THE DECLINE OF SUMMERFALLOW ON THE CANADIAN PRAIRIES

The largest and most economically and environmentally significant chan ge in agricultural land use in the Canadian Prairies during the past q uarter-century has been the decline of summerfallow. To understand the geographical patterns of this decline, the reasons why fallow became so widespread and important in the Prairies are examined. Moisture acc umulation, nitrogen release, weed control, the proportion of cropland sown to wheat, and marketing controls appear to be the main factors, a lthough they vary in relative importance throughout the region. Geogra phical patterns of decline since 1971 are largely accounted for by inc reasing use of manufactured fertilizers and herbicides, expanding mark ets, and a growing awareness of the disadvantages of fallowing. Overal l, the patterns of fallow have become more closely aligned with soil m oisture zones.