Thanks for reaching out with this question. I work out of the local Garfield County Extension office in Rifle. You are welcome to reach out directly to the office for additional specific questions or for assistance with soil sampling and testing. The office phone is<personal data hidden>.

Below is an overview of some soil factors that may help answer your concerns. The region in and around the town of New Castle, CO tends to have soils primarily made up of sand and silt particles; However, soil types do vary quite a bit, so there is not an short answer to your question.

If your horse is actively seeking out and ingesting soil I would recommend consulting with a veterinarian in order to discover if there is an underlying medical condition that is leading to this behavior. No matter the soil type, excessive consumption of soil can lead to and be caused by a variety of health issues. 

However, if I am reading your question correctly, it seems that you are curious to know if there is a negative impact of the soil your horse may incidentally ingest by eating feed off of the surface of the ground.

Texture of the soil does impact how much soil may be consumed by an animal while ground feeding. Sand Colic is a concern of ground feeding horses, in part because sand is easier to inadvertently ingest than other soil textures. The good news is that you can gain some understanding of soil texture without a laboratory soil test. I have attached a PDF of how to estimate soil texture by feel and am happy to help assess a sample for you.  

Sand, Silt, and Clay are the three main terms used to describe the texture and structure of soil. Size is the factor that distinguishes between these terms: Sand particles range in diameter from 2 mm to 0.05 mm, are easily seen with the unaided eye, and feel gritty. [One millimeter (mm) is about the thickness of a dime.] Silt particles are between 0.05 mm and 0.002 mm and feel like flour. Clay particles are smaller than 0.002 mm and cannot be seen with the unaided eye

In addition to the structure of the soil, overall soil types take into account the water and air that occupy the area between minerals, the organic matter (dead plant and animal matter along with the macro and micro organisms that inhabit soil). All of these factors influence the others. 

Soil aggregate, or how the partials "cling" on to one another is a significant component of texture. The particle size comparisons above also help illustrate the natural ease of aggregation, compare how wet sand and wet flour hold together differently. Soil organic matter can increase aggregation as well as water holding capacity, so not all soil made up of sand-sized particles always presents the same texture. 

Sand colic is typically a concern for ground feeding horses when there is loose sand and other soil particles that can easily be ingested due to the lack of "cling" between the particles. If you are observing very loose and dusty soils, feed on areas of the run where the ground surface has more "cling" or consider using a large rubber bin or mat for ground feeding. 

I am assuming that texture is your main concern, but if you are also concerned about the mineral compensation of the soil having impacts on your horse, I would recommend sending a soil sample to a lab to better understand the presence and concentration of different nutrients in the soil. 

Best,

Mariah Foley

Agriculture Specialist

Garfield County CSU Extension