made therein without departing from the scope of the invention as defined in the appended claims and equivalents thereof . for example , the radar unit need not be mounted on the tractor , and may be mounted on the applicator itself . it is not essential that the output signal of the radar unit be used to control directly the setting of the control valve : the radar unit may provide a readout to the operator of the applicator , who then manually adjusts the rate of application through the operator console . alternatively , the radar unit may be used to generate a record representing the value of the depth d as a function of position in the field to be fertilized , and this record may be subsequently used to control application of fertilizer to the field . the record might be a punched tape that is used by a tape reader to control the valve setting , or it might be in human - readable form and provided to the operator of the applicator to enable him to adjust the setting of the valve using a conventional control console as the applicator is towed over the field . the invention is not restricted to fertilizer as the crop enhancing material . for example , the potency of herbicides is dependent on clay content and organic matter content , and if the radar unit &# 39 ; s output signal were representative of clay content or organic matter content , the signal could be used to control application of herbicides . the radar unit could also be used to locate hardpan and different soil types .
a ground penetrating radar unit is scanned over the surface of a field , and provides a signal representative of the variation in the depth of soil along the scanning path . the signal is used to control the rate of application of fertilizer to the soil .