When strolling through horse pastures with a keen eye, one often comes across familiar plant species. Some of these plants provide valuable information about the soil conditions, offering insights into the pasture’s needs. These plants, known as indicator plants, exhibit specific ecological tolerance or preferences. Beyond soil conditions, they can reveal valuable clues about soil nutrients, air and water pollutants, as well as the availability of light.
For instance, certain plant species exclusively grow on specific soil types, while others thrive on multiple or mixed soil types, requiring closer examination of the conditions in each case.
One prominent concern in horse pastures is nitrogen levels. There are indicator plants that thrive in nitrogen-rich soils, as well as those that only grow in nitrogen-poor soils. Horse owners are likely familiar with one such nitrogen indicator: the stinging nettle. There’s a good reason why elderberry is also commonly found around muck heaps or dry lots, and it’s not only because horses don’t like to eat it: it has a preference for nitrogen-rich soils. Blackberries and thistles, especially goose thistle, are frequently observed in stables as nitrogen indicators. Moreover, some plants tolerate nitrogen well but, unlike indicator plants, do not rely on high levels of nitrogen. Common mugwort is an example of such a plant. On the other hand, autumn crocus, scentless chamomile, and European yellow-rattle are typical indicators of nitrogen-poor soils, also known as “lean soil.”
If an abundance of nitrogen-indicating plants is present in the pasture, the issue of nitrate becomes a concern. Nitrate serves as a nitrogen source, enabling plants to produce sufficient protein for their growth. Consequently, nitrate-rich soils yield lush green and “juicy” vegetation, while plants on nitrate-poor soils tend to remain small, tough, and dry. High levels of nitrate should be viewed critically in terms of equine health. Nitrate contamination is particularly high in areas where heavy fertiliser use occurs. This includes not only liquid or artificial fertilisers, but also horse manure and urine left on pastures. They are washed into the soil by rain.
Aside from nitrate, heavy metals can pose a problem. Certain plants, known as metallophytes, have the ability to absorb and accumulate heavy metals, potentially affecting the horses’ metabolism if consumed. For example, Welsh ryegrass readily accumulates lead, which remains in the soil, especially near highways and roads due to the era of leaded petrol. Bladder campion, often found in Alpine regions, also has a tendency to store heavy metals. Rockcress, often mistaken for shepherd’s purse, can accumulate nickel, cadmium, and lead.
Soil compaction is another issue frequently encountered in horse pastures, as the weight of horses exerts pressure on the soil at specific points through their hooves. Studies have shown that horse hooves cause more soil compaction compared to tractors. The degree of compaction depends on factors such as soil type, moisture levels, the number of animals per area, and pasture management. The German Federal Soil Protection Act even emphasises the importance of ensuring sustainable soil fertility and performance while avoiding soil compaction.
Compacted soil reduces soil life due to limited habitat and oxygen availability. Consequently, fewer nutrients are accessible to plants, impacting vegetation growth. Soil compaction is often observed on dry lots for the winter, where water from rain or snow struggles to seep into the ground and instead runs off superficially. Consequently, water is not available to plants as soil moisture or groundwater in the following weeks and months, but instead contributes to sewage pollution during heavy rainfall.
Due to the compacted soil on dry lots, trails or pathways, the plant’s roots aren’t able to deeply penetrate the soil, consequently having worse access to nutrients and less resistance against drought and damage by the horse’s gnawing. The most known indicator plants for compacted soil are broadleaf plantain, dandelion (in its small, flat-growing form), and creeping buttercup. Common couch grass, often mistaken for ryegrass, also falls into this category.
On moderately dry soils commonly found in horse pastures, indicator plants such as sorrel can be found. Sorrel thrives in acidic and nutrient-poor soils. Horses typically don’t eat it fresh or dried in hay, but rather avoid eating it. As sorrel often stays on the fields until after the seeds have formed, more and more sorrel can be found on the pasture every year. Unfortunately, the only remedy against this invasive plant is the laborious task of removing it manually with a special digging tool, as it can propagate through both seeds and roots.
