Soil is the cornerstone of forest ecosystems, providing the essential nutrients that trees and plants need for growth. However, soil fertility is the ability of soil to supply nutrients to sustain forest growth. Soil fertility varies widely in forests and largely depends on its “parent material”, the original geological material from which the soil is derived. This parent material can include a wide range of geological substances, such as volcanic rocks, ancient glacial deposits, or sedimentary layers laid down over millennia. As these materials break down over time through weathering, they form soils with different chemical properties, which directly influence the types of plants that can grow and thrive in them.

The abundance of these essential nutrients is crucial to understanding the link between geologic origins and soil fertility. Professor James Moore and his team conducted a comprehensive study across 154 forest sites in the Inland Northwest, a region known for its diverse geologic history. This area encompasses central and northern Idaho, western Montana, northeastern Oregon, and central and northeastern Washington—each with a distinct geological backdrop that has shaped its soil characteristics over time.

The researchers were particularly interested in how different types of parent material affect soil nutrient levels and cation exchange capacity (or CEC for short), which measures a soil’s ability to hold and exchange essential nutrients. Their approach involved collecting soil samples from each site and analyzing them for key nutrients, including phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg), as well as assessing the CEC and pH levels, which influence nutrient availability.

One of the most striking findings from Prof. Moore’s research is the significant variation in soil fertility depending on the parent material. Soils derived from volcanic rocks, such as basalt, were found to be particularly nutrient-rich. Basalt and other volcanic materials break down into soils that are high in phosphorus, potassium, calcium, and magnesium—all essential nutrients for plant growth. The abundance of these nutrients is crucial.

For instance, phosphorus is vital for plant energy transfer, photosynthesis, and the formation of DNA. Without sufficient phosphorus, plants struggle to grow and develop. Potassium regulates various physiological processes, including water uptake and enzyme activation, making it crucial for overall plant health and resilience. Calcium plays a key role in building and maintaining cell walls, which gives plants their structure and stability, while magnesium is a central component of chlorophyll, the green pigment in plants that captures sunlight for photosynthesis.

Volcanic soils in this study exhibited a high CEC, which means they can retain and supply nutrients to plants more effectively. The high CEC in these soils allows for a steady supply of nutrients, even in challenging environmental conditions, making volcanic soils highly fertile and conducive to robust forest growth.

In contrast, soils formed from plutonic rocks, such as granite, were found to be less fertile. Granite is composed mainly of quartz and feldspar, minerals that weather into soils with lower nutrient content. These soils tend to be less rich in potassium, magnesium, and calcium compared with their volcanic counterparts. Additionally, granite-derived soils have a lower CEC, meaning they are less capable of holding onto nutrients. As a result, trees and plants growing in these soils may struggle to obtain the nutrients they need, leading to slower growth and potentially reduced forest productivity.

The study also examined soils derived from glacial deposits and sedimentary rocks, which presented a more complex picture. Glacial soils are often a mix of materials transported and deposited by glaciers, leading to significant variability in nutrient content. In some cases, these soils were found to be rich in certain nutrients such as potassium and calcium, depending on the types of rocks ground up by the glaciers. However, the variability in these soils makes them less predictable in terms of fertility.

Sedimentary soils, formed from ancient layers of sediments, showed moderate levels of nutrients. Depending on the specific sedimentary rock type (such as sandstone or shale), these soils can vary in fertility. Shale-derived soils, for example, tend to have higher levels of nutrients than sandstone-derived soils. The study found that these soils typically had a moderate CEC, offering a balance between nutrient retention and availability.

The results of Prof. Moore and his team’s study have significant implications for forest management, particularly in the nutrient-poor regions of the Inland Northwest. Traditional forest management practices often involve applying fertilizers to boost tree growth. However, this study suggests that a more nuanced approach is needed—one that takes into account the specific geologic history and nutrient profile of each forest site.

For instance, in areas where soils are derived from granite, which are typically lower in essential nutrients, forest managers might consider applying fertilizers that are rich in potassium and magnesium to compensate for these deficiencies. On the other hand, in regions with volcanic soils, the need for such interventions might be less critical.

Moreover, the study highlights the importance of monitoring soil pH levels, which influence nutrient availability. For example, volcanic soils often have a slightly acid pH, making certain nutrients more available to plants; whereas carbonate bearing soil parent materials may require acidification to increase nutrient availability. In contrast, very acidic forest soils might require lime or other treatments to adjust the pH and optimize nutrient uptake by trees.

Beyond fertilization, the research underscores the need for sustainable forest practices that protect soil health. Practices such as removal of tree limbs and tops during forest harvesting can remove future soil nutrient pools, potentially leading to long-term fertility loss. This is of particular concern on soils inherently deficient in plant essential nutrients such as metasedimentary quartzite. By understanding the specific nutrient pools found in different soils, forest managers can adopt practices that minimize soil disturbance and preserve the natural nutrient cycles that sustain forest ecosystems.

In conclusion, the research led by Prof. James Moore provides invaluable insights into the complex relationship between geology and soil chemistry in forest ecosystems. By revealing how the geological origins of soil influence its fertility, this study offers practical guidance for forest management in the Inland Northwest and beyond. Tailoring forest management practices to the specific soil deficiencies can lead to healthier, more resilient forests that are better equipped to face the challenges of climate change and ever-increasing demands for a variety of ecosystem services.