# Soil scientists study groups of soils in sequence ## Soil forming factors explain why soils vary How lucky we are to live among the vast diversity of soils on Earth. The soil taxonomic system in the United States has identified over 20,000 [[Soil names are like a puzzle| soil series]] that describe unique soil forms around the country. This variation in soil types arises from differences in 6 soil forming factors: climate, organisms, parent material, topography, time, and humans [^lewis]. Unique combinations of these factors influence the [[Soils are always forming, and so are we| soil forming processes]] and ultimately controls the diversity of soil types we find worldwide. ![[soil forming factors-1.png]] When soil scientists want to understand how a soil forms and functions, we begin by putting that soil into the context of the soil forming factors. We ask ourselves: what is the climate in the area, past and present? What kinds of rocks underly this region? How old is the soil? What kinds of plants, animals, and human influences are present? Where on the landscape does the soil exist? Some of these questions are more straightforward to answer than others. Uncovering the answers is necessary to understanding and describing how a soil formed over its' history. We use this knowledge to infer how a soil will continue to form and change into the future. Knowing this context helps a lot when making decisions about whether and how to manage or manipulate a soil. But, there's an inherent challenge in studying the soil forming factors: the characteristics of a given soil arise from interactions between the soil forming factors. Soils emerge from these often complex interactions, so it can be difficult to isolate whether a soil property (like[[Soils are colorful natural wonders| soil color]] is due to the present or historical climate, or if instead it has something to do with the minerals weathering from geologic material. In reality, it's usually a "yes and" situation where several soil forming factors influence a soil property. Multiply that by all the many physical, chemical, and biological properties that we can observe in a soil, and it's clear how every soil body becomes a unique product of its' environment and history. [^lewis]: I like Lewis Fausak's graphic describing the soil forming factors in a concise way. https://www.instagram.com/p/CvYYUQ-vqIX/?img_index=1 ## Soil sequences are natural experiments How do soil scientists disentangle the impacts of one soil forming factor over another? Like in any other scientific experiment, our goal is to control for all variables expect the variable of interest to be able to directly test to influence of just that one variable on the response. Imagine attempting to perfect your apple pie recipe for your siblings' birthday celebrations next month. If you decided to take a scientific approach, you would only change one ingredient at a time. Double the cinnamon, prepare the pie, and then observe the outcome (I bet it's delicious). If you double the cinnamon and use a different type of apple in the same bake, you may not know if the improved outcome is from the cinnamon or from the apples (maybe it's both). You can't isolate the effect of the cinnamon from the effect of the apples. To study soil formation, we need to find a way to isolate each soil forming factor (the ingredients), control for all the other factors, and study how soil properties (the pies) change. The problem is, soil form over long timescales and we can't really "watch" a soil form in real time. Instead, we have to find clues in the soil properties as they are now to infer how a soil has changed over time. We need to find a place in nature where soils under different circumstances already exist. This is called a natural experiment. To do this kind of natural experiment, we use a group of soils called "sequences"[^jenny]. A sequence is a group of soils in which all but one soil forming factor remains constant. One soil forming factor varies across the sequence, and we can use this gradient to isolate the influence of that one factor. There are five types of soil sequences. ![[types of soil sequences.png]] We then measure properties in [[Soils form in profiles and horizons|soil profiles]] within the sequence and are able to evaluate how a given soil forming factor influences those properties. With this kind of careful examination of soil sequences, we can begin to disentangle the role of each soil forming factor. This observational approach requires that we assume that each soil in the sequence represents a different stage of formation along the gradient we are studying. Some natural experiments meet this assumption better than others. It's often difficult to find soils that are truely identical in all the soil forming factors but one, because conditions often vary together. For example, some plants are only found in certain climates. Even so, the sequence approach is one of the best tools we have to study soil formation using the puzzle pieces of nature that we are able to observe in the present. [^jenny]: The idea of studying soil formation by holding all but one soil forming factor constant was formalized by Hans Jenny in his book Factors of Soil Formation: A Quantitative Pedology in 1941. ## Take home message Interactions between the soil forming factors are responsible for the diversity of soil types, forms, and functions around the world. Each soil is a product of a unique combination of interactions between soil materials and the environment over time. We study how these factors influence soils using natural experiments where soils vary along gradients we call "sequences". The context in which a soil is forming is just as important to knowing a soil as the soil features themselves. #### Postscript > This essay is part of [[One thing about soil - an educational series]] created by Dr. Yamina Pressler. The essay was originally shared on substack. *updated Aug 19, 2023*