Gs. Francis et al., Soil degradation under cropping and its influence on wheat yield on a weakly structured New Zealand silt loam, AUST J SOIL, 39(2), 2001, pp. 291-305
Results from the first phase of a long-term experiment showed that, after 6
years under pasture, several soil quality attributes had improved compared
with soil cropped annually. The objectives of this study were to quantify
the effects of pasture-induced increases in structural stability and organi
c matter (N fertility) on wheat grown in 3 successive seasons following pas
ture cultivation. Growing winter wheat after the ploughing of land that had
previously grown perennial grass resulted in gradual reductions in soil or
ganic C and total N. Reductions in soil microbial biomass C and earthworm p
opulations were much more rapid. Soil aggregate stability declined rapidly
in the first year after ploughing, but more slowly after that. Soil macropo
rosity increased after ploughing, mainly due to the relief of compaction ca
used by sheep treading during grazing. The contrasting soil conditions that
existed at the end of the first experimental phase significantly affected
the harvest yield of the first and second wheat crops, with yields 2-3 t/ha
greater after perennial grasses than after annual crops. Variations in har
vest yield and N uptake were explained by differences in soil N fertility a
nd soil structural conditions. Treatment effects on yield were not detected
in the third wheat crop. For the structural condition and N fertility of t
his soil, the extent of improvement during 3 years under perennial pasture
was similar to the extent of decline under 3 years of cropping. This sugges
ts that similar lengths of pastoral and arable cropping are needed in crop
rotations for the long-term maintenance of these properties in weakly struc
tured silt loam soils in New Zealand.