The relative effects of using light (2–3 Mg) versus heavier (5–7 Mg) tractors, shallow (15 cm) versus
deeper (25 cm) ploughing and on-land versus in-furrow wheel placement during ploughing were
investigated from 2003 to 2006 in organic rotations (wheat or barley, green manure, oats with peas) and
conventionally fertilized barley. Trials were located on loam soil in south-eastern Norway and silty clay
loam in central Norway. Ploughing was performed in spring, when the topsoil moisture content was at or
below field capacity, using single furrow ploughs that allowed alternative wheel placement and resulted
in complete coverage of the surface by wheels each year (ca. 3 times the normal coverage during
ploughing). Low tyre inflation pressures (80 kPa) were used throughout. The use of a heavy tractor
increased topsoil bulk density slightly in the loam soil, and, in combination with in-furrow wheeling, it
reduced air-filled pore space and air permeability at 18–22 cm. On the silty clay loam, the use of a heavy
tractor did not increase bulk density, but it reduced air-filled pore space throughout the topsoil. Infurrow
wheeling reduced air-filled pore space in this soil also, compared to on-land wheeling.
Penetration resistance was in this soil always greater at 15–25 cm depth after shallow than after deep
ploughing, especially with in-furrow rather than on-land wheeling. Shallow ploughing led on both soils
to marked increases in perennial weed biomass compared to deep ploughing. Earthworms were hardly
affected by the treatments, but in the loam in 2006 a higher number of individuals were found where the
light rather than the heavy tractor had been used. Few significant treatment effects were found on grain
yield and quality. Deep ploughing with a light tractor gave the highest wheat yield and protein content in
2 years on the loam soil, and on the silty clay loam the yield of conventionally fertilized barley was higher
after deep than after shallow ploughing. In summary, limited evidence was found to support the use of
on-land rather than in-furrow wheeling when ploughing is performed at favourable soil moisture and
with tractor weights < 5 Mg. There is, however, reason to be wary of using heavy tractors (>5 Mg), even
under such conditions. With regard to ploughing depth in organic rotations dominated by cereals, the
need to combat perennial weeds by deep ploughing weighs probably more heavily than any possible
beneficial effect of shallow ploughing on stimulating nutrient turnover