The effect of organic matter amendments on soil surface stability in conventionally cultivated arable fields

In this study, new and traditional organic wastes (green waste compost, farmyard manure (FYM), anaerobic digestate or straw) were ploughed into an arable field experiment at a range of rates (1–3 t C ha−1) and under spring and winter cropping rotations for 5 years. The stability of the soil surface structure (<5 cm) was assessed in Years 3, 4 and 5 to guide the use of organic wastes in arable field management. Aggregate stability was determined by immersing each soil sample in water (fast-wetting test, relevance to summer storms) and measuring the size distribution of the aggregates (mean weight diameter, MWD). The MWD value was compared with the different classes of aggregate stability (very stable, stable, medium, unstable or very unstable) to classify each result. All the arable field samples were classified as unstable, and there were no significant differences (p > .05) in MWD after any treatment. Potentially, this approach is too imprecise to detect differences, so pore-scale analyses using CT scanning were used. There were no significant differences in porosity or pore shape (features associated with microbial activity) between the control and organic waste treatments. The results contradicted published data from the Broadbalk experiment (established in 1843), which was the basis for this study, so its soils were studied to understand this discrepancy. Broadbalk soils were classified as unstable, including soils treated with annual FYM applications. Instead, we found a significant relationship between MWD and clay content and previous interpretations had not taken this variable (23%–39% clay content) texture into consideration. In conclusion, to achieve a stable soil surface structure, a 150% improvement in aggregate stability would be needed here and ploughing in organic wastes was not a successful management approach on these arable field experiments.