The animals may destroy the hard crust and disturb soil surface, but at the same time it will compact the soil beneath the crust. Additionally, disturbing soil crust is not always good; since it might, on the long term, negatively affect soil microorganisms. Generally, grazing is adding another problem to the problems already exist in the dry lands, and those lands should be protected and reclaimed by a very strict and effective management. 

Grazing (better overgrazing due to bad management) is the main cause of desertification in semiarid regions of the world through loss of the vegetation cover, replacement of species favoring non-palatable species, change in the fire regime,  and soil disturbance. This last effect is highly related with body weight.   

I agree with Enrique. It is not grazing, but overgrazing due to mismanagement that negatively impacts the vegetation and the soil. In fact controlled grazing is necessary to maintain optimal vegetation diversity in a given grassland. So, I dont think that controlled grazing by small ruminants will result in compaction with a negative effect on the soil property.  

Over grazing is the main problem, for sure. The soil compaction (mainly in the first 5 cm) is created not by the physical action of the animals feet but due to the lack of time for the root system of the forrage to be recovered after defoliation. Every time animals graze, part of the root system die and a new one grows. After several short cycles of defoliation the root system is 1/4 of the normal size and the organic matter of the first layer of soil decreases fast. With fertilization and good management (rotational) such problem can be solved without mechanical intervention.

Just to enrich discussion the concept of holistic resource management lead by Allan Savory promotes animal hoof action to disturb soil:

- Break soil crusts. Trackers have observed this for thousands of years. The effect is more pronounced when animals are concentrated in large herds as they do when under threat of predation from pack hunters. The broken crust allows soil to absorb water and to breathe, enabling more plants to germinate and establish.

- Compact the soil under their hooves. Compaction is required for good seed to soil contact to increase germination. This is why gardeners tamp down the soil around seedlings or seeds or some farmers put a heavy roller over certain crops after planting.

Keeping in mind that trampling for too long powders soil, lead to increasing erosion by wind and water. Trampling for too long, especially when soils are wet, also causes compaction in deeper layers that is adverse to plant growth, thus requiring longer recovery times between such tramplings. And dung and urine, like most things in excess, become pollutants as feedlot animal producers soon learn.

From “Science” and “Methodology” Underpinning Holistic Management and Holistic Planned Grazing by Allan Savory

I think 3 cases are exist in range land areas:

- Protected areas: animal grazing forms a positive effect on soil and natural plant cover    because it is causes seed shedding and simple plowing by their hoovers.

- Deteriorated areas: which are subjected to overgrazing.and forms a negative effect on soil and plant cover because it is causes soil compaction by their hoovers. Also it may be cause erosion by wind and rainfall water.

- Improved range lands (fodder shrubs planting in contour ridges): The contour ridges will be damaged by livestock movement and need contentious maintenance.

Skip Holistic grazing.  It does not include five very important issues about domesticated animal grazing--

1.) Mining by the animals of soil nutrients.  When you slaughter the animals or remove their milk, you take those nutrients out of the ecosystem, in effect doing soil nutrient mining. 

2.) Selection and truncation of the grassland ecosystem.  Grasslands world-wide are losing their other half, the wildflowers and other dicots.  For example, the wildflowers in California are 99.99% spatially extinct, after only 250 years of grazing.  See my paintings at http://www.ecoseeds.com/

3.) Soil nutrient mining drawing down the levels below which native grassland plant seedlings need for survival.  See http://www.ecoseeds.com/good.example.html where the grazing/nutrient mining of the soil caused the seedlings to sprout then die from starvation when 4 cm. tall.

4.) Removal by the animals of the local Pseudomonas host plants, which is the bacteria that help create rain clouds for that area.

5.) Native animals including carnivores are usually excluded from grazing areas, further narrowing the areas where those animals have habitat to survive. 

The bottom line, is the crust soil you are concerned about, have enough nutrients and organic matter to produce native seedling survival?  

Take a soil sample from the top 5 cm. back to the lab in a dozen plastic pots, and plant in each pot a different native plant seed, and see if they sprout and survive past the seedling stage?  If not find out what nutrients are missing, add them and see if you can achieve seedling survival. 

Sincerely,  Craig Carlton Dremann - Restoring native grasslands for 45 years.