EFFECT OF CROP ROTATION AND N AND P FERTILIZER ON YIELDS OF SPRING WHEAT GROWN ON A BLACK CHERNOZEMIC CLAY

The effects of crop sequence, rotation length, and fertilization on yields of spring wheat were examined for 14 crop rotations over a 25-yr period on a Black Chernozemic heavy clay soil at Indian Head, Saskatchewan. Plots that were fertilized with N and P received the generally recommended rates for the region during the first 18 yr but in the last 7 yr fertilizers were applied based on soil tests. Yields of fertilized wheat grown on fallow were similar for the 2-yr fallow-wheat and the 3-yr fallow-wheat-wheat rotations (25-yr avg. 2505 kg ha−1). Yields of fertilized wheat grown on stubble were also similar within the monoculture rotations. During the first 18 yr, yields of fertilized stubble-wheat averaged 1656 kg ha−1 or about 64% of comparable fallow-wheat yields; but, during the last 7 yr, stubble-wheat yields were generally similar to those obtained on fertilized fallow. Removal of the straw each year from a 3-yr fallow-wheat-wheat rotation did not affect fallow- or stubble-wheat yields. Application of recommended rates of N and P fertilizer increased the 25-yr fallow-wheat yields by 11% (from 2254 to 2505 kg ha−1); the yield increases were significant about 70% of the time. On stubble, application of N and P fertilizer increased wheat yields by 47% (from 1130 to 1656 kg ha−1) during 1960–1977 and by 142% (from 935 to 2263 kg ha−1) during 1978–1984. The yield increase from fertilizing stubble-wheat was significant in 24 of 25 yr. Including grass-legume forage, or legume green manure crops in the rotation increased yields of wheat grown on the unfertilized partial fallow by 15–24% and on unfertilized stubble by 33–71%; the yields were similar and sometimes higher than those obtained on fallow in the well-fertilized monoculture wheat rotations. In comparison, the yields of unfertilized stubble-wheat in the cereal-forage rotations were generally similar to those obtained on fertilized stubble in monoculture rotations during 1960–1977, though they were lower during 1978–1984 when the monoculture rotations began receiving fertilizer based on soil tests. Yields of wheat grown on flax stubble that received fertilizer at the average rate of 68 kg ha−1 N plus 22 kg ha−1 P2O5 since 1978 were generally similar to yields obtained on fertilized fallow (avg. 2546 kg ha−1). These yields averaged 13% higher than yields of wheat grown on cereal stubble in monoculture rotations that received slightly more fertilizer N. Yields of fallow- and stubble-wheat were generally maintained over time with the application of recommended rates of N and P fertilizers, or by inclusion of legume-forage crops in the rotation, but yields of unfertilized stubble-wheat declined with time possibly reflecting declining soil fertility.Key words: Wheat, nitrogen and phosphorous fertilizer, crop sequence, cereal forage rotations, legume green manure crops