Structural crust is formed from water drop impact. If there is sufficient organic matter on the soil surface, the impact of rain will be lessened and soil structure, microbiology and porosity will be maintained.
It takes time for disrupted soils to become fully active in diverse microbial populations. Organic matter on the soil surface will encourage microbiological activity. Dropped leaves, for example, are ideal mulch over the compost-and-nutrient-mixture we have applied already. On immature or unprotected soils, the rain drops break apart the soil aggregates. These aggregates are formed by the exudates produced by the individual soil biology organisms that then act to adhere particles of soil to one another; when these aggregates have little organic matter in the surrounding or surface soil (e.g., no protection) a structural crust results.
The porosity of soil is, thereby, reliant upon a high level of organic matter and healthy microbiology – the makings of soil aggregates which are THE prevention of erosion.
It is important to leave the soils undisturbed as much as possible (e.g., no raking). The soil biology resides primarily in the top 5 cm of soil; any disturbance, such as raking, disrupts the expansion of a healthy soil foodweb.