Chemical changes in standing cereal straw residues in chemfallow and its relationship to N, P, and S availability in two cropping systems
In recent years, conventional mechanical fallow has slowly shifted to chem-fallow on the prairies. With the chemical system of weed control, the straw is not mixed with the soil until seeding the following spring; meanwhile the nutrient status of the straw may change during the fallow period. The C:N, C:P, and C:S ratios may be expanding, resulting in a potential for increased immobilization once the straw is worked into the soil. To determine how the nutrient status of the standing straw residue may be changing, four chem-fallow treatments were chosen and the standing cereal residue sampled at six week intervals starting in June of 1993. Two treatments are located in the Brown soil zone (site 1) and two are located in the Black Soil Zone (site 2). At site 1 there is a treatment of wheat-fallow and a treatment of barley-fallow. At site 2 there is a wheat-fallow treatment and a treatment of wheat in rotation with flax, peas, and fallow. Site 1 is pre-tilled prior to seeding while site 2 is direct-seeded. Results for the first three months of the chem-fallow period reveal that water extractable sulfate, phosphate, and total nitrogen decrease over time accompanied by a widening of the C:N, C:S, and C:P ratios. This may suggest a greater immobilization potential per unit weight of straw when worked into the soil.
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