Changes in soil structure under various cropping systems

Abstract

The percentage volume of soil occupied by pores larger than 100 µm in equivalent diameter (ε100) was used as an index of topsoil structure. The effects of variables on ε100 porosity values were examined to give information of use to a conceptual model. It was found that root systems, crop canopies and earthworm burrowing may have an effect on preserving ε100pores, but any effects in changing ε100 porosity were small compared with tillage, compaction by traffic, dairy stock treading or slaking and slumping.

It was found that ε100 porosity was as well correlated with other topsoil physical properties as total porosity. Since the measurement of ε100 porosity shows the relative volume of pores that may be readily exploited by the lateral roots of cereals and grasses it was concluded that ε100 porosity is a useful index of topsoil structure.

The effects of various crop species on a silt loam topsoil structure in Canterbury, New Zealand were examined. Three grass species were used (perennial ryegrass (Lolium perenne), Italian ryegrass (Lolium multiflorum) and tall fescue (Festuca arundinacea), as well as clover (Trifolium repens) and wheat (Triticum aestivum). Bare fallow treatments were associated with a significant (p<0.01) decline in aggregate stability, organic matter and total porosity at 0-2 cm depth. Wheat was associated with an intermediate effect between bare fallow and grasses/clover over the two years of study (1984 & 1985). This effect was thought to be due to the ground cover provided by these plants.

In the top 5 cm of soil, ash-free-root-dry-matter weights in the second season at anthesis were significantly lower (p<0.01) for wheat than for any of the grasses or the clover. Clover and tall fescue produced the largest root weights at 0-5 cm depth (0.90 and 1.18 t ha⁻¹ respectively). It was concluded that the low surface root weight of wheat might be unfavourable in maintaining surface organic matter compared with the grasses or clover.

Populations of earthworms were surveyed in selected silt loam topsoils of Canterbury, New Zealand. It was found that mulching compared with burning cereal residues was associated with higher populations of 2 to 3 times. Direct-drilling compared with regular ploughing was associated with higher populations of 1.5 to 2.5 times. Severe compaction by machinery and dairy cattle treading was associated with lower populations of less than 0.5 times those of comparable uncompacted sites.

Controlled laboratory experiments were conducted to study the burrowing of Aporrectodea caliginosa (Sav) at 15, 10 and 5 °C. In the homogeneous silt loam topsoil used, it was found that there were significant (p<0.01) negative correlations between antecedent bulk density and the rates of burrow formation and soil ingestion by this species. Rate of casting above the soil surface was greatest between bulk densities of 1.1 and 1.4 g cm⁻³ (relative densities of 33 and 73%).