Short-term effects of management on the soil structure in a deep tilled hardened volcanic-ash soil (cangahua) in Ecuador

In the Ecuadorian Cordillera, the hardened volcanic ashes (cangahuas) account for 15% of the cultivated area. The soil resulting from the fragmentation of these materials, generally by heavy machinery, shows an apparent stable millimetric structure. However, this new structure is highly susceptible to disintegration under rain, because it contains no organic matter and little clay, and the material itself is readily eroded in consequence. We studied the evolution of soil aggregate stability in two factorial experiments during five cultivation cycles with two kinds of soil preparation and five fertilization treatments. The aggregate stability was not influenced by either kind of soil preparation, nor by large additions of cattle manure (80 t ha^?1) or green manure (10 t ha^?1), nor by growing a perennial grass. The variation in the aggregate stability seemed to depend on the components inherited from the original volcanic material: in the plots with larger clay content, and with swelling clay minerals, the aggregates were less stable than those composed of isometric fine silt particles.