Evaluating the soil quality of long-term crop rotations at Indian Head
Crop rotations which differ in fallowing frequency, residues returned and fertilizer additions were hypothesized to have measurably altered the soil physical and biological properties that contribute to a quality soil. This study was initiated to evaluate the role of crop rotation in determining soil organic matter levels and concomitant changes in soil properties. The rotation site was started in 1958 on a Black lacustrine soil at the Agriculture Canada Experimental Farm, Indian Head, Sk. Rotations of unfertilized fallow-wheat(FW), fertilized fallow-wheat-wheat(FWW(N+P;+straw)), fertilized fallow-wheat-wheat with straw baled (FWW(N+P; -straw)), unfertilized fallow-wheat-wheat-hay-hay-hay (FWWHHH), and fertilized continuous wheat (cont.W) were maintained in a modified randomized complete block design. Cultural practices were fairly consistent over time. Fertilizer additions, after 1977, increased to levels recommended by the Saskatchewan soil testing laboratory. However, the overall mean fertilizer additions from 1960 to 1984 varied only slightly among fertilized rotations. Systematic transects across the experimental site revealed a major change in soil type occurring in the northern ranges, which was useful in determining a uniform sampling area. Ap horizon thickness and depth to carbonates suggested that topsoil was being removed from the plot areas and accumulating on the grassed roadways. Continuous wheat and FWWHHH rotations maintained the highest organic C and N concentrations. Soils under FW and FWW rotations contained 13% less organic C and N on average than the cont.W or FWWHHH soils. The amount of light fraction (LF) organic matter and the C:H ratio of the LF were closely related to biological turnover, the LF is a readily available portion of the soil organic matter. Soils under cont.W and FWWHHH contained 1.5 to 2 times more of this active organic matter than those under FWW and FW. Baling straw, reducing fallow frequency, and adding fertilizer did not have a clear impact on total organic matter or the LF. Mineralization of C, N, and S was statistically more sensitive to reductions in fallow frequency, improved fertility and residue removal. Nitrogen and S mineralization followed a ranking similar to organic C and LF-C contents, with cont.W = FWWHHH > FWW (N+P;+straw) > FWW (N+P;-straw) > FW. Soil biological properties in the 7.5 to 15 and 15 to 30 cm depths were not affected by long-term crop rotation, except for higher rates of C mineralization under cont.W and FWWHHH. Soil aggregates were larger and more water stable in the less frequently fallowed cont.W and FWWHHH rotations. Soil organic matter and fertility characteristics were related to soil aggregation, likely through enhanced crop growth and production of roots and fungal hyphae. Sorptivity of water under suction was a sensitive indicator of pore structure. Soils in frequently fallowed rotations had the fewest large pores, whereas cont.W and FWWHHH soils had the most large pores. Long-term crop rotation did not affect soil aggregation below the 7.5 cm depth, except where rotations were sampled directly after forages. Erosion, estimated by comparing 137Cs content in the topsoil, was highly variable but suggested that FW incurred the largest topsoil losses, followed by FWW and cont.W, with the FWWHHH rotation least eroded. Reconstruction of the Ap horizon also indicated that crop rotation had influenced the amount of soil lost. Forage periods and improved trash cover, associated with infrequently fallowed rotations, probably reduced wind erosion. Soil loss from frequently fallowed rotations may be exacerbated by tillage and lateral movement of soil from the plots onto the roadways. Loss of organic matter rich topsoil was negatively related to the level of 137Cs, implicating soil loss as an important process in depleting organic matter. Soil quality as indicated by nutrient supply, soil organic matter content and lability, and soil tilth was best in the least eroded, most productive FWWHHH and cont.W rotations. Frequently fallowed FW or FWW rotations resulted in the lowest soil quality, with straw baling and inadequate fertilizer additions of lesser importance to the overall soil quality.
soil quality, fallow frequency, crop rotation
Master of Science (M.Sc.)