A survey of the agricultural surface irregularities are reported as sources of tractor vibrations. Farm surfaces are generally liable to change with repeated travel over them. The changes of field characteristics by traffic are evaluated by the changes of frequency distributions, power spectral densities and cone indexes. Power spectra of hard farm roads are measured and compared with ISO's generalised road classification. Power spectra of deformable farm lands are compared with several reports of other investigators. (1) Power spectral densities of farm fields had several conspicuous peaks in the road frequency range 0.2-2.5 (c/m). The peaks were caused by tractors or other agricultural machines. (2) On the deformable farm lands, the changes of the power spectral density curves by traffic were noticeable in the range of 0.2-2.5 (c/m). The slopes of curves plotted on log-log scales were steeper in proportion to the numbers of passes of the same rut. (3) A frequency distribution curves of the farm field roughness gave a straight line when plotted on normal probability papers. There was not a bit of difference between the frequency distribution curves of the rut profiles for single tractor pass and those for two passes. (4) Power spectra of measured arables showed that all can be approximated by the well known simple relationship P(F)=CF^<-n> (cm^2/c/m). As the undulations of them were severer, the value of C was smaller and that of n was larger. The range of C was from 1.0 to 10.7 and that of n was from 0.6 to 1.5. Power spectral densities of unsurfaced farm roads fell in the lower end of class E in the ISO's classification. The mean values of C and n were 0.48 and 1.74 respectively.