8032: Tilth formation, aggregate hierarchy, and self-organization in differently managed, arable soils

Elmholt, S.; Schjønning, P.; Munkholm, L.J. and Debosz, K. (2006) Tilth formation, aggregate hierarchy, and self-organization in differently managed, arable soils. [preprint]^*

Summary

This paper presents experimental data on tilth-forming agents in arable humid sandy loam soils, addressing two field pairs (FP1 and FP2) from neighbouring farms. FP1 included two dairy farm soils, one organically cultivated with high inputs of nitrogen and dry matter and a diverse crop rotation with grass. The other conventionally cultivated with no grass but high inputs of nitrogen and dry matter. FP2 included an organically cultivated dairy farm soil with high inputs of dry matter but less grass and nitrogen than the organic FP1 soil. The other was conventionally managed with no animal manure, continuous growth of small grain cereals and rape and a low input of dry matter. Soil sampling and fractionation aimed at yielding soil fragments that had detached from the bulk soil in the planes of highest mechanical weakness. Aggregates of 4-8 mm, 0.5-1 mm and 0.063-0.25 mm were used for analyses of microbial biomass, ergosterol, clay dispersibility, hot-water extractable carbohydrates (CHWE), and hyphal length.
Our results did not support the aggregate formation theory by {Tisdall & Oades 1982 430 /id} that the different biotic bonding and binding agents act at different aggregate sizes. They rather indicated that differently sized aggregates in arable, frequently tilled soils are random fragments of larger-sized soil bodies. This was most clearly shown for fungal hyphae, which were equally abundant in micro- and macro-aggregates.
FP2 had higher levels for all biological variables in the organically than conventionally cultivated soil. In management terms, the poor conditions in the conventional soil were most likely caused by the low input of dry matter. Both soils in FP1 and the organic soil in FP2 were all in a state, where the biotic bonding and binding agents interacted with the soil mineral particles to create a stable, yet friable structure. Given the high levels of organic inputs to these systems, they can resist the disturbance they experience from e.g. tillage.
The conventional FP2 soil had significantly higher aggregate stability than its organic neighbour despite less biotic tilth forming agents. This indicates the action of other aggregating agents and our results point to clay. Much of this clay will be dispersed into the pore water, when such a soil gets wet, resulting in an unstable and muddy soil. When drying up, the dispersed clay will cement the aggregates to mechanically hard clods. This was supported experimentally.

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