2.5 Soil Aggregate Scale
When we look closer still, we can see that the soil solids are often organized into three-dimensional assemblages ranging in size from approximately 0.1 to 10 cm. These assemblages are called aggregates, and their presence and characteristics are the defining features of what is commonly called soil structure. Small, rounded, highly-porous aggregates are indicative of soil structure that is well-suited for plant growth; whereas, large, angular aggregates with low porosity indicate structural conditions that are likely to limit plant growth. Often,a wide array of different aggregate sizes are present in a soil at the same time, as illustrated by the photo below. Soil structure and soil aggregates can change over time and are often altered, for better or worse, by human management practices such as tillage, crop planting, or preparing soil for construction projects. Properly-timed tillage practices using suitable implements can improve soil structural conditions near the surface, but these improvements are often temporary. Poorly-timed tillage or traffic by equipment or livestock can degrade soil structure resulting in surface or subsurface compacted layers that may last indefinitely.
Soil aggregate size distributions can be measured while the aggregates are either wet or dry, depending on the purpose of the measurement. Researchers often use a stack of sieves to measure the size distribution of aggregates remaining intact after soaking and shaking initially dry aggregates of a specific size range (e.g. 4-8 mm) in a dilute solution. This process gives the size distribution of water stable aggregates which is intended to represent a soil characteristic called aggregate stability.
Fig. 2‑8 shows size distributions of water stable aggregates for two soils in France [3]. The soil from the forest had greater aggregate stability as shown by the fact that the >2 mm size fraction dominates the distribution. The cropland soil had lower aggregate stability as shown by the larger mass fractions in the smaller size classes (<2 mm), and data from other sites showed that the aggregate stability was lower the longer the soil had been farmed. This trend of decreasing aggregate stability was likely due to a similar decreasing trend of soil organic carbon which varied from 52.6 g kg-1 for the forest soil to 8.4 g kg-1 for the soil which had been farmed for 100 years. These data illustrate the general tendency for aggregate stability to increase as soil organic carbon increases. Soils with higher aggregate stability are typically less susceptible to surface crusting and erosion by natural rainfall, topics addressed in a later chapter.
For educational purposes, a simpler procedure can be used to demonstrate aggregate breakdown due to wetting or to show differences in aggregate stability between soils (Fig. 2‑9). For this photograph, two initially dry aggregates from different soils were immersed in water. The aggregate on the left is more stab