An ongoing 34-yr crop rotation experiment being conducted on a heavy clay at Indian Head, SK, allowed a snap-shot assessment to be made of the influence of fertilization, legume green manure and legume-grass hay crops, and frequency of cropping to monoculture hard red spring wheat (Triticum aestivum L.) on deep leaching of soil nitrate (NO3). The results showed that NO3 leaching was greatest when summer fallowing was most frequently employed. Although N fertilization may increase NO3-N leached, using a soil test as a guide to choosing fertilizer application rates and cropping annually will minimize NO3 leaching. Further, we found that, although deep-rooted forage crops such as alfalfa (Medicago sativa L.) remove NO3 and water to a depth of 2.4 m, and because legumes increase the N supplying power of the soil, considerable NO, leaching can still occur under these systems. This is especially true if legume plowdown is followed by a fallow period, because during the fallow period, copious net N mineralization may occur together with increased soil moisture storage. These results do not imply that NO3 leaching is pervasive on the Canadian Prairies, but they do confirm certain well-known principles: viz., that NO3 leaching can be expected whenever conditions favor NO3 accumulation and water buildup in soil. Consequently, it is important to keep the land cropped for a large proportion of the time and to use fertilizers at moderate rates based on soil tests.
